Honda-SOHC
SOHC.co.uk Forums => CB350/400 => Topic started by: Ken4004 on May 02, 2026, 05:27:33 AM
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Had an issue with the bikes engine overheating after going up a step hill so when I stopped at traffic lights the bike died, left it 20 mins and I was able to ride the 20 miles home.
I suspect this is due to either the bikes engine overheating running a bit lean and or timing slightly out.
I did think the bike was running a little rich so because I had run out of adjustment on the air screw I decided to move the carb needle up one notch to lean it out but I think that was too much of an adjustment so will put them back to the middle position.
I have been able to do this without removing the carbs , a little fiddly but possible and means I can complete the task in under two hours for all four carbs which
Isn’t too bad.
So the plan is adjust needles
Reset air screw to standard position
Recheck points
Reset timing and strobe
And finally sync carbs
Probably would be an idea to check the valve clearance as well while I am at it.
Still running the bike in after engine rebuild so covered 280 miles, I think there is a bit of a flat spot on performance after 5000 revs so hopefully all the work about will improve this, I did consider changing the plugs because I am using NGK DR8ES plugs and the manual says DR8ES-L so not sure what difference that will make.
I did also check the ignition auto advance assembly for the points to see if that was sticking but it seemed to be OK last time I looked and using the strobe I could see it advancing when the revs increased.
Sorry about all the waffle probably just thinking allowed while typing this, I think another problem is that I am normally riding my Triumph 1050 so I might be confusing the sluggish throttle response with the fact that the performance of the 400 is so much lower so comparing 120 horses with 37 so I really need to ride another bike to make a good comparison.
That’s my day planned out unless the wife has a job for me !
Well you will be glad to here I finished waffling for one day !
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If you raise the needles upwards, that will make the mixture even richer. The needles are tapered, so the higher position admits more fuel earlier as you open the throttle.
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A flat spot beyond 5k RPM is essentially the opposite of the way these bikes typically act so clearly something is not as it should be. I'm curious about the "running out of adjustment' comment re the air screws. The normal adjustment range is fairly narrow and "running out of adjustment" sounds to me like the low speed circuit is clogged/otherwise not functioning properly.
As far as above 5k RPM, retarded ignition timing could cause poor high speed response. Retarded ignition will also cause overheating. A clogged muffler can cause both as well.
I agree that returning everything to the stock settings is the best way to start the troubleshooting process. Of course that assumes that the various components - ignition system/carbs are all adjusted to spec/working correctly. So that might require carb disassembly for a thorough cleaning/setup. Carb float level and fuel flow from the tank should be checked as well as the vent on the tank's cap. A clogged cap vent can cause a lack of power under high engine load - sometimes it can be bad enough where the engine will idle but not run well in local low-speed traffic.
FWIW re fuel flow - I once had an issue with an add-on aftermarket fuel filter causing a restriction. Removing that filter eliminated the problem. I no longer add any fuel filters; I rely on the ones that came OEM! Re that - pull the OEM filter from the tank and check its condition/replace if necessary.
In any case, good luck!!!
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How was the fuel level in the tank? Was it on the verge of needing to be on reserve? And the strep hill altered how the fuel sits in the tank
Regards
Dave
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Hi Guys thanks for all the replies let’s answer the questions
I dropped the needle once notch because it was running I thought a bit rich so will move it back to the middle position so stock position
I believed I had run out of air screw adjustment because I adjusted it in small intervals and went out for a ride and I believed it was still running rich.
I had completely stripped the carbs and cleaned everything, fitted new carb kit all except the original needles because they looked good ( what was I thinking school boy error)
I took the carbs apart and fitted the new needles and put the cir clip one notch up to make it run leaner I should have really just fitted the new needles and then tried it.
I have read that regarded ignition can cause overheating but it did look good when I strobed it but will definitely recheck.
I have a pre filter on the tank so this can be removed and see what happens
The fuel cap vent being clogged is a good possibility as I have never checked this .
And finally I did have plenty of fuel in the take when I went up the hill the bike stopped when O was on the level just pulling up to the traffic lights, I also like the sound of the muffler being blocked as it did have quite a lot of loose rust in it which I thought I had cleaned out which is not easy to do.
Well you have given me a lot to think about but as I said first get it back to stock adjustment then carry on from there cheers guys I keep you updated regards Ken
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There's a huge difference in torque between those two bikes :) and quite different driving style to go with that. The little four really doesn't suit anything towards lugging, but thriving on revs.
Its possibly the most sensitive (of these fours) in serious consequences, to jetting and especially the effects of running too lean. Certainly good to establish a standard scenario to then evaluate from there I'd agree with.
