Peculiar thumping\detonation issue slowly becoming more noticeable

[Andre1150]

Just completed a valve job and the bike has been running great, idle is a bit high, but overall, it humms along.

 

What's been happening more frequently is this detonation / thumping issue that happens at all speeds, but is more apparent when I'm idling.

 

At idle, every so often there's a powerful thump or detonation in the engine that drops the RPM's about 500rpm, at which point the engine recovers almost immediately and stays at idle. It's like a miss, but with a pronounced thump. I think it might be my advance is too far forward and the spark detonated the fuel early, which stops the forward motion of the piston. Or at least it feels that way. .

 

Any ideas?

[dirtrider]

Evening FireKit

 

It doesn't sound like an ign timing problem as you would think that IF the ign timing was advanced enough to stop a piston that it would do it on every firing not just once in awhile. (I'm not saying your spark timing is not too far advanced just that it doesn't sound like your problem)

 

Can you tell if it is thumping in the intake or in the exhaust system? That might point us to something.

 

You might have a leaking or tight valve, or a rocker arm that is to tight on side play & sticking a valve open once in awhile.

 

Or maybe a dripping fuel injector.

 

Or maybe something sticking into the combustion chamber that is glowing causing early ignition.

 

You didn't get your o2 sensor pig tail zip tied up too close to the R/H lower spark plug wire did you?

 

I wish I could tell you more but this problem sounds difficult to find even when leaning over the engine.

 

 

[Andre1150]

I pulled a plug on both sides and it appears that there's heavy carbonation on the piston head. .

 

I'm going to open up the cylinder heads and see if anything is sticking

[dirtrider]

Morning FireKit

 

If a valve job was just done I presume the cylinder heads & piston tops were de-carboned at that time, where did all the "heavy carbonation on the piston head" come from?

 

Is your engine an oil burner?

[Andre1150]

I adjusted the valves, yes. But, I did not decarbonize the heads. (not sure how to do that) And yes, the bike has always been a heavy oil burner, as long as I've had it.

[Andre1150]

On this topic, has anyone ever performed a slow water intake through a vacuum line to decarbonize?

[dirtrider]

I adjusted the valves, yes. But, I did not decarbonize the heads. (not sure how to do that) And yes, the bike has always been a heavy oil burner, as long as I've had it.

 

Afternoon FireKit

 

OK, so you didn't do a valve job (grind the valves) you just adjusted them, correct?

 

Using water to de-carbon is iffy but will usually work on soft carbon, potential for engine damage is always possible.

 

If it is hard (baked hard) carbon then water will probably not do anything as that carbon is about as tough as ceramic.

 

If hard carbon then even removing the cyl heads & going after it manually is a big /big task.