bobwilson67
CB750 New member
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?
thanks,
Bob
- 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?
thanks,
Bob