The plugs, and that L designation. The L should push the plug tip out further into the combustion chamber, that to in effect give a grade of 7 1/2 in NGK terms. This was, in my understanding, to try and mitigate fouling by raising the tip temperature without going to a 7 graded plug. I don't feel that would help here without further analysis, but raise your risk in overheating effects further, which you don't want to do.
If anything, it may help to go to a NGK 9 rated plug to give more safety in margin for plug tip temperature (plug runs cooler) during this evaluation. To ultimately be reviewed when you've established more comprehensively the mixture you've got in practice.
If you're running with E fuel, then the fuel contains, relative to original fuel when jetting etc was specified, more oxygen to impact the decision now being made.
To err on the side of caution in jetting and evaluation is priority for the experience you've just had. It's easy to compromise these engine otherwise.
A question, running out of travel on airscrew, which direction was that, in or out end ?
The flat spot at revs above 5000rpm does tally with lean mixture, the mix being reluctant to fire competently, then opening the throttle with revs not rising (classic flat spot) will drive these slide carbs toward even more lean as vacuum drops and less fuel is pulled from float bowl.
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I had similar issues with the wear in the emulsion tubes giving blackish plugs, confirmed it by swapping in a spare set followed by a new OEM set. The bike would stumble on off on the throttle at low revs in traffic , made it a bit of a handful. My son behind me on the road said it stunk of petrol.
There was corrosion pitting around the top of a couple of tubes. I could easily see the difference by dropping a needle in and seeing how much it wiggled.
No other changes were made other than swapping out the tubes and needles followed by a static balance.
All caused by leaving the bike for a couple of years with fuel residues in the carbs.
The D8EA was the replacement when the ESL was discontinued.
Regards
Dave
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There's a huge difference in torque between those two bikes :) and quite different driving style to go with that. The little four really doesn't suit anything towards lugging, but thriving on revs.
Its possibly the most sensitive (of these fours) in serious consequences, to jetting and especially the effects of running too lean. Certainly good to establish a standard scenario to then evaluate from there I'd agree with.
The plugs, and that L designation. The L should push the plug tip out further into the combustion chamber, that to in effect give a grade of 7 1/2 in NGK terms. This was, in my understanding, to try and mitigate fouling by raising the tip temperature without going to a 7 graded plug. I don't feel that would help here without further analysis, but raise your risk in overheating effects further, which you don't want to do.
If anything, it may help to go to a NGK 9 rated plug to give more safety in margin for plug tip temperature (plug runs cooler) during this evaluation. To ultimately be reviewed when you've established more comprehensively the mixture you've got in practice.
With regards to running out of air screw adjustment it was out trying to make the bike leaner, so my theory which I think was incorrect is to adjust the air screw go out for a ride then check the plug colour so kept doing that and still had sooty plugs so decided to change the needle height and put the air screw to 1 1/2 turns out and that’s when I had the over heating issue.
Just to clarity when the bike died it would not even turn over until it had cooled down , never tried to kick start because it was so hot.
Let it cool down for about half an hour and it started with no hesitation.
Carbs now back to stock needle position and air screw at 1 1/2 turns out from fully in , going to see what it runs like and recheck the timing / advance etc.
If you're running with E fuel, then the fuel contains, relative to original fuel when jetting etc was specified, more oxygen to impact the decision now being made.
To err on the side of caution in jetting and evaluation is priority for the experience you've just had. It's easy to compromise these engine otherwise.
A question, running out of travel on airscrew, which direction was that, in or out end ?
The flat spot at revs above 5000rpm does tally with lean mixture, the mix being reluctant to fire competently, then opening the throttle with revs not rising (classic flat spot) will drive these slide carbs toward even more lean as vacuum drops and less fuel is pulled from float bowl.
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From the experience with my 400F over the past few years, the bike will run great on E10 fuel with totally stock settings on everything - jet size, needle position, airscrew turns, STOCK type muffler and stock air filter/airbox. As noted, leaving untreated fuel (without some sort of fuel storage additive) in the tank and especially in the carbs for extended period will have bad results.
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Since splashing out on OEM brass ware 15 years ago I take no chances now. I used an ethanol protection additive at each fill up. I keep a 8 ml bottle of it in my jacket pocket. Then at the end of the season I flush the carbs through with white spirit and then refill them with white spirit with a dash of oil in it.
Even with these precautions the brass ware still goes dull. Yet brass ware out of old carbs that's not seen unleaded fuel still looks like brass and the float bowls look like new inside.
Mine got into such a mess before the when it had been parked for 3 years when I worked at sea. Carbs had been emptied but not flushed out.
Regards
Dave
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Here is a picture of one of the spark plugs I took after I got home from my ride when it over heated, it looks normal to me but you might not agree .
Light tan colour which according to the manual is the normal colour , all spark plugs look the same.
