Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: Graeme77 on July 29, 2011, 05:55:51 PM
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First ride of the 750 today, took it for MOT and it passed.
However, there were 'issues' LOL.
Bike stutters, power comes in fits and starts. It did smooth out a little after a while, but not great.
Then, it cut out and wouldnt re-start. It did this a couple of times, cutting out dead when pulling up to a junction.
It would re-start, but only with a bit of choke, then off choke imediately as it doesnt like running on choke.
First thing felt like it was lean, or maybe ignition.
Then, stopped for petrol and it wouldnt re-start. Eventualy drained the battery. There is no way this bike will start on the kickstart, but normaly starts straight away on the starter.
Got it trailored home, pulled a plug, and its properly sooted up. Didnt get a good run on the bike so hard to tell, but my guess is its fouling up the plugs to the point where there not firing.
Air filter is suspect, ill remove it for now, see if that helps, but ill look into the carbs again.
Do you think the dodgy power thing could be running rich?, plugs fouling? It did feel a bit like that.
When i went over the carbs i noticed i have 2 different types of emulsion tubes. One set where the row of 4 holes are all the same, and another set where the bottom 2 holes are a fair bit smaller than the top two.
Any idea which ones my 750F1 should have?
As the chains are a bit noisy, and the starter clutch chatters, the AA man thought it had slipped the timing chain. I find this highly unlikely though. I think he just didnt want to know.
Ill look into the starter clutch though.
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Oh yeah, and the clutch bite point changes as you turn the bars ::)
Cable routing around the fairing bracket must be suspect. You can feel it in the lever. Makes pulling out of junctions 'interesting' ;D
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Good stuff getting your test done.
Sounds from the plugs as if it's rich. It would be sensible to have the emulsion tubes the same as a good start point.
Rich usually gives the feel of abruptness and sort of chang, chang, chang as it tries to tick over then eventually dies if badly rich, also you get a much stronger unburnt fuel smell from exhaust.
Lean usually feels much less precise and sort of wafty with delayed response and usually gets very hot.
Don't let the clutch cable compromise your safety especially moving out of T junctions, that's gonna hurt you if you get caught out.
At least you can now ride it to get it sorted.
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I posted on the other site, but ill add my new findings here.
I have been trying to sort the running issue.
Its hard to start, have to play with the choke then get the choke off quick. It never has liked running on choke.
If i turn the idle up to 2500+ it runs fine, but try and turn the idle down and as soon as it gets to around 2000rpm it just dies.
Ive checked timing, tappet, and had the carbs apart agian. All the passageways are clear, all jets are fine, float heights are all spot on.
116 main jets and 40 idle jets.
It did idle ok at 1200rpm before this first ride. I dont get what could have changed.
Clutch cable will be sorted, but need to get it running properly first.
I have no air filter in it right now as i thought maybe its clogged.
Maybe i should mess up the float heights again, stick the mix of 132 and 152 mains back in and unbalance the carbs. It at least ran when it was like that (but badly)
Runs great, on all 4, above 2000 ish.
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If it is a CB750F1 then the main jets should be #105's, my F1 hates running on choke, it needs it to start but then I push it down as fast as possible and get moving.
Got this from the US site I think.
Float Main Slow JN Screw
75 CB750F B750A & 7A 2-Jet Mechanical Slide Pre-EPA 26 105 40 3 1
657A & 657B 2-Jet Mechanical Slide Pre-EPA 26 105 40 4 1
76 CB750F 069A 2-Jet Mechanical Slide Pre-EPA 26 105 38 2 1
Cheers
Den
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Yeah, the 116 are a bit rich, but the smallest i have. It did run ok with these though.
Im not bothered about it being a bit rich at the moment, im more concerned why it used to idle and now it wont even think about running under 2000rpm.
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I think it might be a float issue.
Ive just checked everything agian ::)
All is good, but it just dumped some fuel out of the carb drain unexpectedly. There is also fuel in the carb mouths when i take the filter off.
