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Cylinder 4 plug carbon fouling


CB750 Enthusiast
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Nova Scotia Canada
I am having problems with my cylinder 4 spark carbon fouling on my 79 750k.

Coils, wires and plugs have all been replaced, valves set to 0.005-0.007", carbs cleaned, pilots adjusted out 2 turns (new washer and o ring), new air cut offs, carbs were bench synced and vaccuum synced. Compression is 130 after running for 5-10 minutes. Float levels were adjusted as well.

Bike is running stock air box with stock jets but has a 4-1 delkevic exhaust. Idle is rough under 2k and barely stays running. All other plugs are fine? (No fouling).

What am I missing that could be causing that cylinder to run rich? I have tried putting the pilot out only at 1 turn and it make no differnce. It is not wet, just dry but fouls within minutes.

So an update... I attached drain tubes to carbs 1, 3, and 4 and filled the carbs to see where the float levels were and the tube on the very left is from carb 4 (the one my plug is fouling on).

Checked the distance from the gasket surface to the bottom of the float and I was at 18.5mm (15.5mm is spec) which should be lean?? Anyways, I adjusted the tang to spec, tested the bike and its still fouling.

Again, any help would be greatly appreciated.


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I'm a little confused. On your first post you said all the floats were adjusted, then your second post you said #4 was out @18.5. How did that change?
I am not sure. I am starting to think that I might have missed that float when I did my initial float height measurements because 3mm is quite a difference.

Today I installed all new plugs and cylinder 4 did not foul as it has been doing and the idle was more steady.

Bike seems to be struggling to rev up past 6k in combination to everything else.
Did you do a rebuild on the carbs or just a clean up? Did you change the needles? If it is struggling to rev, that could be a few things such as slides not opening properly, running too lean etc. Now that you have changed the pipes you may have to start looking at jets. What do the plugs look like? Do they look lean?
I replaced the cut offs, accelerator pump kit, jets, nozzles, pilot jet (spring washer and o ring too), but am using the same needle.

I only ran the bike for probably 5 minutes after putting the new plugs in so it's probably too early to tell, but no they looked spotless.
Is it running better since you re-adjusted the float on #4? You could have a leaking needle if you are using the old ones.
Adjusting the float didn't seem to make any difference.
New plugs definitely helped with the idle but not high rims.
I should be more specific here... after I adjusted the float, I still had the old plugs in which were pretty dirty (I didn't think they were bad enough to limit spark but I could have been wrong) and I didn't see any improvement. Then, after the plugs were replaced, I noticed a difference so the improvement could be a result of the float being correct and the new plugs?
Yup.. all were adjusted to within 0.005 to 0.007. Compression is 130.

I have vb42b carbs on the bike instead of the vb42a carbs that came on the k model and if I'm not mistaken the vb42b are supposed to have 100 secondary jets instead of the 102s so maybe that is having an impact along with the 4-1?

I was pretty confident the only difference between the vb42a and 42b was the secondary jet size.
Less jet will hurt for sure. With the different exhaust it might run better with more than stock, not less. If you have the plug fouling on one cylinder fixed then you might be better off with more jet.