Compression test for '82 GS1100GL?

[Guest]

Hi, I am having some trouble getting a good test. Or maybe my compression *is* really bad!

I am getting 140 on the first cylinder and only 100 on the other three. But I need to get the battery charged up and try again. Maybe I'll start with #4 instead next time. Seems #1 is always strong and as the battery goes so do the compression numbers. On the first test I did with Tom MLC we had 158 on #1 and 100 on the rest, though he thought maybe the low numbers were because the battery was low. (Tom if I screwed up any part of this report feel free to correct me!

Is it better to check hot or cold or doesn't matter much?

Thanks guys!

[TheCafeKid]

NEEDS to be hot to get accurate numbers...Throttle WIDE open.

[mike_of_bbg]

The only reason to do a compression test cold is if the motor won't run. Otherwise heat that puppy up. The battery should be fully charged, as compression numbers will go down the slower it turns (or you can do like I did and jump it from a running car). You should also be certain your valves are properly adjusted or you might be losing some compression through valves stuck barely open.

[gearhead13]

The only reason to do a compression test cold is if the motor won't run. Otherwise heat that puppy up. The battery should be fully charged, as compression numbers will go down the slower it turns (or you can do like I did and jump it from a running car). You should also be certain your valves are properly adjusted or you might be losing some compression through valves stuck barely open.

BTW I heard bad things about jumping bikes from a running car, like frying electrical components.

[TheCafeKid]

Yeah...as in DONT DO IT!

[chef1366]

It doesn't matter how slow the motor turns it will build to the maximum compression eventually.

[duaneage]

Don't have the car engine running when jumping and it will be fine. Pull all plugs to reduce the strain on the starter. You need at least 50 RPM (normal cranking speed) and five complete strokes to get an accurate reading. Should be over 100 and within 15 PSI of each other for best results.

Valve adjustments greatly affect compression tests. If not done yet check the clearance and retest if changed are made. Weak cylinders get a tablespoon of ATF fluid down the spark plug hole and if compression improves the rings are the issue. If not, it's a valve that's the problem.

Retorque of a head might improve things too, but that's a hail mary pass.