Honda-SOHC
SOHC.co.uk Forums => CB350/400 => Topic started by: Mikep328 on July 22, 2023, 09:55:34 AM
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My new to me CB400 does the "won't return to idle" thing. I noticed this the first time I rode it. It will idle OK after starting/warmed up but when out riding, it will hang at 2500+ RPM. Blipping the throttle doesn't help. I have found extensive posts here re that issue so I'm going work my way through the suggestions/opinions.
I had a Norton Commando that did the same thing and it turned out the mechanical advance weights were sticking in the advanced position when the throttle was reduced to the idle position. So I'll check it with a strobe light and verify that the weights are functioning properly.
I don't know if the carbs were worked on previously. There are receipts and mechanic's notes for an engine rebuild but no mention of carbs and no parts receipts for carb parts. So I need to at least check them for proper linkage adjustment, etc. Also will check sync though when it is idling at 1200 RPM after start up, it runs smoothly so I'm thinking the sync is reasonably close. Popping the throttle open when running down the road at say, 3000 RPM will result in a major bog; opening the throttle less aggressively it pulls well. I should go through the carbs I guess but if I can sort out the not-returning-to idle with external adjustments, I'll delay that..
Also noted a seep from the oil pan gasket. I checked and all bolts were all adequately/evenly tight so seems like a new gasket is in order.
A problem I have for mechanic work is that our bikes and car are in an apartment building multi-car garage. So there is no workbench or electricity and everything I do has to be contained within the parking slot which also contains our MiniCooper and two other motorcycles. Obviously, things like the carbs can be removed and worked on indoors but much care has to be taken with any operation that might end up with oil or fuel being spilled on the community garage floor. Replacing that oil pan gasket might be tricky under the circumstances... :(
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Double check to make sure it is the sump gasket that's leaking. I chased the same leak years back only to find it was an oil seal, gear change I think, that was leaking and weeping down and around the join of the sump pan.
Also, if you do take your carbs apart, I sell complete O ring kits in Viton which you will need to prevent issues with Ethanol in fuel.
Where do you live?
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Thanks! I'll check to verify where the oil is seeping from. We live in Bath.
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Thanks! I'll check to verify where the oil is seeping from. We live in Bath.
Not local to us then 😂😂😂. I was going to say if you live fairly local you could come and work on your bike in our workshop but Bath to Lincolnshire is a bit far 🤣🤣🤣
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Im in Compton Martin near Chew Valley Lake (BS40) and Multiman Tim isnt far from you either. Let me know if you need a help or bit of space, we're all here to help and youre smack in the middle of both Tim and me if youre struggling.
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As Roo says I'm close, Corsham.
I had similar idling issues with my 400/4 and between Roo and myself we eventually got it sorted.
Very happy to come over and help remove the carbs and get them stripped for you.
Inspection of the brassware and rings should reveal where the problem is.
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My new to me CB400 does the "won't return to idle" thing. I noticed this the first time I rode it. It will idle OK after starting/warmed up but when out riding, it will hang at 2500+ RPM. Blipping the throttle doesn't help. I have found extensive posts here re that issue so I'm going work my way through the suggestions/opinions.
I had a Norton Commando that did the same thing and it turned out the mechanical advance weights were sticking in the advanced position when the throttle was reduced to the idle position. So I'll check it with a strobe light and verify that the weights are functioning properly.
I don't know if the carbs were worked on previously. There are receipts and mechanic's notes for an engine rebuild but no mention of carbs and no parts receipts for carb parts. So I need to at least check them for proper linkage adjustment, etc. Also will check sync though when it is idling at 1200 RPM after start up, it runs smoothly so I'm thinking the sync is reasonably close. Popping the throttle open when running down the road at say, 3000 RPM will result in a major bog; opening the throttle less aggressively it pulls well. I should go through the carbs I guess but if I can sort out the not-returning-to idle with external adjustments, I'll delay that..
Also noted a seep from the oil pan gasket. I checked and all bolts were all adequately/evenly tight so seems like a new gasket is in order.