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I did have a look at the breather hole in the fuel cap but couldn’t see a way of taking it apart easily.
Rain stopped play this afternoon but managed to get the carbs back to standard settings , c it checked the valve clearances which all look good, reset the points gap so will if I get time tomorrow recheck the timing snd rebalance the carbs.
If I get time I’ll take off the silencer and give it a tap with a rubber mallet and see if any more rust comes out.
I don’t think you can remove the baffles out of this Silencer as it appears to be welded together unless anyone knows different.
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Here is a picture of one of the spark plugs I took after I got home from my ride when it over heated, it looks normal to me but you might not agree .
Light tan colour which according to the manual is the normal colour , all spark plugs look the same.
I would call that black/sottey
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I had similar issues with the wear in the emulsion tubes giving blackish plugs, confirmed it by swapping in a spare set followed by a new OEM set. The bike would stumble on off on the throttle at low revs in traffic , made it a bit of a handful. My son behind me on the road said it stunk of petrol.
There was corrosion pitting around the top of a couple of tubes. I could easily see the difference by dropping a needle in and seeing how much it wiggled.
No other changes were made other than swapping out the tubes and needles followed by a static balance.
All caused by leaving the bike for a couple of years with fuel residues in the carbs.
The D8EA was the replacement when the ESL was discontinued.
Regards
Dave
Been there and seen that, good post and good advice.
Absolutely agree...
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I don't think fuel flow is the issue here, you're getting more than enough.
Time to check the easy bits before tearing stuff apart too much
Spare bit of pipe on the fuel tap into a clean recepticals and try the fuel tap in both pisitions
Choke flaps fully open once the leaver is clicked in the open position
Corrosion inside the post where the main jet sits, or fuel getting by the o ring
Float heights are the float valves sealing or allowing the fuel level to rise too much
Clean plugs with a blow torch before trying again. There's always a risk that you have cleared the problem but not burned the crud off the plugs during normal running.
Sounds like you are going through the same mill I did 15 years ago. I replaced all the brass ware in the rebuild kit but the needles and emulsion tubes are not in the kit.
Regards
Dave
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From this "Just to clarity when the bike died it would not even turn over until it had cooled down" do I understand correctly that it had partially seized, which stopped you turning the engine over ken4004 ?
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It was like the starter motor didn’t even attempt to turn the engine over, in hindsight I should have tried the kick start.
There may be a possibility that there may not be a sufficient enough gap on the piston rings causing excessive friction as the engine heats up but I would have thought if that was the case the engine would have completely seized by now it had done 280 miles since the rebuild.
I would have thought I would have heard the starter solenoid clicking but as I said pressing the starter button absolutely nothing which is a bit strange.
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Setting the valve clearances today the engine turns easily as before, with an engine seizure you would think the engine would be difficult to turn over by hand at the nut on the points !
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I think you have an electromechanical gremlin that stoped the engine and starter didnt work
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I think you have an electromechanical gremlin that stoped the engine and starter didnt work
Still in gear and clutch lever not pulled all the way in. We have probably all done it much to the annoyance of cars behind us.
Dave
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From this "Just to clarity when the bike died it would not even turn over until it had cooled down" do I understand correctly that it had partially seized, which stopped you turning the engine over ken4004 ?
That sets alarm bells ringing especially after a recent engine rebuild.
A partially blocked oil pump strainer gauze screen (possibly with surplus gasket
sealer etc) can cause restricted oil flow to the cylinder head, over heating and eventual camshaft seizure.
I'd pull the sump pan off and inspect the strainer gauze and check the oil filter at the same time.
Good luck
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I think you have an electromechanical gremlin that stoped the engine and starter didnt work
anything is possible I am not ruling anything out and will continue double checking everything.
I think however that the issue is heat related at its root because it has never happen before in over 200 miles it was a hot day and the hill I went up was quite long and steep so if found out the weakness in the bike.
As I did day earlier once it had cooled down enough I took it easy and made it home without any issues so it’s a bit strange.
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Here is a picture of one of the spark plugs I took after I got home from my ride when it over heated, it looks normal to me but you might not agree .
Light tan colour which according to the manual is the normal colour , all spark plugs look the same.
I would call that black/sottey
OK you can see why I was looking at the needle heights to make it leaner
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I don't think fuel flow is the issue here, you're getting more than enough.
Time to check the easy bits before tearing stuff apart too much
Spare bit of pipe on the fuel tap into a clean recepticals and try the fuel tap in both pisitions
Choke flaps fully open once the leaver is clicked in the open position
Corrosion inside the post where the main jet sits, or fuel getting by the o ring
Float heights are the float valves sealing or allowing the fuel level to rise too much
Clean plugs with a blow torch before trying again. There's always a risk that you have cleared the problem but not burned the crud off the plugs during normal running.