The float heights are set to 26mm and ive checked them with a bit of clear tube on an old drain screw. Levels are all the same, fuel level is around 6mm below the top of the bowl.
I think ill drop the fuel height and see what happens.
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Im begining to think this must be an ignition issue.
A quick look on the boyer site shows a fault finding page.
This is what mines doing...
The Engine Runs Fast At Idle, Kicks Back on Starting
Poor fuse connection or wiring running low or variable voltage to the ignition. As the alternator charges into the system with increasing speed the problem can clear.
So, not really sure how i go about diagnosing this, i guess i just need to check over the whole electric system.
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Checked everything over, all is fine. I did make the kill switch work again though, it had been badly bypassed.
Still the same though, i think my ignition box may be fubard.
Im sure this is the problem, its not carbs, its displaying symptoms boyer themselves say are possible, but all the electrics are fine.
Seems like the only conclusion is a fried ignition box.
I have a couple of points ignition sets, but im missing some of the advancer mech. I have the advancer plate thing, and the cam, i just dont have whatever springs and bob weights are in there.
Anyone have any spares?
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Check it with a FULLY charged battery, see if the charging circuit is working properly
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I think its cahrging properly, but ive had the battery fully charged since this issue started and its still the same.
As far as checking the charging system, i think maybe i should explain what ive checked already.
I checked it out of curiosity a few weeks back. Voltages rose with revs but not quite high enough. I tweeked the voltage screw to get them up a little. I also re-did the core gap.
When i checked it today it was showing 15.5v at 3000rpm, so i tweeked it back down a little.
Im wondering if this voltage was high enough to kill the ignition unit. I guess i have no way of knowing if it may have gone even higher.
If the regulator was bad, what symptoms would show?
Battery is back on charge now, but it reads over 12.5v at the moment anyway.
Im sure the ignition box is holding the idle up, but nothing seems wrong, so i think its broken.
It did once idle at 1500, it then gradualy rose up to 2500, stayed there for a while, then died.
Im sure the idle stop on the carbs is right, the box is making it idle high, then when i try and tweek the idle down im just shutting the carbs off.
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I recharged the battery and tried again.
No different.
It was idling at 2500-3000rpm. I got a timing light on it was half way between F and the full advance marks.
However, it doesnt change with revs.
I got the engine to briefly run at 1500rpm but the advance was the same.
The box is stuck somehow and not advancing/retarding.
This took 5-10 mins, at which point i noticed the headers for 2/4 were glowing cherry red!
I shut it down and walked away :(
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Could i take the whole bikes electrical system out of the equation by running the electronic ignition from a seperate, fully charged battery?
See how it runs then.
That would remove all possibility of any problems with connections etc.
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I would look at replacing the main jets No 105 as previous posts, and check for leaks in the inlet rubbers.
I think the early CB 750 No 120 jets have been fitted
Good luck
Alan
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If the advance was not going back down to base level you would get those sort of effects i.e.fast idle and too much heat (from advance with no increase in fuel flow, making it leaner than ideal).
You should be able to rig up a power supply from a good car battery maybe just to the ignition pack to take that variable out of the equation if the pickups are not earthed back to the engine, I'm unfamiliar with the circuit used on these.
If no advance function, can you swing the timimg plate back to retard it and see what response this has at tickover?
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Jets will get changed, but i dont think there anything to do with the main issue.
No leaks on inlet rubbers.
Ill try using the battery off my car, its easy to isolate the ignition system from the rest of the bike. Ill give it a go.
Cant retard it enough to get it right, it seems to just do what it damm well pleases LOL.
Trying to get an advance unit for the stock points, i think going back to points will solve this.
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Speaking of the jets, i dont seem to be able to find any 105 mains.
I guess im just looking in the wrong places, or being blind!
Any pointers?
