CB750K0 Carburettor Problems

[Steph550]

Thanks again for all the replies!

The bike has NGK plug caps as recommended by K2-K6: two straight ones in the middle and cranked ones on the outers. However we will check that they're not shorting.

Surely, though, if it was an electrical problem it would affect the bike all the time, across all RPM ranges? We can balance the carbs, and the bike will run beautifully for 5-10 minutes, then go out of balance again. However even when out of balance it behaves well at higher RPMs. I would have thought if the problem was electrical it would show at least some symptoms at high RPM too? It also idles very nicely. 

Cheers,

Steph.

[tarmac]

Steph,..after checking the plug caps and leads for sparking and you find that all is well in that department,
 why not do as Jamesh suggests and  do a impedence test,....I have just done mine and it took less than 5 minutes and the results are.

---Plug caps=5.2 kohms..
---coils with plug caps off=14.1kohms.

 please note the coils are old ones but the caps are new.

another point to mention but you probably already know,...connect your meter to the plug leads to 1 and 4 cylinders,and then to 2 and 3 cylinders.

    hope this helps,....cheers terry

[K2-K6]

It's probable that there is not much wrong, just that finding the real cause can be a bit of a chase around.

Adjusting the carb's balance may be masking the real issue and giving you a wrong lead, possibly. If a cylinder is not running well then it's vacuum reading will be disturbed so that if you then adjust the carb slide position for that one cylinder it may not then be correct i.e. you could just have moved it away from what was a ideal setting. You can only really set the final carb vacuum values after verifying igniton etc are correct, in other words vacuum balance is really just the icing on the cake.

The question you pose about running ok at bottom range and also at the top is quite possible, the transient stage i.e. the middle range is used to accelerate the bike's mass and has the highest demand of both ignition and carb jetting, if either is defficient then it will most likely show here (making the assumption that the idle jets are correct and the main jets are able to supply the maximum flow required) so if the ignition is in any way suspect, this being the most demanding sector will show a fault......hope that makes sense.

If you don't get anywhere, I'd set the carbs manually by putting fingers into the intake to make sure all the slides are starting to lift at the same time and then leaving them alone while you work through other items. this should take any variance out of this area.

Just a quick thought about plug gaps, what are they set? and do you have a spare set of plugs you can swap into it?
Also what plugs are in it? The whole number written on the plug.

It's a bit more difficult to go through stuff when it's not in front of you, so apologies for so many questions.

[fisjon]

I had a brand new CB750K1 back in 1971. (cost £719)
When it rained, the bike just cut out. When I took off the metal plug cap covers it improved but occasionally misfired. So I covered the caps with heat resistant insulating tape and that cured the problem. The thing is that the metal covers on the original equipment were there to stop radio interference! Nowadays of course the caps come with resistors.

The original points, coils and caps were total rubbish and produced a puny spark. If you have these on your bike then you should consider putting electronic ignition on. You simply will not believe the difference. It will run smoother, have more power and do more miles per gallon. The difference between the spark on original fit and Boyer electronic ignition is the difference between striking flints together and a bolt of lightning. I speak from experience, you don't want to be holding a spark plug against the engine to 'test for spark' when it is attached to a Boyer kit.

Having a fat spark will eliminate a lot of problems associated with carbs. Good combustion will take full advantage of any carburettor.

What you describe however could be something simple........advance and retard..............remove the points plate, take the advance retard off, dismantle it and give a thorough clean. Reassemble with plenty of engine oil, (not grease as this dries up and causes it to jam). Note, it should have two springs attached that are different. One is stronger/heavier gauge that the other. What happens if the advance retard is not working properly is that it temporarily jams and of course changes the ignition timing for any given amount of revs, this will cause loss of power pulling away but will be ok at higher speeds. If you balance the carbs when they are jammed open, when they release the carbs will need rebalancing again.

It could also be the transition between the jets that is causing the problem. I would get the carb bodies ultrasonically cleaned to make sure that there are no blockages or small particles of crud bobbing up and down in the drilled airways. If you can't find anybody local to do them I can do them for you.

Hope this helps
 

[Steph550]

Ok, so we've just spent some time investigating the electrical system. First we checked the spark from each plug with the engine turning over on the starter, and found them all to be fine. We then checked the coils and HT leads which gave 14.6 kOhms . The outer two plug caps were also good, at 4.7 kOhms. However, we could get no reading at all from the centre two plug caps. We checked one of the middle plug caps from my 550 to check our technique was ok and got a good reading.

Would these two plug caps not working properly be enough to cause the symptoms I've described? The spark from those plugs was fine just turning the engine over but I guess it would struggle under load?

Thanks,

Steph.

[mick]

well it won't help can you borrow the 2 caps off your 550 and try them on your KO? see how it runs/behaves? cheers Mick.

[tom400f]

If it does have the metal type plugs caps, as the others say, they never really worked when new....nearly everyone with any intention of have the bike run reliably used NGK, two straight ones in the middle and two cranked ones on the outside cylinders.