A problem I have for mechanic work is that our bikes and car are in an apartment building multi-car garage. So there is no workbench or electricity and everything I do has to be contained within the parking slot which also contains our MiniCooper and two other motorcycles. Obviously, things like the carbs can be removed and worked on indoors but much care has to be taken with any operation that might end up with oil or fuel being spilled on the community garage floor. Replacing that oil pan gasket might be tricky under the circumstances... :(
Do you have pods fitted ? They can give problems with bogging ;)
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Thanks guys for offers to assist: I really appreciate it!
I did a bit of checking this morning. The timing per strobe is dead on and the advance mechanism is working correctly. I futzed around a bit with the airscrews/idle speed adjustment and managed to make a bit of improvement. It didn't hang up at high RPM when revved up/back to idle but since it was raining I didn't care to go out to see how it acted on the road. If it's dry tomorrow I'll do that and, if no change, pull the tank/examine the various linkage/cable settings/adjustments. I'll check the valve clearances as well.
Again, thanks!
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Depending on your ignition system might be worth checking the timing on 1&4 and 2&3 to check they are synchronised before taking the carbs off.
For how easy it is to get at compared to the carbs removing the mechanical advance - clean & lube if you have not already done so. Something that is fine when cold can start binding when hot.
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Thanks guys for offers to assist: I really appreciate it!
I did a bit of checking this morning. The timing per strobe is dead on and the advance mechanism is working correctly. I futzed around a bit with the airscrews/idle speed adjustment and managed to make a bit of improvement. It didn't hang up at high RPM when revved up/back to idle but since it was raining I didn't care to go out to see how it acted on the road. If it's dry tomorrow I'll do that and, if no change, pull the tank/examine the various linkage/cable settings/adjustments. I'll check the valve clearances as well.
Again, thanks!
Hopefully in addition to the investigative pot, the "idle air screw" on these carb can play significant part in this performance, tickover and throttle opening response.
They effect the mixture much further up into rev range than appreciated by some. If not rich enough they can give that diving into a hole flat spot described. Also too lean and the idle takes it's own good time to get back down to base.
Honda manual advice is to wind them out (one cylinder at a time) to hear maximum rpm, slowly a little further until it only just starts to falter from being too lean, then turn the screw back in again (making it richer) until the revs drop by 100 rpm from the peak you obtained. Follow that with successively each of the other three cylinders.
If this doesn't work in a linear fashion (no response possibly) it would indicate that in that idle circuit there's some restriction that you'd have to get sorted to be fully operational.
The good thing about this (as opposed to just setting them 1 1/2 turns out) is it allows you to see and hear if one or more is not working properly.
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To add detail, the idle circui, both air and fuel, have to be ABSOLUTELY on the money for clean unimpeded flow on these carbs, else you'll chase your tail forever :)
They meter really fine amounts, are very accurate and work impeccably when all clean and set well.
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I've owned my 400 for over 30 years, and I've found that because there is virtually no flywheel effect, it does not like 'ticking over' until really warm. I made the mistake of increasing the idle speed using the large thumbwheel under the right side of the tank. This was fine until the engine was hot, and then, like yourself it was "idling" at 2500 revs! If your choke lever is set up right, there is a fast idle position as you start to lift it. You say that it's possible the carbs may have been removed, might be worth checking the operation of the two throttle cables, when I replaced mine it took a couple of days to get them settled in correctly. Hopefully this might help before you have to go tearing everything apart. Good luck. Mike.
PS. I heartily second the comment re. the gearbox oil seal!!
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Good info, thanks! Today I'm going to check the fact idle/choke linkage and throttle cable adjustment. Regarding adjusting the carb airscrews...is there any reason to re-install the four black plastic airscrew limiters that were on the carbs? I don't really see any need for them now.
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"Regarding adjusting the carb airscrews...is there any reason to re-install the four black plastic airscrew limiters that were on the carbs? I don't really see any need for them now"
No, they are not needed. Don't throw them away though, keep them safe.
