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Bad compression - why?

  • Thread starter Thread starter moonstoner
  • Start date Start date
M

moonstoner

Guest
Just did a compression test on my 1981 GS650G that I can only get running with the full choke (and boy does it smoke).

cylinder 1 - 100 lbs
cylinder 2 - 60 lbs
cylinder 3 - 50 lbs
cylinder 4 - 50 lbs

OK you experts, where do I look, what do I do now.
 
Did you test the compression with the engine hot? Have you verified the valves are in adjustment? Did you hold the throttle open during the test? How many miles are on the bike and has it been ridden regularly? Regarding the poor running, are the carbs clean and are the rubber O-rings fresh both inside the carbs and on the intake boots where they fit to the cylinder head?

Lots of questions need to be answered before we can understand what is going on.
 
moonstoner said:
Just did a compression test on my 1981 GS650G that I can only get running with the full choke (and boy does it smoke).

cylinder 1 - 100 lbs
cylinder 2 - 60 lbs
cylinder 3 - 50 lbs
cylinder 4 - 50 lbs

OK you experts, where do I look, what do I do now.
Click to expand...

Was it sitting for a long time?

Start with

1. Adjust valves
2. Clean carbs per clean up page
3. New O rings all around
4. Tsp of Marvel Mystery oil in each cylinder
 
moonstoner said:
Just did a compression test on my 1981 GS650G that I can only get running with the full choke (and boy does it smoke).

cylinder 1 - 100 lbs
cylinder 2 - 60 lbs
cylinder 3 - 50 lbs
cylinder 4 - 50 lbs

OK you experts, where do I look, what do I do now.
Click to expand...
Is that blue smoke coming out? if so your rings are shot.
 
Big T said:
Was it sitting for a long time?

Start with

1. Adjust valves
2. Clean carbs per clean up page
3. New O rings all around
4. Tsp of Marvel Mystery oil in each cylinder
Click to expand...

Can't say if it has been sitting for a long time. The bike is in overall good condition and looks like it's been well cared for. I did a carb job on it last weekend and the carbs looked like they were in real good shape. Now they're in great shape.

So, how do I adjust the valves. And would they have likely gone out of adjustment?

Would O rings at the carbs cause low compression?

What is Mystery oil suppose to do? Will seafoam do the same?

Do head gaskets ever blow on these bikes? (I see no evidence of any oil or water leakage anywhere)
 
OOOOk...

Stay with us here..

Valves on these bikes are CONSTANTLY on their way out of adjustment... it happens to all vehicles. As the valves on these bikes wear, they start to receed into the head, and the valve head itself starts to "tulip" This causes the clearance between the cam lobe and the valve bucket shim to decrease, and eventually the lobe is in constant contact with the shim, holding the valve open. Your compression WILL BE EFFECTED BY THIS, obviously, because the valves arent sealing completely, if at all. Adjustment, or at least a check of clearance, is suggested every 3-5K miles... If you dont know when the last time it was done was, its time to do it. Failure to adjust the valves will cause MASSIVE and EXPENSIVE issues, including burnt valves, pistons, rings..etc...thats assuming the bike will even start..

Head gaskets, while not completely common, arent unheard of for going out on ANY bike, especially one that as sat for any length of time. What it boils down to are these simple facts:

In order to get your bike running, the following MUST, an ABSOLUTELY MUST, be taken care of:
The valves MUST be adjusted FIRST
The carbs must be removed, stripped, dipped, and rebuilt with NEW orings
The electrical system must have all connections cleaned and greased or replaced if necessary
The charging system should be checked for proper charging, and dealt with as needed if found faulty.
Ignition should be checked and dealt with if found faulty.
Petcock should be replaced (its over 20 years old, it needs it, working or not, it probably wont much longer)

If you DONT do these things, you WONT get the bike to run as it should, and you WONT succeed. Refurbishing and returning one of these old beauties to the road costs more than alot of people realize. Motorcycling is NOT a cheap habbit/hobby. Unfortunately, many people think they'll get off cheap when they find an old bike, and once they realize they wont, cant, or dont want to do the work and put the time and money into it, they go back in a shed or get sold or scrapped again...
 
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Mostly these bikes have sat, rings get stuck with time, cylinders get a little rust, whatever. The valves are usually out of adjustment, usually too tight. These engines mostly have low compression until they get a few thousand miles of running, without sitting for years in between.... and a good valve adjustment.
Very few GS 650s have had enough miles to wear out the rings. He didn't mention it being well over 150,000 miles.
Most all of the dead GSes I have restored have had the compression come up after a lot of riding over a few weeks..
 
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gearhead13 said:
Is that blue smoke coming out? if so your rings are shot.
Click to expand...
Rings may NOT be shot if the bike has sat a while, rings can stick, which will cause compression numbers to drop, but running the bike for a while can also unstick the rings, and suddenly things are fine. Secondarily, blue smoke is NOT always a sign of bad rings. Blue smoke under throttle is more likely rings. Blue smoke at idle is possibly rings, but could also be valve seals. Untill the valves are adjusted and the necessary things are attended to, its impossible, and silly to speculate on the cause of the blue smoke and low compression numbers. Tearing the motor down on a hunch or guess is most times a waste of money. If the other things (which will need to be done anyway) are done, and the bike is still displaying the same symptoms, then it comes time to start looking at a top end rebuild. But, untill you know for sure, youre talking about throwing 3-500 dollars or more at a problem that 1-200 bucks and some time could possibly fix.
 
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