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Discussion Starter · #1 ·
Recently I was doing some work with the engine out and when I got the bike all back together, I found that by right cylinder was at 50psi compression. Put a little oil on the cylinder and no change to the compression. I didn't have a leak down gauge at the time so I just did a quick and dirty "leak down" test by blowing air in the cylinder (at TDC) and listening for a leak. Sounded like a had a good size leak from the exhaust valve so I pulled the engine thinking I had some valve troubles.

When I got the engine on the bench I just double checked valve clearance, which was off. I finally got a leak down gauge and the test showed barely any leakage (well under 10%). This time, I did not feel any air coming from the exhaust, and whatever leakage there was came through the crankcase.

So right now it seems like I might have bad compression but good leak down test results. However I have not retested the compression (I don't want to put the engine back in and I'm not sure how to test compression out of the bike).

Could it be that improper valve clearance dropped my compression to 50psi and readjusting fixed my problem? What else could possibly result in low compression with a good leak down test? The bike ran well before pulling the engine the first time (150psi on both cylinders). I had only pulled the engine to put in some heilcoils in various spots.
 

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Check the leak down not only at TDC but also at DBC.. there could be a issue somewhere in the cylinder past when the piston is at TDC. Also check and see if if the jugs are leaking into each other at the head gasket..
 

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An improperly adjusted valve (too tight) could very well have caused your problem.

If you have a solid stand to hold the engine you could spin it with the electric starter to do a compression test. Unless you can run the engine on that stand you'll be doing a cold test so expect the numbers to be low.

A leakdown at BDC won't tell you anything since one valve or the other will always be partially open at that point.
 

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Can we assume that, when you did the compression test, the throttles were held open, or the carbs off? A cold test with throttles closed will be low.
 

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Discussion Starter · #5 ·
Can we assume that, when you did the compression test, the throttles were held open, or the carbs off? A cold test with throttles closed will be low.
Yes, throttle open. I had also done a test with the carbs pulled and that came back at 50psi as well.
 

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Too bad. Maybe the rings are just jammed up with deposits. Sometimes a soak with a cleaner, like Seafoam, can free them; you have to give it time, though.
 
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