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Black airscrew limiters, I'd view them as obviously limiting movement, but think they look to do tbat specifically in relation to situations that we probably don't encounter in these lowlands :)
For screws set at sea level, then riding up to high altitude, they seem to give a set adjustment to compensate without having to do the "turns counted out" thing. In other words you could easily set them at one end or t'other in that scenario. And maybe an early nod to restriction on adjustment for emissions legislation in some market.
Outside that, they dont seem to do much, but some may want them for originality purposes.
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They were for the Yanks to stop people "fiddling" in those states with emmisions rules
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One advantage of the black stoppers is that they give you a visual reference point - i find it easy to loose track of how many turns the screw has moved especially if you have been tinkering with the settings over a few days.
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Black airscrew limiters, I'd view them as obviously limiting movement, but think they look to do tbat specifically in relation to situations that we probably don't encounter in these lowlands :)
For screws set at sea level, then riding up to high altitude, they seem to give a set adjustment to compensate without having to do the "turns counted out" thing. In other words you could easily set them at one end or t'other in that scenario. And maybe an early nod to restriction on adjustment for emissions legislation in some market.
Outside that, they dont seem to do much, but some may want them for originality purposes.
I have never understood this sea lever and high altitude ::)
I have rode old Honda's though out the world, starting at sea level, up to 2500 meters and back down to sea level over and over again. On all these trips i have never tinkered with the carbs and the bike has always run spot on.
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I believe it is due to the density of the atmosphere being rarer at height that leads to engines effectively running rich due to lack of oxygen - iirc it is at extremes of altitude and worse in highly tuned engines. Might be that as bikes rely on gravity rather than fuel pumps the effect is mitigated.
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My experience I never had problem driving highly tuned engine cars with fuel pumps from the UK, crossing the Austrian alps, the full length of Yugoslavian mountains and crossing the northern Greek mountains. Diving a Noble M12 or a UVA sandrail that had twin choke Webber's ;)
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My experience I never had problem driving highly tuned engine cars with fuel pumps from the UK, crossing the Austrian alps, the full length of Yugoslavian mountains and crossing the northern Greek mountains. Diving a Noble M12 or a UVA sandrail that had twin choke Webber's ;)
I think you need to get to much higher altitudes before you'll notice any loss of performance. The highest roads in the Himalayas, for example. For most of us we'll never need to worry about it.
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On a related topic all my bikes & cars drove much better after a decent downpour of rain - probably explains a brief period when ICE manufacturers toyed with water injection.
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The not-returning-to-idle seems to have been caused by the high-idle stop being out of adjustment! The manual calls for .012"/.3mm clearance and there was none. It was holding the throttle slightly open which got worse as the engine heated up. I adjusted the clearance per the Honda specs and, once the engine was good and warm - which takes more time than I would have thought - I adjusted the idle stop screw to 1200 RPM. From that point on the engine returned to that RPM when the twist grip was returned to the idle position. But the idle is a bit rough and the engine still bogs when opening the throttle quickly.
further re the bog - I haven't found any info in the Honda service manual or a Clymers that mentions changing the position of the needle to adjust the mixture in the range the needle controls. I'm NOT saying there isn't any info, just that I haven't found it. I'd think that raising the needle might reduce/eliminate the bog. Any thoughts/experience re this?
I'm definitely going to pull the carbs/check/adjust everything out but we're going out of town on Tuesday for a week so that will wait until we return.
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I've felt the loss of power in a modest vehicle at 9500 feet altitude in Colorado. There are suggestions of losing 3% of power per 1000 feet of altitude gain, also that engines can run on lower octane fuel in higher altitudes than elsewhere. In certain parts of Colorado fuel stations have sold petrol with octane levels as low as 85.
I guess the impact of altude is felt more on lowly poweredengines. Presumably manufacturers set engines up to run optimally at some average altitude above sea level. No idea whether it varies by manufacturer or intended market. Tibet might be require some adjustments with the lowest city over 9000 feet and an average altitude over 14000 feet. Probably not many 400 fours there though.
