1979 CB750K Cyl #3 Plug fouled, black soot


CB750 New member
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Kingston, NS
Hi all, I have an issue with cylinder #3 plug getting fouled at idle with black soot, not wet with gas, dry soot.

- plugs 1,2,4 look good with a tan brown color, not lean or rich.
- carbs were sync. and the mixture screws are at 2 turns for all 4. The bike actually likes 3 turns for all four, but going from 2 turns to 3 would make it rich anyway, so I left it at 2 for now.
- plugs are new, correct gaps and I switched plugs to verify
- checked resistance on the coils, wires and caps, all good
- spark was also checked to gnd for #3, but I didn't checked the spark length yet.
- timing was checked, pulsers are aligned correctly
- I was through the carb twice, reset the floats (even did the clear tube method to ensure they were not too high), cleaned all jets, float valve seats are good, etc) Doesn't appear the carb vacuum is pulling in too much fuel, but it does seem the bike is using a lot of fuel during these tests at idle. I removed the carb's again to ensure the float valve seats were not leaking and that the floats were at the correct height. At first I was sure it was a fuel issue with the amount of fuel being used.
- doubled checked the air cutoff valve for #3 carb, it's clean and working, no backfire on decel either.
- compression test was 150psi across all cylinders.

Now for the puzzler. I just checked the valve clearance for all 16. All the valve clearances are in spec. other than the inlet and exhaust valves for #3. What has me puzzled is the clearance is .006" and .007" for the exhaust and .005" and .006" for the inlet, being too loose on the spec. Wouldn't a tight valve clearance cause the plug to foul with soot, making the valve close too soon and not burn the fuel? Seems my valves for cyl #3 being loose on the spec should make the valves seat better? The only thing not in spec. or correct that I can see is the valves for #3.

I ordered the valve shim tool to correct the clearances, 1 week out for that.

Any thoughts? Would the loose valves cause this problem?


Very thorough explanation.......and you've eliminated most of the more common causes of the symptom.

My guess, at this point, is that the valves on #3 have carbon or rust build-up on the seat and aren't completely sealing. That could've happened if they were left open during long-term storage. The build-up could account for the extra clearance you found. I see you've checked compression and that could show the valves not sealing, but not always - a leak-down test may show it though.

Sometimes one can remove the header and intake and clean those valves - in place -with a brush (like used in rifle cleaning). The alternative, of course, is disassembly.
Thanks for the reply, that makes sense. I put a scope in the cylinder last night and from what I could see it's full of carbon deposits. I'll pull the header off like you said to see how much I can clean it and recheck the clearances. The bike also sat last year so you're likely right on point. I also rechecked the compression numbers I had written down and #3 was actually 145, 1,2,4 are 149-150. Not a huge difference, but 3 is def lower. Thanks again, I'll update the thread after I'm done.