Strange compression readings

[Anonymous]

My 1981 550L (21K miles) has been sitting in the garage since December, and before I laid it up I squirted oil (30W) into each cylinder. Today my new compression tester arrived so I tested the bike while cold (it hasn't run since December) with the carbs on (the throttle wide open) with the each cylinder reading about 80 PSI.

I rechecked the pressures after removing the carbs (I'm rebuilding them) with these results: 1=130, 2=110, 3=130, 4=120. I then squirted about 15cc of 30W oil into cylinders 1 & 2 and each now developed 225 PSI.

Before being laid up for the winter the bike ran well and didn't smoke. I'm rebuilding the carbs because it lacked low RPM power.

Any thoughts?

[debby]

I don't think the carb rebuild is gonna help. sounds like you're ready for new rings. they say if any cylinder is < 120 or if the diff between cyls > 20 it's time to tear it down. you could probably run it for some time but it's not going to be performing real well. Sorry...

debby

[earlfor]

Re: Strange compression readings

A spread of 110 to 130 is servicable. I would put everything back together and ride the bike for a while to see if anything changes/improves. You cant lose anything by giving it a try. Dirty carbs and a poor synch could be the cause of loss of low end power.

Earl

My 1981 550L (21K miles) has been sitting in the garage since December, and before I laid it up I squirted oil (30W) into each cylinder. Today my new compression tester arrived so I tested the bike while cold (it hasn't run since December) with the carbs on (the throttle wide open) with the each cylinder reading about 80 PSI.

I rechecked the pressures after removing the carbs (I'm rebuilding them) with these results: 1=130, 2=110, 3=130, 4=120. I then squirted about 15cc of 30W oil into cylinders 1 & 2 and each now developed 225 PSI.

Before being laid up for the winter the bike ran well and didn't smoke. I'm rebuilding the carbs because it lacked low RPM power.

Any thoughts?

[Anonymous]

Earl,

With regards to the compression readings, does it make a difference that the bike was cold and hadn't been run for several months?

Kelly

[earlfor]

I think it would, but I cannot say how much difference it would make. If the engine was warm, the piston to cylinder wall clearance could be tighter, and the rings could be compressed a bit more. Air cooled engines are manufactured with greather clearances because they expand and contract more than water cooled engines. A cold engine would be "loose". If the engine had not been run for a couple months, I would assume the cylinder walls would be dry. If the engine had been run, there would be an oil film to some extent and that too could make a difference.

My advice is to ride the bike for a bit and let things "settle in", then after a tank of gas, re-check the compression before making any decision.

Earl

Earl,

With regards to the compression readings, does it make a difference that the bike was cold and hadn't been run for several months?

Kelly

[Anonymous]

Thanks Earl.

Kelly

[redliner1973]

Maladjusted valves can also cause differing readings.... :twisted:

[earlfor]

If the valves are out of adjustment, adding oil to the cylinders will not change the compression readings.

Earl

Maladjusted valves can also cause differing readings.... :twisted:

[SqDancerLynn1]

DID you have the throtle open on your original readings, I would go ahead and do the carbs I would also check the cam chain tensioner and check the chain for stretch

[redliner1973]

He stated his pressures went from low 100's to 225psi after oil...that much of an increase is not a fault of the engine. that is an increase from getting too near to the point of hydrolock, from too much oil. a 100 psi increase is an awful lot. his readings, while slightly low are within tolorance, and valve adjustment is what I would check first. :twisted:

[argonsagas]

He stated his pressures went from low 100's to 225psi after oil...that much of an increase is not a fault of the engine. that is an increase from getting too near to the point of hydrolock, from too much oil. a 100 psi increase is an awful lot. his readings, while slightly low are within tolorance, and valve adjustment is what I would check first. :twisted:

A 100PSI increase?


This is a good point.

At 225PSI you are about 17 to 1 compression ratio, and you are into diesel pressures.

This is not a reasonable number for a stock GS.

[earlfor]

A reading of 225 and borderline hydrolock makes that value invalid as a compression test.

Earl

He stated his pressures went from low 100's to 225psi after oil...that much of an increase is not a fault of the engine. that is an increase from getting too near to the point of hydrolock, from too much oil. a 100 psi increase is an awful lot. his readings, while slightly low are within tolorance, and valve adjustment is what I would check first. :twisted:

[saaz]

I have never had readings go up that mcuh by putting oil in the cylinders.
The bike should be warm before taking readings, and the throttle held wide open when doing the readings. Unless it is using oil I would not worry about it just yet...

[Anonymous]

The bike was dead cold and the readings were taken with the carbs off the bike. I'll blow some of the oil out of the cylinders today and try the wet readings again.

[Anonymous]

One month later.

I've got the valves adjusted, carbs rebuilt and synced. With the bike hot here are the compression readings:
1=145
2= 126 (added oil and it went to 136)
3=130
4=140

The bike runs great after it warms up, less vibration and stronger pulling in the lower rpm's. I pulled the spark plugs today and they all look perfect. I think I'm ok here.