Hmmm, I have these. They do seem to be working OK but at the very least are very ugly. Think I'll change them

[K2-K6]

I'd also go along with swapping the two centre caps (new if you don't want to disturb your 550) as they shouldn't be too expensive. If you get a failure of caps at all it's usually these two as if they have been used in the wet anytime before they take all sorts of road crap from the front wheel.

Unsure if you've taken them off the leads, but if not they will ordinarily unscrew from the HT lead and just turn them until they detach......then reverse for the new ones.....make sure you use the new rubber covers that go on the leads first and prevent moisture from entering the feed end of the cap.

I like to find a reason that could be the cause before spending money in the hope of curing something, bringing these back to spec will either get you going or at least take a faule out of the equation to see other things a bit clearer. They certainly have the scope to give the problem you've described.

let us know how you get on.

[tarmac]

those 2 centre plug caps would certainly give you the problems you mentioned,....those were the 2 culprits
that gave me grieve,...in fact I am looking at them now,....one has 2 cracks at the top under the rubber hat and is burned black,and both have corrosion on the screw connecters,...


                 regards ,,terry

[Steph550]

Unfortunately it looks like we're back to square one.

Changed the centre plug caps to brand new NGKs which work fine. However, Nigel (my other half) rode the bike yesterday and it's no better at all... He was really struggling in traffic and pulling away from junctions. 

Looks like we may be back to a carburation issue.

Steph.

[mick]

are there any forum members in your area that could lend you a set of "known good carbs" for you to try ?, rather than buying some,

I doubt anyone will have a spare set of the KO carbs so you would probably have to try some newer carbs and you may have to change the throttle cables to ?,

I know it a hassle but it may help you sorting out your running issues  cheers Mick. 

[tarmac]

damn!!,..I thought a change of plug caps would have solved your problems,but i hold my hands up and admit I got it wrong.

it looks like it's back to the drawing board then???

   tarmac

[MarkCR750]

Sounds very similar to a problem I had with my zxr 400, in fact the symptoms are identical, it was float heights, when the engine was cold it would run fine, after riding for 5 minutes or so it would have the symptoms you describe, the only thing that was changing in that time was engine temperature, this lead me to think that it was over fuelling (running rich) , so by trial and error I adjusted the float heights, I couldn't just set the floats to the factory settings because the intake, carbs and exhaust were modified so the standard settings weren't necessarily the correct settings, at the 5th attempt the bike ran perfectly, so I guess the question is after you balanced the carbs and it ran well for 10 mins did you subsequently let the bike cool to stone cold ?, and did it run ok for 10 mins after that subsequent cold start up?, I'm absolutely sure it won't be carb balance btw, cheers Mark.

Another thought would be a vacuum forming in the fuel tank due to a blocked breather, not so likely but easy to check, just go for a ride with the fuel cap off (well life is no fun without a little danger is it!), this happened on my XJR1300 a few weeks ago, after a few miles of imitating John McGuiness the engine started to cut out, I opened the fuel cap and there was a huge rush of air into the tank, I got home by keeping the revs below 3k and found that the breather hole in the cap was blocked, no problems since cleaning it out, I doubt your bike has this problem but it's a quick check.

[K2-K6]

At least you've now taken that particular area of concern out of the way, you'd have got more frustrated if you did a load of work in another area only to find something simple like defective plug caps!

They are such nice bikes to ride so hopefully with you and us lot on here we should be able to get it.

Another question, if you warm it up by riding and then turn it off and let it sit for perhaps 15mins without turning the fuel tap off.....then start and ride it, will it accelerate normally at that point or is the fault apparent immediately? i.e.will it give you one good acceleration run and then the fault returns? or does it just display the fault straight off?

[MarkCR750]

Sounds very similar to a problem I had with my zxr 400, in fact the symptoms are identical, it was float heights, when the engine was cold it would run fine, after riding for 5 minutes or so it would have the symptoms you describe, the only thing that was changing in that time was engine temperature, this lead me to think that it was over fuelling (running rich) , so by trial and error I adjusted the float heights, I couldn't just set the floats to the factory settings because the intake, carbs and exhaust were modified so the standard settings weren't necessarily the correct settings, at the 5th attempt the bike ran perfectly, so I guess the question is after you balanced the carbs and it ran well for 10 mins did you subsequently let the bike cool to stone cold ?, and did it run ok for 10 mins after that subsequent cold start up?, I'm absolutely sure it won't be carb balance btw, cheers Mark.

Another thought would be a vacuum forming in the fuel tank due to a blocked breather, not so likely but easy to check, just go for a ride with the fuel cap off (well life is no fun without a little danger is it!), this happened on my XJR1300 a few weeks ago, after a few miles of imitating John McGuiness the engine started to cut out, I opened the fuel cap and there was a huge rush of air into the tank, I got home by keeping the revs below 3k and found that the breather hole in the cap was blocked, no problems since cleaning it out, I doubt your bike has this problem but it's a quick check.

PS I'm sure your right to suspect that carburation is the problem, it doesn't sound at all like an electrical fault.