jwing
CB750 Member
Hoping to get some input on an issue I’m having with my ‘81 CB750k. Running stock airbox with 4into1 exhaust. I believe it has about 20k miles on it.
Seems to be missing on cylinder 2. She smokes pretty bad (blue) out of the exhaust on deceleration. Spark plug is fouled.
Here’s what I’ve done recently (500-600 miles ago) in an attempt to resolve the issue:
-New piston rings & cylinders honed
-carbs cleaned and vacuum synced
-valves adjusted (0.05 +/-0.01)
-new spark plugs and wires
-new ignition coils
-new intake boots
-leak down test shows about 10% leakage on each cylinder including #2.
I have the engine out of the bike right now. Planning to pull the head and inspect valve guides and valve guide seals. My thought is that if one of those are bad, oil could be getting into the cylinder whenever that valve is opened. Any opposers to that?l theory?
Another odd thing that I noticed is a discoloration of the intake walls. Now I run ethanol-free gas and it has a yellow tint to it, so I believe that’s why 1, 3 & 4 are stained yellow. But if #2 is not stained as shown in the picture I attached, that would lead me to think it is not getting fuel from the carb. If cylinder 2 isn’t getting fuel, could that cause the plug to get oil fouled (even though it has pretty good compression)? OR if my first theory is correct and the intake valve(s) on cylinder 2 are leaking oil past the seals, the oil may be coating the intake walls and thus not letting it get stained yellow by the fuel.
I’ve attached a few pics of the fouled #2 plug, the discoloring of the intake walls, and the carbon build up on the exhaust walls for reference.
Thanks for any help!
Seems to be missing on cylinder 2. She smokes pretty bad (blue) out of the exhaust on deceleration. Spark plug is fouled.
Here’s what I’ve done recently (500-600 miles ago) in an attempt to resolve the issue:
-New piston rings & cylinders honed
-carbs cleaned and vacuum synced
-valves adjusted (0.05 +/-0.01)
-new spark plugs and wires
-new ignition coils
-new intake boots
-leak down test shows about 10% leakage on each cylinder including #2.
I have the engine out of the bike right now. Planning to pull the head and inspect valve guides and valve guide seals. My thought is that if one of those are bad, oil could be getting into the cylinder whenever that valve is opened. Any opposers to that?l theory?
Another odd thing that I noticed is a discoloration of the intake walls. Now I run ethanol-free gas and it has a yellow tint to it, so I believe that’s why 1, 3 & 4 are stained yellow. But if #2 is not stained as shown in the picture I attached, that would lead me to think it is not getting fuel from the carb. If cylinder 2 isn’t getting fuel, could that cause the plug to get oil fouled (even though it has pretty good compression)? OR if my first theory is correct and the intake valve(s) on cylinder 2 are leaking oil past the seals, the oil may be coating the intake walls and thus not letting it get stained yellow by the fuel.
I’ve attached a few pics of the fouled #2 plug, the discoloring of the intake walls, and the carbon build up on the exhaust walls for reference.
Thanks for any help!
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