Sounds like you are going through the same mill I did 15 years ago. I replaced all the brass ware in the rebuild kit but the needles and emulsion tubes are not in the kit.
Regards
Dave
Hi Dave when rebuilding the carbs I checked the float heights and they were all spot on, did have an issue with float valves and had fuel leaking out the drains so I have changed all the valves and all looks good .
I have drained the tank a couple of times through the tap and flow was good but will check again .
Can you please clarify what you mean by emulsion tubes ?
I am going on holiday for three weeks on Monday so running out of time but will pick this up again when I get back.
Would you be interested in meeting up when I get back for a coffee at a neutral venue it would be nice to meet you face to face, I am only in Copthorne near Gatwick and I see your only in Wimbledon so not far away from each other.
Cheers Ken
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I think you have an electromechanical gremlin that stoped the engine and starter didnt work
anything is possible I am not ruling anything out and will continue double checking everything.
I think however that the issue is heat related at its root because it has never happen before in over 200 miles it was a hot day and the hill I went up was quite long and steep so if found out the weakness in the bike.
As I did day earlier once it had cooled down enough I took it easy and made it home without any issues so it’s a bit strange.
When I checked the valve clearances there was plenty of oil around the cam shaft snd the oil light always goes out straight away but I will check the filter and gauze as I said earlier I am not ruling anything out, I don’t really find this frustrating just interesting and will get to the bottom of it.I see this as a learning curve the more I dig into this issue the more I learn so all good experience.
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I don't think fuel flow is the issue here, you're getting more than enough.
Time to check the easy bits before tearing stuff apart too much
Spare bit of pipe on the fuel tap into a clean recepticals and try the fuel tap in both pisitions
Choke flaps fully open once the leaver is clicked in the open position
Corrosion inside the post where the main jet sits, or fuel getting by the o ring
Float heights are the float valves sealing or allowing the fuel level to rise too much
Clean plugs with a blow torch before trying again. There's always a risk that you have cleared the problem but not burned the crud off the plugs during normal running.
Sounds like you are going through the same mill I did 15 years ago. I replaced all the brass ware in the rebuild kit but the needles and emulsion tubes are not in the kit.
Regards
Dave
Hi Dave when rebuilding the carbs I checked the float heights and they were all spot on, did have an issue with float valves and had fuel leaking out the drains so I have changed all the valves and all looks good .
I have drained the tank a couple of times through the tap and flow was good but will check again .
Can you please clarify what you mean by emulsion tubes ?
I am going on holiday for three weeks on Monday so running out of time but will pick this up again when I get back.
Would you be interested in meeting up when I get back for a coffee at a neutral venue it would be nice to meet you face to face, I am only in Copthorne near Gatwick and I see your only in Wimbledon so not far away from each other.
Cheers Ken
Hi Dave sorry I was being a bit thick I know what you mean by Emulsion tubes , when I stripped the carbs they were all partially blocked with hard deposits. I have some very fine drill bits at home so cleaned out the holes with a slightly smaller drill bit and then followed through with the correct size without removing any of the tube material but they were pretty bad but they are clean on all carbs.
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From this "Just to clarity when the bike died it would not even turn over until it had cooled down" do I understand correctly that it had partially seized, which stopped you turning the engine over ken4004 ?
That sets alarm bells ringing especially after a recent engine rebuild.
A partially blocked oil pump strainer gauze screen (possibly with surplus gasket
sealer etc) can cause restricted oil flow to the cylinder head, over heating and eventual camshaft seizure.
I'd pull the sump pan off and inspect the strainer gauze and check the oil filter at the same time.
Good luck
It does for me too, and similarly feel caution in checking possible cause is time well spent.
My thoughts also go into running lean, the situation described with high heat long hill, flat spot (classic lean consequence) and motor coming to a stop. Then not re starting but free when cooled .... suggest "nipped up" to me, if no poor oil supply etc that points to classic partial bore seizure.
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I don't think fuel flow is the issue here, you're getting more than enough.
Time to check the easy bits before tearing stuff apart too much
Spare bit of pipe on the fuel tap into a clean recepticals and try the fuel tap in both pisitions
Choke flaps fully open once the leaver is clicked in the open position
Corrosion inside the post where the main jet sits, or fuel getting by the o ring
Float heights are the float valves sealing or allowing the fuel level to rise too much
Clean plugs with a blow torch before trying again. There's always a risk that you have cleared the problem but not burned the crud off the plugs during normal running.
Sounds like you are going through the same mill I did 15 years ago. I replaced all the brass ware in the rebuild kit but the needles and emulsion tubes are not in the kit.
