Cold cylinder compression numbers...

[TheCafeKid]

is there such a thing? or is it an imaginary useless number?? I started tinkering on my newly aquired GS1000G tonight, and after pulling the plugs and checking the valves (which are all tight, pretty much as i expected though) the plugs are all pretty carboned up, B7ES plugs (hotter than stock, I think the PPO was probably having issues because the valves werent closing all the way) Cylinder 4 however looked just a touch shiney...may be my imagination, paranoia or the like, but I hooked my compression gauge up to it, even tho i cant crank it except by hand and since the carbs are off obviously it would be a cold test... Im not wise in the ways of compression testing, so this maybe a completely worthless endeavor but I got 35PSI on that cylinder, dead cold, cranked by hand.

Now, couple years ago i bought a low miles GS750 8V motor off ebay, and had it shipped to me. After going thru the valves first thing, I was horrified to find that ALL of the intake side were at LEAST TWO shim sizes too tight. Paranoid that the valves were burnt, I hooked up my compression gauge the exact same way, cold hand crank test, and I DO remember them all testing out at about 35PSI plus or minus...At any rate, that didnt quell my fears and I ended up pulling the head to check the valves...all ok...but since i had the head off...Well, I ended up rebuilding the top end just for kicks since i had already bought the gasket kit (didnt do the rings, they all specked out) It was a learning experience for me...AT ANY RATE...so that 35PSI number...coincidence? Worthless? Or does it mean anything at all??

Experts??

Thanks in advance


TCK

[mike_of_bbg]

Worthless. Particularly the hand-cranking part.

All cylinders will leak past the rings a bit; they do all have gaps after all. But with the starter motor there's usually not enough time for the compressed air to escape but a little. If you crank by hand, there's plenty of time for your pressure drop. I cranked my GS550 by hand and got about 30 psi. Cold is about 110-115. Hot was 125-130.

[Nessism]

Hey Josh,

Just sent you a PM but missed the part about hand cranking the engine. I think those compression numbers are useless. Best to get the valves adjusted and test again with a good battery installed. Actually, I wouldn't even bother with a compression test. Just do the maintenance the bike needs and go flog the crap out of it for a thousand miles or so. The compression will be fine unless the bike has been abused. Check compression after you put some miles on the engine if you want confirmation that everything is ok inside.

[tkent02]

Hey Josh,
Just do the maintenance the bike needs and go flog the crap out of it for a thousand miles or so.

A good flogging will tell you what you need to know much better than a compression test.