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Here you go
http://www.davidsilverspares.co.uk/parts/by-part-number/ (http://www.davidsilverspares.co.uk/parts/by-part-number/)
Cheers
Den
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As Spitfire, DS has the 105 main jet (an old ref No. from 2009 invoice 991142461050) I had refurbed my carbs with complete kits from www.npr-carbs.com (http://www.npr-carbs.com) all the componants where fine except the main jets where 120. On rebuild it was a pig to start (not at all on the kick), overheating, would not tick-over unless RPM was above 2000
Strip down and fitted the 105 main jet's, starts no problem, tick-over 1,100 RPM. I did use a vacuum set to complete the final balance
Alan
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Correct address www.nrp.carbs.com (http://www.nrp.carbs.com)
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Great, thanks.
Odd you had the same issues and the jets solved it, the mains shouldnt be doing anything at 1800 and below.
Did it ever idle right with 120's? Mine idled just fine before.
It even idled with 132's.
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Out of curiosity i checked the cam timing. Can lift the cover enough with the engine still in to see the end of the cam :)
As predicted, its fine, spot on in fact.
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120 jets, Checked the electric's/timing/points/needle hight as yourself, still no joy! Replaced the main jets to 105 (forgot to mention I changed the air box to carb rubbers, existing had hardend and shrunk) checked all clamps and its been great to this day
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Spoke to a very helpful tech guy at boyer and he said its either a bad connection in the power feed to the box, or the box is broken.I said about runing it from another batty and he agree'd that would confirm either way.
Run it from my car battery and no difference so the box is indeed faulty.
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If you had no ignition, would you get together a stock points set up, or go electronic bith boyer?
Cant decide whether to perservere trying to get an advancer, or get another ignition box.
Pity i dont have the reciet or it would be replaced under warranty.
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In my experience they are a good points system on these and very rarely give any significant problems so it's not a bad route to follow and easy to work on too.
As far as I'm aware the electronic kits may have an advantage at high rpm/compression/competition motors but it's far from clear cut on a standard road motor that's not thrashed to hell, so you're unlikely to miss-out much if you do run points.
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Thats what i figured.
Points are more complicated, well, more moving parts, but simple to work on and fixable at the roadside.
EI is all well and good, but my bikes just proven it can sometimes just break and leave you stranded.
Ill carry on trying to find the advancer.
Anyone know if a cb400 advancer will work? It has a different number (333 rather than 300) but the difference could be in the cam, which i have, springs/weights could be the same.
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It runs!
Fitted the mech advacne and points (spark a little, condensors are good, ill order new points) set the timing up, charged the battery hopefully for the last time ::) and it runs spot on. Idles nice at 1200, picks up well, sounds nice.
Charging ok, got a smidge over 14v at 4k. I might tweek that up a tiny bit more.
Fitted 105 jets and a new air filter this morning too.
So, second ride in a bit and hopefully it will pull well.
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Well done! you'll enjoy it a bit more now ;D
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Great news, enjoy !
Cheers
Den
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Just had a 15 mins jaunt around some local roads, max limit of 40mph so no real testing, but seems ok.
Not quite as quick as i was expecting, but as its my first proper bike thats no bad thing :)
It hesitates sometimes when you whack the throttle open from low speeds but i seem to remember reading this is tunable with the air screws so ill have a little play with those.
Ill go a bit further this afternoon, see if i can build up some confidence in it.
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Hi Greme glad to hear the part helped 8), hope you get the rest of the little niggles sorted, cheers Mick.
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Yeah, thanks alot for that, without that advancer it would still be stuck in the garage.
Front brake feels a bit poor. Its been rebuilt, has HEL lines and new pads. Im hoping the pads will bed in a bit.
If not, ill convert to twin discs.
Not sure how it should be, doubt i ever will unless i get to ride another cb750.
The brake on my old 125 (i know, LOTS less weight!) was great, could get the front tyre chirping with perfect controll, could lift the back wheel if i wanted too.