Regards
Dave
Hi Dave when rebuilding the carbs I checked the float heights and they were all spot on, did have an issue with float valves and had fuel leaking out the drains so I have changed all the valves and all looks good .
I have drained the tank a couple of times through the tap and flow was good but will check again .
Can you please clarify what you mean by emulsion tubes ?
I am going on holiday for three weeks on Monday so running out of time but will pick this up again when I get back.
Would you be interested in meeting up when I get back for a coffee at a neutral venue it would be nice to meet you face to face, I am only in Copthorne near Gatwick and I see your only in Wimbledon so not far away from each other.
Cheers Ken
Hi Dave sorry I was being a bit thick I know what you mean by Emulsion tubes , when I stripped the carbs they were all partially blocked with hard deposits. I have some very fine drill bits at home so cleaned out the holes with a slightly smaller drill bit and then followed through with the correct size without removing any of the tube material but they were pretty bad but they are clean on all carbs.
Emulsion tubes on a CB400F, also referred to as needle jet set.
https://www.cmsnl.com/honda-cb400f-england_model14639/zestaw-igiel-odrzutowiec_16012377004/ (https://www.cmsnl.com/honda-cb400f-england_model14639/zestaw-igiel-odrzutowiec_16012377004/)
I had the same symptoms on my F1 as Dave so nicely described on his earlier post above.
Went into denial mode when I saw the price, but when I eventually bit the bullet and renewed them it felt like I was riding a different bike. Very happy.
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From the experience with my 400F over the past few years, the bike will run great on E10 fuel with totally stock settings on everything - jet size, needle position, airscrew turns, STOCK type muffler and stock air filter/airbox. As noted, leaving untreated fuel (without some sort of fuel storage additive) in the tank and especially in the carbs for extended period will have bad results.
I don't see particular problems either, but here with possibly lean mixture (flat spot etc) it could help compound any effects by containing more oxygen in fuel mix.
There's reasonable margin (there usually is on non accelerator pumped slide carbs) as they have to be set with bias to slightly rich to run correctly (the bog std setup) but with needles dropped plus E10, there's some measure of compound effect that eats the margin provided by standard settings.
My view is that these engine are very sensitive to being run in the correct air fuel range, when everything is fine. Outside that though, unintended consequential problems quickly build.
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From the experience with my 400F over the past few years, the bike will run great on E10 fuel with totally stock settings on everything - jet size, needle position, airscrew turns, STOCK type muffler and stock air filter/airbox. As noted, leaving untreated fuel (without some sort of fuel storage additive) in the tank and especially in the carbs for extended period will have bad results.
I don't see particular problems either, but here with possibly lean mixture (flat spot etc) it could help compound any effects by containing more oxygen in fuel mix.
I am always running the bike on E5 my Triumph I alternate between E5 and E10 so would that be E7.5 ( sorry little joke)
I have had a busy morning , removed exhaust and it’s clear, remove oil , removed oil filter, removed exhaust , remove sump and checked sump filter which had a little debris but nothing I would say would restrict oil flow around the engine.
All back together snd going to re do the timing and re sync the carbs and thats me done for the day.
There's reasonable margin (there usually is on non accelerator pumped slide carbs) as they have to be set with bias to slightly rich to run correctly (the bog std setup) but with needles dropped plus E10, there's some measure of compound effect that eats the margin provided by standard settings.
My view is that these engine are very sensitive to being run in the correct air fuel range, when everything is fine. Outside that though, unintended consequential problems quickly build.
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https://youtube.com/shorts/fo1BwZChoxk?si=zYPBbhgGbNOJM228
Bike carbs sync these don’t look bad to me they were well out but not surprising as I had the carbs apart
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Everything set up again today so just to recap :-
The fuel cap breather is working correctly
The oil filter and oil strainer are clean
The spark plugs look gaps are correct
The timing has been reset and checked with a timing light
The advance mechanism is moving freely and advancing the timing when revel up.
Carbs have been balanced with pro tune
The fuel is flowing freely from tank, removed fuel Tap a cleaned filter just a few bits on the filter.
Bike starts straight away and idles nicely at 1200 rpm
Bike revs up to 6000 freely ( haven’t gone past that as still running in) when not in gear
Wife has been very patient with me so need to get reading for our holiday so that’s it till I get back in a couple of weeks.
Jobs to do
Remove spark plugs and clean with a blow torch to clean up then go out for a run and see how she feels, thanks everyone for your input.
One last thing I think I need some new leads for my coils they are too short on number two cylinder and the spark plugs caps need changing, there is a possibility that I might be getting an occasional misfire on number two cylinder because of the ht leads I think.
Can anyone recommend a new set of coils for my 400 4 that are decent , original coils are ok but leads are moulded in so cannot be changed so ideally I would like some coils that I can change the leads on when they get ropy.
