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Curtis B.
Curtis B., Technician
Category: Small Engine
Satisfied Customers: 13581
Experience:  have worked on and around most engine models for 35 yrs./Polaris ATV expert
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You answered my question regarding Honda GX 340 carboning

Customer Question

You answered my question regarding Honda GX 340 carboning the plug last night. I removed the exhaust, screen, and muffler, washed it out, and then tested it by breathing into each end (wouldn't come apart). Breath unrestricted. Left off the air filter and ran it at various speeds for 10 minutes. Ran great, but the plug was already covered in carbon. Had I continued, would have been walking again.
Submitted: 1 year ago.
Category: Small Engine
Expert:  Curtis B. replied 1 year ago.

You didn't run it with exhaust off?

Customer: replied 1 year ago.
Again I say; I did not run the engine without the exhaust manifold. When I could breath freely through either end with no restriction, I felt certain the muffler was not blocked. This was after I had removed the screen (threw it away) and washed out the muffler.
Expert:  Curtis B. replied 1 year ago.

If there is no way the exhaust is plugged. Then it's too much fuel/too little air, or the timing is off. The carbon on the plug only comes from incomplete combustion.

Customer: replied 1 year ago.
I have eliminated the "too rich" theory several ways: new carb., adjusted both ways, removed air filter. I have eliminated the restricted exhaust theory (which I really thought would help). I have been pondering the possibility of valve adjustment or as you now suggest timing. The engine has never had valve adjust or overhaul, nor any breakdown other than the nagging carb. problems and now this carbon issue. What is your next guess?
Expert:  Curtis B. replied 1 year ago.

Try setting the valves. .006 inch intake, .008 exhaust.

Expert:  Curtis B. replied 1 year ago.

Then if no help, replace the flywheel timing key.

Customer: replied 1 year ago.
What you now suggest is out of my league as I don't know how to adjust the valves. I really don't think the flywheel key is likely since there has never been a traumatic experience that might have caused a key to half shear. You tried and thanks for the effort.
Expert:  Curtis B. replied 1 year ago.

Thank you.