Compression significantly different between throttle open and closed…why?

‘80 KZ650-F1. Pods. 

I’ve been struggling with cylinder #2 not firing for months now. I’ve been completely unsuccessful diagnosing the issue. I feel like I’ve tried everything. However, today, while testing compression once again, I noticed that the three working cylinders will read around 135-145 psi cold and throttle closed. I panicked when I saw only 75 on cylinder #2 but that could explain it. However, I did it one more time with throttle open and….jumped quickly up to 145psi. 

Can somebody explain to me why this would happen?! I’m pretty confused.
 

Have you adjusted valve clearance?  And if so, did you find any valves with zero clearance?

I just finished up a valve seal job and verified all the clearances were in spec as well. So I think we're good there.

With the throttles closed, the piston going down makes negative pressure in the intake tract, thus not fully filling the cylinder giving the low reading. The compression test must be done with the throttle wide open to give an accurate reading.
Edit-Oops, did not see your reply about the valve job.

I know throttle open is ideal but usually we're talking 10-20psi difference. This was on the order of 70psi whereas the other 3 pistons were maybe 10 psi different when measured with throttle open. 

A definitive "answer" can be attained by squirting a tablespoon or two of engine oil into that plug hole and measuring again...
An increase in pressure will indicate an issue with the bore and / or rings, being either worn and / or rings seized in the ring grooves.

My 2c

H.

 

......or/and maybe the synchronization is not great, and the butterfly of #2 is almost closed...with not throttle, related to the others.

Throttle closed means the slides are blocking air from flowing through the carbs and into the cylinders.  

wrote:

I know throttle open is ideal but usually we're talking 10-20psi difference. This was on the order of 70psi whereas the other 3 pistons were maybe 10 psi different when measured with throttle open. 

You are right, I got nothin. I do know that the oil in the cylinder is a controversial method and that a leakdown test is more informative. 

As long as I am pissin in the wind, maybe there is a sticking valve in that cylinder.

wrote:

wrote:

I know throttle open is ideal but usually we're talking 10-20psi difference. This was on the order of 70psi whereas the other 3 pistons were maybe 10 psi different when measured with throttle open. 

You are right, I got nothin. I do know that the oil in the cylinder is a controversial method and that a leakdown test is more informative. 

 

The "wet test" method does work but you need to be sparing with the oil to avoid filling your tester with it 

Shop air dialed down to 80 psi pumped it the cylinder is a poor mans leak down test. Make sure piston is at bottom dead center on power stroke. Listen at crankcase, exhaust, and carbs fo air leakage.