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Alternatively if someone has a set of coils they are not using I could fit them to my bike and modify the originals in slow time
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No disrespect, but even when running in, if you do not go above 6000RPM, then you are not doing it any favours, they don't even start breathing properly till about 7000 or 7500, so if you were going uphill in top gear at 6000 or less then, yes, you could overheat as you are virtually labouring it. You mentioned your other bike which is no comparison whatsoever with the 400, which has very small pistons, very oversquare dimensions allied to no flywheel. They do need to be kept spinning at a reasonable rate.
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Hi Tricky that’s a very good point , I didn’t really think about this particular road being an issue but from what I have read on this forum this bike needs to be reved hard so really I wasn’t giving it a chance.
I haven’t wasted my time re running through the set up because things had changed and needed to be tweaked while the bike settled down after the rebuild.
Had a lot of issues fitting the exhaust the first time because it was from another bike but once fitted and heated up it was relatively east to remove and refit.
Next time I go out for a ride I’ll give it some more revs just being over cautious I suppose.
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Everything set up again today so just to recap.
Can anyone recommend a new set of coils for my 400 4 that are decent , original coils are ok but leads are moulded in so cannot be changed so ideally I would like some coils that I can change the leads on when they get ropy.
I fitted Dyna S coils as part of their electronic ignition kits, I adapted the existing coil brackets to make the Dyna coils fit.
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We've all been there Ken, but if it's any consolation, I've just dug out my original owners handbook, and it advises for the first 600 miles, do not exceed 8000RPM, and that's with everything being brand new. Enjoy it!!!
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Thanks For the info I’ll open her up a bit more, I must say I am very impressed with the handling around the bends it really inspires confidence.
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This is an interesting looking bit of kit - not cheap, with Boyer-Bransden name.
www.ebay.co.uk/itm/364680555365
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I noticed yesterday that no 2 exhaust pipe warmed up a little slower than the test although it seemed to start OK so I believe the cable / plug on no 2 is occasionally breaking down.
The leads on all plugs are a bit short like they have been shorted a number of times so time for new hardware .
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I noticed yesterday that no 2 exhaust pipe warmed up a little slower than the test although it seemed to start OK so I believe the cable / plug on no 2 is occasionally breaking down.
The leads on all plugs are a bit short like they have been shorted a number of times so time for new hardware .
That rings alarm bells for a poor running cylinder. Given that your other investigations have not given anything confirmed as probable cause, that partially missing cylinder needs investigating for both ignition and idle circuit fuel supply.
If you have a borescope it would be worthwhile looking in there to see if there's anything untoward. The cheap type for phone, with front & side facing camera are really good value to get decent inspection.
Hope you have a good holiday, anywhere interesting ?
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I noticed yesterday that no 2 exhaust pipe warmed up a little slower than the test although it seemed to start OK so I believe the cable / plug on no 2 is occasionally breaking down.
The leads on all plugs are a bit short like they have been shorted a number of times so time for new hardware .
That rings alarm bells for a poor running cylinder. Given that your other investigations have not given anything confirmed as probable cause, that partially missing cylinder needs investigating for both ignition and idle circuit fuel supply.
Off to Kefalonia in Greece for three weeks , going around the whole Island staying one or two nights in several places so should see the whole island,
I was looking at endoscopes a couple of months ago and thought it might be useful bit of kit and it appears that I was right , I did a very detailed and meticulous clean of the carbs when I had them off the bike but as I said before I am not ruling anything out.
I was going to change the coils anyway in the future because of the dodgy looking leads so this is a good a time as any to get it done and cross another possibility off the list.
Thanks for the advice regards Ken
If you have a borescope it would be worthwhile looking in there to see if there's anything untoward. The cheap type for phone, with front & side facing camera are really good value to get decent inspection.
Hope you have a good holiday, anywhere interesting ?
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I think you have an electromechanical gremlin that stoped the engine and starter didnt work
Still in gear and clutch lever not pulled all the way in. We have probably all done it much to the annoyance of cars behind us.
Dave
I have just got back from holiday and went out for a ride and 10 miles in the bike overheated again and died at a junction and you were spot on Dave , the bike would not initially start but I then remembered what you said and realised that I was in gear and started it again straight away but it was hesitant and would stall at low revs so nursed it home.
I did have my suspicions about no two cylinder not firing properly and so decided to look at the plugs again and every plug looked fine except no two cylinder which was sooty.
On another post I did talk about changing the HT leads on the coils as per Ash’s modification and have bought all the parts needed for the mod.
So just to let you know what I found , whilst preparing the coils reading for fitting the new cables I thought I would first check the resistance between the leads between 1 and 4 which came out at about 24 ohms with the caps fitted and 14 ohms with cable only so what I expected.
