Cyl. #2 fouling help

jwing

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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!
 

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Last edited:
Have you tested the strength of the spark on the "suspect"cylinder?
Good question! I did check for spark when I replaced the plugs and coils. However, I didn’t check with one of those in-line testers to make sure it’s getting spark under compression. And I don’t recall if I really compared the spark to the other plugs to make sure it was truly strong. That’s my lazy ass fault.
And it could be an intermittent electrical problem so I’m not ruling this one out yet. If I can rule out the engine side of things while it’s out of the bike I’ll circle back to spark/ignition/wiring. Appreciate the suggestion!
 
Well blue smoke and the build up on that spark plug is obvious oil usage. Could be an incorrect hone job, possible broken ring on assembly, bad valve guide seal.....they get really hard and brittle on these motors, valve guide has excessive clearance. Some thing people never think of.....when you "rebuild" the bottom end you are restoring compression...and vacuum. This causes much higher vacuum in the engine which can cause more oil to be sucked through worn valve guides and seal. In the automotive and motorcycle repair field we stopped doing bottom end "rebuilds" only because of this oil usage. We only did repairs with bottom end and complete cylinder head rebuilds. Another thing to consider that will cause blue smoke is excessive fuel washing the cylinder down. This would also explain the one clean intake port.
 
Well blue smoke and the build up on that spark plug is obvious oil usage. Could be an incorrect hone job, possible broken ring on assembly, bad valve guide seal.....they get really hard and brittle on these motors, valve guide has excessive clearance. Some thing people never think of.....when you "rebuild" the bottom end you are restoring compression...and vacuum. This causes much higher vacuum in the engine which can cause more oil to be sucked through worn valve guides and seal. In the automotive and motorcycle repair field we stopped doing bottom end "rebuilds" only because of this oil usage. We only did repairs with bottom end and complete cylinder head rebuilds. Another thing to consider that will cause blue smoke is excessive fuel washing the cylinder down. This would also explain the one clean intake port.

Thanks for the input. All good points. I do have newer valve guide seals, but that’s not to say one isn’t cracked already. My initial thought was valve guides, so I’ll do some further inspection once I pull the head.
Good point on restoring vacuum in the cylinders and the excessive fuel. It’s possible I have a carb issue. Maybe my intake ports being stained yellow are normal and the clean #2 port is from excess fuel keeping it clean. I was thinking it was lack of fuel to that cylinder, but could certainly be the opposite. Thanks again for the input
 
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