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CB750K0 Carburettor Problems

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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.

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