Just checked compression with a Taiwan made compression tester. 105 exactly on both cylinders. Good or Bad?

105 would be pretty weak. Did you hold the throttle open, and pull it over 4 or 5 times? If so, I would first check it with another gauge to make sure. How many miles on this engine?

I didn't hold the throttle open. about 2400 miles since last rebuild.

If you don't know what it read with the same guage after the last rebuild, then this new number does not mean too much. Does it start easy and run good? Both being the same is more important than the actual number, so if it starts and runs good then it probably is good. Make sure you hold the throttle wide open and give it five good pulls when you check it again.

on my newly rebuilt 580 i have 125-130 psi on both cylinders but all compression testers are a little different just as long as they are both equal on the guage you should be good to go

Get this, borrowed buddies Craftsman (made in the USA) compression tester, 122psi both cylinders. I should have learned by now to not buy foreign tools. JUNK!!! Needless to say i'm not going to bother opening the motor on this sled.

i had the exact sled about 5 yrs ago. i siezed it one cold morning driving about 60 mph. sat along the trial for about 30 minutes. gave her a crank didnt the thing start right up. couldnt believe it. drove it home, checked the compression and one side was down a bit but not anof to worry about. sorry but i cant remember what the compression was. for peace of mind i took the heads off and checked the pistons. one had stuck but not enought o hurt it. i drove it one more season that way. i think i put at least 3000 miles on it after it was siezed and it worked as good as it ever did.

Originally posted by zr580s rule@Aug 21 2003, 04:43 PM
but all compression testers are a little different just as long as they are both equal on the guage you should be good to go
This is not necessarily true. If compression reads different on each cylinder, this would usually indicate damage on one cylinder, but compression can be down on both cylinders an equal amount, due to wear. The variance between the two is only one thing to consider, it also matters what the peak reading actually is.