Bad News on Compression

[Norseman]

Well, having run the compression test now has resulted in accute depression .

Regular test with 3 readings per cylinder:
#1 = 135 psi
#2 = 165 psi
#3 = 160 psi
#4 = 130 psi

Wet test (oil in cylinder) with 2 readings per cylinder:
#1 = 210 psi
#4 = 210 psi

Harbor Freight compression tester broke apart before wet-testing #2 and 3# cylinder.

Obviously the head comes off and cylinders need to be re-bored with over-sized rings. Now for my question: Who to do this? Who would you folks trust? Dealership? Vintage bike shop?

How much should I expect to pay? Is it good sense to do the valve job at the same time, and how much would that cost?

Appreciate any guidance here...

[p_s]

I don't know if I would trust your HF compression tester if it broke halfway through. Are these hot compression numbers? Valves adjusted? 210 psi sounds like you put in too much oil.

[KRISTI'SGS550]

Not sure what type of bike you have, but in reference to my service manual they seem OK. Also what p_s says how can you trust a tester that broke during testing. I think you need to test again.

Cory

[Norseman]

Well, the compression tester seemed to work fine until the adapter hose fitting blew off the hose, so I think the readings were relatively accurate.

Too much oil? I would tend to agree because it seemed an awful lot of oil to pour in. But my Clymer manual stated 15cc for the wet test and I put in only 10cc :shock:.

Regardless of the exact psi difference, though, going from 130 to 210 with oil is a heck of a difference, and according to the manual, indicates the piston rings are shot.

What do you folks think?

[p_s]

I don't know what bike you have, but the compression chambers aren't that big to begin with and making them effectively 10cc or 15cc smaller will do a lot to your numbers. Actually I'm not sure if all the oil stays above the rings, but you get the idea.

I get 140-148 cold on my bike, but it's the hot numbers that really matter.

[Nessism]

Your numbers are not that bad; worst thing is the high low difference. Have you adjusted the valves lately? Did you test with the engine hot and at wide open throttle?

[Norseman]

I apologize guys - I guess I need to pay more attention as I thought I had mentioned that the bike is a 1981 GS650GLX

Okay, I did the valve adjustment about 1000 miles ago (last year), and had them spot-on in the center of the range, or 0.05.

I did the compression testing today after a short ride - so it was hot. And I did hold the throttle fully open for the testing and kept cranking until the needle stopped moving up (roughly 4 to 5 sec.). And for each repeated test, the gauge read the exact same numbers per cylinder.

Clymer (GS650 Fours 1981 - 1983) states the compression range should be 142-199 psi with no greater difference than 28 psi. So that's why I got so concerned.

So do I park the bike until I can afford the work, or is it driveable? Thanks for your help, all.

[Nessism]

I apologize guys - I guess I need to pay more attention as I thought I had mentioned that the bike is a 1981 GS650GLX

Okay, I did the valve adjustment about 1000 miles ago (last year), and had them spot-on in the center of the range, or 0.05.

I did the compression testing today after a short ride - so it was hot. And I did hold the throttle fully open for the testing and kept cranking until the needle stopped moving up (roughly 4 to 5 sec.). And for each repeated test, the gauge read the exact same numbers per cylinder.

Clymer (GS650 Fours 1981 - 1983) states the compression range should be 142-199 psi with no greater difference than 28 psi. So that's why I got so concerned.

So do I park the bike until I can afford the work, or is it driveable? Thanks for your help, all.

I think your manual is missing a key point, the lower limit. The factory service manual typically states the service limit is in the 128 psi range. Your readings are within this range when you consider the gauge error that is sure to be present.

I say ride your bike and enjoy. Check it again after a good summer of thrashing. Keep track of oil usage and make notes. Report back in fall unless your bike burns a lot of oil and/or starts smoking. You will not hurt it by riding it.

Good luck.

[49er]

I apologize guys - I guess I need to pay more attention as I thought I had mentioned that the bike is a 1981 GS650GLX

Okay, I did the valve adjustment about 1000 miles ago (last year), and had them spot-on in the center of the range, or 0.05.

I did the compression testing today after a short ride - so it was hot. And I did hold the throttle fully open for the testing and kept cranking until the needle stopped moving up (roughly 4 to 5 sec.). And for each repeated test, the gauge read the exact same numbers per cylinder.

Clymer (GS650 Fours 1981 - 1983) states the compression range should be 142-199 psi with no greater difference than 28 psi. So that's why I got so concerned.

So do I park the bike until I can afford the work, or is it driveable? Thanks for your help, all.

How many miles on the clock? Clymer differential is a guide only. If it starts and runs ok, keep running it until you decide that you want more performance. You can then rebore and up the compression to widen your smile!

[Norseman]

You Guys Are Great!

Thanks so much, Guys. The bike has 25,100 miles on her. Smokes minimally upon start-up and cold.

I'm gonna keep on riding - keeping an eye on things - and later down the line pull her apart and rebuild.

I really appreciate your help - all of you! \\/

[uudfourty]

Rock and Roll.

[earlfor]

The 450 I'm in the process of refurnishing had sat up, not running for nearly 8 years. It shows 135 psi when cold. It runs fine. I would just ride what you have until you feel like doing additional work. Low compression is not going to hurt anything mechanical. The worst is you might be a little down on power and gas milage, but nothing bad is going to happen.

Earl

Thanks so much, Guys. The bike has 25,100 miles on her. Smokes minimally upon start-up and cold.

I'm gonna keep on riding - keeping an eye on things - and later down the line pull her apart and rebuild.

I really appreciate your help - all of you! \\/

[Norseman]

Thanks again, uudfourty and earlfor, I appreciate the info and restoring my confidence in continuing to ride the bike - until I tear it down this fall for a complete frame-up rebuild. At that time I'll probably do something performance-wise as well.

[psyguy]

as a referrence my 850 used to start and run just fine at only 90psi
unless you're planning on performance upgrade, you re likely to get good results with just the new rings and light honing of the cylinders