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High compression in 3 cyl

rotten_1

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I picked up an ugly CB750 for Freeeeeeee.

I started going through the motor checking spark and compression and came across something strange. Cyl 1,2,&4 all show 135-140. Cyl 3 shows 235+ psi compression.

What could cause it? I pulled the valve cover to see if I could see if the exhaust valves are openings but everything seems to be in order with the cams rotating and the tapets rising after the lobes press them down.

What am I missing?
 
It either has some excess carbon build up or has some oil or fuel in that cylinder causing high compression. Are you doing the compression test with all spark plugs out and throttle held wide open? Have you double checked the reading to make sure you dont have a compression tester issue?
 
I completed the tests in order 1-4 and then ran the tests again. 1-4. I did add some oil to each cylinder to help seal the rings and not make them cycle dry. (lord knows how long this bike has sat). I will re-run the compression tests again today.
 
Good chance you got too much oil in one cylinder which will cause high compression. It takes very little oil to affect compression.
 
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