When checking the 2 and 3 coil no resistance so open circuit and after stripping back the insulation found that the HT lead was broken half way down and showed signs of corrosion, just pulling on the lead it was stretching so I knew the wire had completely broken.
So in a nut shell when originally checking for a spark all plugs looked good so assumed the leads were OK but a little short , when starting the bike for the first time noticed that number 2 cylinder exhaust pipe was not getting hot straight away then finally after a few minutes got up to temp leading me to suspect the HT lead.
Bike has done about 200 miles after the rebuild and didn’t seemed to me to be pulling as it should so must have been only running on three cylinders most of the time which would explain a lot.
Anyway in the process of repairing / replacing all HR leads on all coils which will take a couple of days due to waiting for expoxy and putty to dry and then I’ll get it all back together clean the plugs check the HT resistances and take it for another spin and see how it goes.
I am not really sure if running the bike on only 3 cylinder would cause the engine to overheat but sounds very plausible because the other working cylinders are working extra hard .
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Hopefully you'll get to the bottom of it with the coil rebuild, then verify its firing properly on all 4 cylinder.
Running on three will cause it to be a heat risk, as you've found. They've only about 24 ft/lb of torque @ 7500rpm when healthy, so starting with approx 18 lb and running below peak torque will likely bring about 15 lb at a, licked finger in the wind, estimate.
Trying to pull uphill like that will severely compromise its output, ultimately to cause lean running through that set of circumstances. Essentially, the crank rpm can not "follow" throttle opening as there's insufficient torque to accelerate the bike and rider. That causes you to open the throttle further, which drops the vacuum affecting jets, to then run permanently lean (if the crank rpm never catches the throttle slide position) up gradient being the biggest risk, as it can never catch up.
"Lean" running, in and of itself doesn't necessarily increase heat as a cause. What happens as the combustion chamber reaches its ability to burn fuel as it approaches the 14.7:1 ideal (stoichiometric) the propagation of flame front outward from the spark plug begins to break down such that the burn phase doesn't spread out to far reaches of tbe periphery of piston/chamber. This results in unburnt "gas" mixture in a ring around that periphery, which then explodes rather than burns as piston comes to tdc. Its this uncontrolled event that gives a shock wave that breaks the boundary layer protecting the piston crown from exposure to high heat and subsequently raises the overall temperature if left unchecked. When more extreme, we can hear this as "pinking" but it starts in virtually inaudible levels before we become aware of it in more severe form (we're isolated from source & with helmet sound insulation too) but it happens and significantly shifts the temperature accumulation, that also being relative to the ambient air trying to cool tbe motor and it's capacity if moving more slowly. Detonation is the process, as there's loss of control thats fundamental to how the fuel air mix is supposed to combust, rather than explode. Essentially it has the effect of advancing the timing to detriment of the engine's desirable running condition.
There's another phase going on from this as too much of the above will likely bring pre-ignition, which is usually catastrophic, and in short time frame too :o this will definitely put a hole in tbe piston for you!
Seems like you've got away with it in recent description, but quite close to serious damage though.
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Hopefully you'll get to the bottom of it with the coil rebuild, then verify its firing properly on all 4 cylinder.
Running on three will cause it to be a heat risk, as you've found. They've only about 24 ft/lb of torque @ 7500rpm when healthy, so starting with approx 18 lb and running below peak torque will likely bring about 15 lb at a, licked finger in the wind, estimate.
Trying to pull uphill like that will severely compromise its output, ultimately to cause lean running through that set of circumstances. Essentially, the crank rpm can not "follow" throttle opening as there's insufficient torque to accelerate the bike and rider. That causes you to open the throttle further, which drops the vacuum affecting jets, to then run permanently lean (if the crank rpm never catches the throttle slide position) up gradient being the biggest risk, as it can never catch up.
"Lean" running, in and of itself doesn't necessarily increase heat as a cause. What happens as the combustion chamber reaches its ability to burn fuel as it approaches the 14.7:1 ideal (stoichiometric) the propagation of flame front outward from the spark plug begins to break down such that the burn phase doesn't spread out to far reaches of tbe periphery of piston/chamber. This results in unburnt "gas" mixture in a ring around that periphery, which then explodes rather than burns as piston comes to tdc. Its this uncontrolled event that gives a shock wave that breaks the boundary layer protecting the piston crown from exposure to high heat and subsequently raises the overall temperature if left unchecked. When more extreme, we can hear this as "pinking" but it starts in virtually inaudible levels before we become aware of it in more severe form (we're isolated from source & with helmet sound insulation too) but it happens and significantly shifts the temperature accumulation, that also being relative to the ambient air trying to cool tbe motor and it's capacity if moving more slowly. Detonation is the process, as there's loss of control thats fundamental to how the fuel air mix is supposed to combust, rather than explode. Essentially it has the effect of advancing the timing to detriment of the engine's desirable running condition.
