81 Gs1000l compression numbers...

[Anonymous]

i'm in the process of fixing up my bike, and i did a compression test to see if the motor needs to be rebuilt. The numbers are with a little oil in the cylinders. here's the numbers:
#1 210
#2 125
#3 70
#4 145
The motor has about 40,000 miles on it. I looks to me like it needs to be rebuild. I was going to do just a piston and rering. Also does anyone know a aftermarket manufacturer of pistons and rings. I onlt found them through the dealer, and was just wondering if anyone else sells them cheaper. I'm relatively new to bikes if you haven't noticed, haha
thanks for the help
tom

[Guy]

Compression should be in the region of 130 to 170 psi with the engine cold and the throttle wide open. The important thing is that they're all in the same ballpark. Your numbers seem way high and way low. You need to be consistant in the way you take your measurements. If any cylinder reads low with a little oil added, it's definitely a valve rather than a piston/ring problem. Check your valve clearances before you take more drastic measures.

[Anonymous]

i redid the compression test after the motor had sat for a couple of hours. Here's the numbers:
#1 155
#2 115
#3 65
#4 125
This test gave more reasonable results. What do you think
thanks
tom

[duaneage]

Possible valve problem on that one, pull the valve cover and check for the clearance between the cam and the follower. Obviously when the piston is on the squeeze stroke both valves should be closed and you should be able to slip a .002 or smaller feeler gauge between the cam shaft on each side and the follower. If not, the culprit may only be a valve lash adjustment by replacing the shim, very cheap and easy.

I would also consider a head gasket, does it appear to be seeping oil around the head and cylinder ? Leaky gaskets are common on these bikes as well. the head gasket could be leaking into the passageway for the cam chain, right down the middle. Remove the breather tube that comes out of the valve cover when it is running and see if a LOT of pressure or smoke is coming out, indicating that combustion is getting past somewhere.

both options are cheaper than pulling the jugs. If it does not smoke then the rings might not be the problem. If your spraying for mosquitos then you need to get real dirty with it.

[Jethro]

Compression should be in the region of 130 to 170 psi with the engine cold and the throttle wide open.

I've always been told to do a compression test with a warm engine. In fact the new comp. tester I got had directions that said the same. Makes sense to me...

[Guy]

I never do what I'm told, but I try to do most things I'm asked.....
It doesn't matter too much if you do it hot or cold, the difference will only be a few psi. The important thing to measure is the difference between cylinders.