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The not-returning-to-idle seems to have been caused by the high-idle stop being out of adjustment! The manual calls for .012"/.3mm clearance and there was none. It was holding the throttle slightly open which got worse as the engine heated up. I adjusted the clearance per the Honda specs and, once the engine was good and warm - which takes more time than I would have thought - I adjusted the idle stop screw to 1200 RPM. From that point on the engine returned to that RPM when the twist grip was returned to the idle position. But the idle is a bit rough and the engine still bogs when opening the throttle quickly.
further re the bog - I haven't found any info in the Honda service manual or a Clymers that mentions changing the position of the needle to adjust the mixture in the range the needle controls. I'm NOT saying there isn't any info, just that I haven't found it. I'd think that raising the needle might reduce/eliminate the bog. Any thoughts/experience re this?
I'm definitely going to pull the carbs/check/adjust everything out but we're going out of town on Tuesday for a week so that will wait until we return.
Good progress with sorting out basic throttle rest position to get at least a consistency there.
This really is tied up in idle circuut competency, they lead the mixture in with a gradient to fully "knit" together with the main jet and needle flow in getting a seamless transition. Without at least verifying the carbs and their adjustment it's hard to get decent view of what is happening.
With those engine, wide open throttle with high gear and low speed is not where you want to operate, but given the correct gear ratio, then wacking the throttle open shouldn't give that bog, it should just run up the revs without complaint.
The scenario that gives the "bog" is insufficient fllow/metering of idle circuit to get the fires going, then to bring the response fully onto main jet. If this doesn't happen and it's gone too lean from idle giving insufficient mixture, then further opening the throttle slides just exacerbates this by then dropping vacuum to also not pull fuel through main jet route. One needs the other to lead it. Usually you need to close the throttle a little to help increase the vacuum under the slide, then it'll start to pull up through the flat spot but you'll normally have to baby it to get through there.
Focus on impeccable idle circuits, then the rest Usually falls into place on these.
Also, idle curcuit are absolutely key to getting smooth idle rpm and quiet running. One out of kilter can make it sound like a bag-o-nails.
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Another scenario that gives the 'bog' is aftermarket needles.
Many cheapo needles have incorrect taper profiles compared to the genuine Honda items.
The transition from idle jet to main jet is affected exactly the way you describe.
This topic has in the past been discussed over on the USA SOHC site.
Good luck
Skoti
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Black airscrew limiters, I'd view them as obviously limiting movement, but think they look to do tbat specifically in relation to situations that we probably don't encounter in these lowlands :)
For screws set at sea level, then riding up to high altitude, they seem to give a set adjustment to compensate without having to do the "turns counted out" thing. In other words you could easily set them at one end or t'other in that scenario. And maybe an early nod to restriction on adjustment for emissions legislation in some market.
Outside that, they dont seem to do much, but some may want them for originality purposes.
I have never understood this sea lever and high altitude ::)
I have rode old Honda's though out the world, starting at sea level, up to 2500 meters and back down to sea level over and over again. On all these trips i have never tinkered with the carbs and the bike has always run spot on.
To be honest I do notice a slight power loss when crossing the higher alpine passes in Austria and Switzerland on my CB750F1 and my Norton Commando. (But I've never tinkered with the carbs)
It's not noticeable on my BMW R1150RT, but it has fuel injection and an engine management system so I suppose that compensates the air/fuel ratio accordingly?
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The slight oil seep I have does not appear to be from the pan gasket. It is dripping from the cover on the left side of the engine adjacent to the pan and then down the side of the pan. So based on an earlier post, the suspect is the gear change seal?
I suppose I could use the oil seep to determine if there is oil in the bike as per old BrittBikes - "If it's not leaking it's out of oil!" But I'd prefer the typical Honda method - no oil leaks at all!
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Mike,
My normally very oil tight 350/4 developed a drip from the sump a few months back, so I removed the L/H side cover and there was quite a misting of oil around the sprocket/gearchange shaft/oil pump. I cleaned the area with brake cleaner and ran the engine for a while. My leak was from the 'crimp' on the oil light switch where the case is rolled over the insulator. Replaced the switch and it's now back to Honda oil tightness. Might be worth a look. It's 20 years since I restored the 350/4, so I can't grumble really and I had a spare switch from my 400/4.