The CB750 front brake only seems to slow it down, feels like the brake on my work colegues crappy scooter LOL.
Ill wait and see if it improves before making any judgements though.
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Graeme did you find the little badge i put in for you? it was tied in the top of the bag the advancer was in? cheers Mick.
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No, but i still have the packaging, ill go check.
thanks :)
Edit, got it, thanks! Love it 8)
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Nice one 8)
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It will kick start now, easily, there wasnt a hope in hell of getting it to kick start before even when it was running ok.
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The front brake on the 750 can be made better with braided hoses etc but it will never meet todays standards, you have to think about braking a little earlier ;), sounds like you have got it sussed if its starting a lot better 8), the bogging on the throttle might just need some fine tuning ;), cheers Mick.
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Im sure ill get used to them (the brakes).
I take it the front brake on these isnt man enough to get the front wheel skipping from say a 40mph HARD stop?
I guess that also depends on your tyres, michelin macadam in this case.
Ill go for a ride with a screwdriver later and play with the air screws. Still need to balance the carbs properly at some point, i should probably do that first really.
My timing light has gone a bit odd too, so ill borrow another next week to check the timing at full advance, but yes, it seems to be almost there, just a little tweeking to perfect.
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Not even with twin discs will you get the tyre to "Howl" mate, they are nothing like modern stuff but WAAAAY better than the Old Brit stuff that was about at the time
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The 'modern' bike was a 1984 kawasaki ar125, not exactly new :)
But as i say, very light so easy to make stop.
Just put another 10 miles or so on it and its getting better. Brake is nice and progressive and has good feel so its not scary or anything.
Might check the cam chain and tappets again, top end is a little noisy.
Doesnt sound like there are any problems, but you can here the chain and tappets working.
Maybe having been in cars for so long im just not used to hearing an engine rather than hearing an exhaust.
Gearchange can be a bit clonky, its fine as long as you change like you mean it. I hit a false neutral between 3rd and 4th which was surprising. It was a reasonably swift but thoughtless upchange though.
Runs nice though, not missed or hesitated again. I tweeked the air screws out another 1/4 and it seems to be happy. Feels stable and controlled in corners but i think the fairing may need to go, its a little restrictive.
Going for a ride to high beech, epping forrest tomorrow with a couple of friends, that should settle everything down well :)
Thanks for all the help getting it to this stage, i think this bike will be fun.
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Been reading the posts and I empathise with you, keep going, doing it carefully and methodically and the time will surely come when the only things you will have to do between years is oil changes, polishing and servicing, riding and keeping the front brake piston free! ;)
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Hopefully your right, at least for the next few months.
Once the weather turns ill pull the bike apart, go through the engine and powder coat the frame.
To be honest, my main concern is loosing the front end in a corner. Did that a few times when i was young and overly enthusiastic. Im overly cautious now but that will probably go reasonably soon then ill chuck it into a corner too fast and it will go sliding down the road LOL.
Hopefully im wiser as well as older now.
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I call these single discs the first anti-lock as you'd have to have hands like king kong to squeeze hard enough to get a lock up in the dry.
As you've found though they are very nicely progressive and give good feel that you can work with without anything scary happening.
Front to rear grip is quite well balanced on these and you are unlikely to get the front to let go in the dry without extreme input, it's more likey that you'll run wide in a corner as they are quite long and lazy steering so watch out for bends that tighten more than you first thought.
My experience of double discs on a K2 was that they would easily make the front tyre howl and lock if you hit it hard, that's with standard hoses and single master-cylinder.
Good you got the ignition sorted, as you've found the std setup works fine for a road engine and you can fix it yourself!
There are usually no tapping noises if all is ok but they always have a softish rustle that's unlike most other fours, guess you'll get more used to what's normal as you go along.
Enjoy getting out and about with it.
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Softish rustle sums the noise up well, it doesnt sound tappety, but you can hear it working.
Im sure its fine :)