There's another phase going on from this as too much of the above will likely bring pre-ignition, which is usually catastrophic, and in short time frame too :o this will definitely put a hole in tbe piston for you!
Seems like you've got away with it in recent description, but quite close to serious damage though.
Many thanks for the detailed explanation , I have modified the coils just waiting for the Milliput epoxy putty to dry then just a matter of rubbing down and painting then checking coils and fit back on the bike snd see what happens.
Will probably check the timing again and the carb sync because this will all be affected if I was only running on three cylinders some of the time.
I did notice that when setting up the timing with a strobe it was occasionally jumping around which was probably due to the misfire on number two spark plug.
This has been an interesting problem and it’s very easy with any issue to jump to the wrong conclusions.
When I originally bought the bike and rode it home from Barnstable to Copthorne Sussex it seemed ok but when I got around the stripping down the engine I found a piston and I think it was two had quite a bit of heat damage where the head of the piston was partially vaporised.
I originally believed was due to a piston ring being broken but it’s all making sense now.
Obviously during the rebuild I did change piston two and fitted new piston rings to all cylinders.
I will try and see if I can get hold of a endoscope camera to look at the head of the pistons thru the spark plug hole to see if there is any damage .
This is my first bike rebuild so it’s been a learning curve , I originally just checked for a spark on all cylinders and didn’t check the coils resistance so lesson learned .
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Hi everyone just a quick update.
I have rebuilt the ignition coils as per the mod by Ash and fitted the coils back on the bike and rechecked the resistance , around 24.5 k on each of the coils so all looks good.
During my investigation released that the incorrect spark plugs were fitted so I had resistor plugs and resistor caps fitted so have changed the plugs to non resistor version so should get a stronger spark.
I have checked all the cylinders with an Edoscope looking through the spark plugs and holes and all look good except for cylinder two which had deposits of carbon on the piston head because cylinder two was not firing all the time.
Just taken it out for a run and it is pulling better, the flat spot seems to have gone , there is not a strong smell of unburned fuel and doesn’t appear to be getting as hot but the jury is still out , I might try and borrow a digital thermometer to check the engine temp after a longish ride.
Was going to take it for a longer ride but the heavens have just opened so looks like heavy rain for the rest of the day , when the engine cools down I’ll check the colour of the plugs and then decide to try and tinker with the air screws again.
Regards Ken
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All plugs removed and checked, no signs of soot all look exactly the same an bike ticks over nicely so will go out for a longer ride over the weekend and see how it goes but all looking positive.
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Good news with progress. Potentially running more in equilibrium now and give better conditions for running rings in.
On that note, I'd not worry at using it up to about 7500rpm during this period. That's the torque peak and avoids "lugging" it to risk heat building exces.
My view is that the components generally don't need running in and associated care, just consideration for the rings/bores in bringing those in. That operation is related to linear speed, of the pistons, ultimately to "hone" the rings with the bore cross hatching. These motor are so short stroke its unlikely you impinge on ideal linear speed for that action by running to the above rpm. Light throttle and let it spin with casual ease to treat it well :)
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Good news with progress. Potentially running more in equilibrium now and give better conditions for running rings in.
On that note, I'd not worry at using it up to about 7500rpm during this period. That's the torque peak and avoids "lugging" it to risk heat building exces.
My view is that the components generally don't need running in and associated care, just consideration for the rings/bores in bringing those in. That operation is related to linear speed, of the pistons, ultimately to "hone" the rings with the bore cross hatching. These motor are so short stroke it’s unlikely you impinge on ideal linear speed for that action by running to the above rpm. Light throttle and let it spin with casual ease
to treat it well :)
Thank you for the advice noted
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Most piston aero engines specify a run-in period with mono grade low detergent oil. Maybe the same should apply to our bike engines..
Essential qualities and characteristics of a high-quality running-in oil include..
High Zinc (ZDDP) Content: Contains elevated levels of zinc anti-wear additives to protect highly stressed components like camshafts and lifters during initial startup and friction.
Low Detergent Levels: Deliberately formulated with fewer detergents and dispersants. This allows microscopic metal asperities to wear away smoothly, rather than suspending them or preventing the necessary friction to "bed in" the parts.
Zero Friction Modifiers: Avoids friction modifiers and molybdenum. Fully synthetic oils or highly slick lubricants prevent proper seating, which is why running-in oils are almost exclusively high-grade mineral oils.
Proper Viscosity: Typically a straight-grade mineral oil (such as SAE 30) selected by the engine builder to provide a specific shear rate that encourages proper seating while maintaining vital oil pressure.