Ian
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A good diagnostic is to remove the side cover, clean up the area thoroughly, puff with talcum powder, run engine and you will see a witness mark of oil coming from whichever seal / O ring is leaking.
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Thanks Ian. I'll take a look under that cover this weekend and see what's happening. Hope no major disassembly work will be required to sort out whatever it is. If such work is required, I'd probably just let it seep for a while and enjoy riding the bike!
Julie, appreciate the talc suggestion; I'll definitely do that!
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The slight oil seep I have does not appear to be from the pan gasket. It is dripping from the cover on the left side of the engine adjacent to the pan and then down the side of the pan. So based on an earlier post, the suspect is the gear change seal?
I suppose I could use the oil seep to determine if there is oil in the bike as per old BrittBikes - "If it's not leaking it's out of oil!" But I'd prefer the typical Honda method - no oil leaks at all!
Sounds crazy but I found new oil especially 10w/40 synthetics would leak oil via the rear main seal on cars - after a few days or a 100 miles they would disappear - I put it down to the thin searching oil & plasticizers that they add to modern oils then fixing the cause.
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Good advice re checking instead of ASSuming the oil pan gasket was the leak. I removed the left side cover and found that the leak is at the lower left (as you face it) of the oil pump. It seeps immediately upon starting the engine. no leaks from the oil pressure sender. I'll check the service manual for the procedure to remove/replace the oil pump seal but if anyone has some specific advice re this, don't hesitate to post!
This added several minutes later: It appears from the service manual that the pump has essentially three Orings - a large one and two smaller ones that appear to fit around the lower bolts AND that the pump can be removed without draining the oil. Is my reading correct?
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There are 5 O rings in total in the oil pump.
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There are 5 O rings in total on the oil pump.
There are 8 in total on the 400 Ted, including the gearbox oil pump O rings.
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Oops it's not a 500 its a 400 they have those extra rings - my Bad - either way more than 3 by quite a few.
TBH I had forgotten just how different the two oil pumps are.
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Try and lean the bike to the right when removing the oil pump in order for any oil to drain back into the sump. That's if you're not dropping the oil that is.
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Good idea, will do!
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Just a thought if your oil pump is leaking oil - could the cause be linked to something else that has caused the leak?
I'm thinking oil pressure valve, oll filter valve?
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Probably just the age of the orings Ted, if they are original they are really old now.
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According to receipts the engine cases were separated to do various work, including a DSS cam chain tensioner "upgrade" so I ASSUME the oil pump was reinstalled with new O rings. Maybe an Oring wasn't seated properly or moved out-of place during assembly? In any case, I hope the cause will be obvious and easily resolved with new Orings.
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Doesn't mean they've removed the oil pump though Mike, if it's not needing to do the work they'd have left it in place
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I took one engine apart and the oil pump o-rings were in a hell of a state, had not been replaced, just smeared with orange silicon gasket. Not everybody puts it back together properly. 😣
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That's interesting - Never having done it (or looked up the procedure), I had assumed that the pump had to be removed to split the cases.
Actually, I hope the pump was NOT removed previously. If it was and is leaking due to some faulty assembly, it will make me wonder about the quality of the considerable engine work that was done...£2,216.85 worth per the receipt!;)
This added a few minutes later: After leaning the bike to the right for a bit then placing it on the center stand, when the pump is loosened/removed will any oil - other than residual oil in the pump itself - be draining out? Will I need a catch pan or will a few rags handle it?
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Always best to have a tray and some rags handy Mike. Ideally you need to keep the bike leaning to the right through the process.
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I use an old cat litter tray as a catch tray plus an old hand towel underneath to catch any errant drips.
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OK! I'll use one of my darkroom trays!!
Totally unrelated... Ted, the Clown Loach is one of my favorite tropical fish!!
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Whatever you do, make sure the bike is secure before working on the oil pump. You do not want the bike moving and falling over! If it's not on the centre stand put it in first gear and chock the wheels.
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OK! I'll use one of my darkroom trays!!
Totally unrelated... Ted, the Clown Loach is one of my favorite tropical fish!!
Until about five years ago my 5 ft tank had a shoal of 30 or so Clown Loach they are beautiful when young as they swim in a shoal becoming more solitary & nocturnal as adults. I have kept them since I was a boy of 9 or 10 only giving up tropical fishkeeping when the maintenance took up too much time. As you will know they are unusual as they have skin rather than scales. Huge variations in the banding as well.
Your bike will be safer on the center stand at least until your side cover is off & the oil pump bolts partially loosened, if you have an assistant to then take it off the stand & hold it whilst leaning so much safer.
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All good advice re handling the oil - a lot more came out than I was expecting so it was good to have a tray available!!! Pump is off, back at the house and disassembled. All Orings are very flat so I'm assuming the new ones Nurse Julie sent me yesterday - (and arrived today, THANKS!!!) will take care of the seep.
However...and there is always a "however," isn't there? I saw something I do not like. When I disassembled the pump, I found that the center rotor was installed with the alignment dot facing up BUT the outer rotor was installed with the dot facing down. IOW, no way to align the dots to check clearance.
I installed them correctly and checked the clearance, which is well within spec but this little error makes me concerned about all the other work that the (alleged) mechanic performed on the engine. TBF, the engine sounds great and, for that matter, I don't know if the mech that did all the work took the oil pump apart or not. All the engine work was done last year and the Orings on the pump appear much older so no telling when the mismatching of the rotors occurred. Oh well, I'm confident it's OK and I'll mount it back on the bike tomorrow.
Thanks again to everyone!!!
30 clown loaches, WOW! I never had more than three!
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I did not in reality make any profit out of the Clown Loach but it did help subsides my hobby back in the day.
Small Clown Loach used to cost around £2-2.50 mark each & I would get a couple of free ones thrown in as they are so hard to count in a net. Around three years later when I had grown them on to a good five- six inches in length they would fetch a decent price (over £20 each depending on size) from Wharf Aquatics who would always buy them off me - payment was by credit note. So the cycle would repeat itself.
I did very frequent 20 % water changes using pre heated/aged water to reduce the hormone levels in the water. I was told a dominant fish secretes a hormone to reduce the growth of others in a small environment. This might be fake news - Roo would know if there is any truth in the story.
Back to the oil pump:-
A few member here have encountered oil pumps that have not had both dot marks facing outwards so it's not that unusual to find.
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Ah...OK. Maybe Honda didn't worry about it either! ;D
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So long as the dots are both on the same side Mike it's fine. IIRC the inner rotates anyway around the outer so the dot moves around the outer as it goes through it's cycle, eventually it returns to align with the outer dot then moves on again.
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Well, the dots are now on the same side! :)
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This is why you can never trust the previous owner. I bought a bike once that the owner said, it has had a total engine rebuild.
After stripping the engine down, found that the piston to bore clearance was anything from 1.5 thou to 2.3 thou, two piston rings in up side down, valves not seated, primary chain knackered, primary rubbers hard as nails, spring and washer on the wrong side of the oil filter, side cover gaskets glued on with bathroom sealant and the list goes on ;)
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Yeah, I can only hope the engine work was properly done. The engine revs freely to red line through the gears out on the road with no odd sounds/behavior but time will tell. ;)
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New Orings on the pump solved the seeping problem. Reinstalled pump, went out on ride, took engine to the redline on several occasions. I had left the side cover off to be able to easily see any seepage - there was none. ;D
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New Orings on the pump solved the seeping problem. Reinstalled pump, went out on ride, took engine to the redline on several occasions. I had left the side cover off to be able to easily see any seepage - there was none. ;D
Great, I bet you're very pleased you didn't take the sump off now 😊😊😊
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Yes, that's for sure. If it hadn't been for advice here re other possibilities, I would have replaced that oil sump gasket...and still had a leak. Thanks for the extremely quick shipping of the O-rings!