Problems with compression test.

[glyn1718]

Full throttle. Plugs out. Check all cylinders and note results. Then insert a teaspoon of oil in each cylinder and retest. If the reading are significantly different you are loosing compression usually via rings. But I say don't worry, if it runs, get out on the road and enjoy it. Only consider compression readings when the engine is playing up as part of diagnostics

 

[Gord Baker]

I hope I don’t get thrown off for this, but I’m having problems with the compression test, but this is on a four-cylinder motorcycle. I didn’t know where else to ask, but I thought you guys could be a big help.

I bought a compression test kit at Harbor Freight and hooked it up as I should in a while. Turning the engine over with the throttle open. It only reads about 65 pounds for each of the four cylinders. I thought the gauge was faulty and I exchange it Harbor Freight for a new one does exactly the same. How can I only be getting 65 pounds. The engine would never run at that level. The engine does run fine, but I’m just trying to take a test to determine how the cylinders are. Thanks for any help you can give me.

Check the Spec's for that engine. Your engine seems OK but low across the 4. It would run at that level I believe.

 

[Gord Baker]

Full throttle. Plugs out. Check all cylinders and note results. Then insert a teaspoon of oil in each cylinder and retest. If the reading are significantly different you are loosing compression usually via rings. But I say don't worry, if it runs, get out on the road and enjoy it. Only consider compression readings when the engine is playing up as part of diagnostics

Yup.

 

[fastbroshi]

It's an interesting question, but it's mostly academic. Your best compression test is to run the motorcycle. If it runs smoothly and accelerates well, you have good compression, and that's all that matters.

It's often the test gague or your procedure. I've seen 80 psi on a chain saw, yet it performed perfectly. Always believe performance over a gague, unless you are a pilot flying on instruments in an aircraft, and in the clouds!
.

I agree with this. The compression may be reading low per spec, but you have to consider you're not using the same tools used by Yamaha when they created their spec. Not to mention if this bike hasn't ran in a while, it's possible it's got some gummed up rings, valves, etc that may affect your reading. Did you ever try adding some oil to the cylinders before a test, as others have mentioned?

 

[IronTom]

A few years ago, 1957, I had a early Matchless 500 single. ( Big 'jump' up from my Whizzer ).
To start it, I coasted down the Vernon street hill, and released the clutch in 2nd gear.
There were many failures,,, push it back up the hill,, try again.
Uncle (machinist) installed a 'flip-valve' by drilling a tiny hole through the head.
It cut the compression down to where little-skinny me could use the kick-starter !
It also would often, shoot fire from under the gas tank !
which resulted in Dad trading it in on a brand-new Cushman Eagle !

Where were we ? Your 65 lbs compression is just enough pressure to 'fire' the engine up with that shrimpy-small battery starter. Once the RPMs increase, the valves return to normal high-compression operation, the electronic ignition advances the spark timing,,, and the beat goes on!
A simple 'what-me-worry' test would require a few drops of motor oil onto each piston, then re-test the compression. If the oil changes the 65 value, that indicates normal piston-ring wear.
Thats not a problem. Write the numbers in your manual,,, 'till next year.
B.T.W.
Do not try to roll-down-the-Vernon-street-hill, that modern engine may throw you over the bars !

 

[Smithsonite]

Some machines have a compression release on the intake cam, deactivated once over 300 RPM or so by flyweights.

 

[awkerper]

I hope I don’t get thrown off for this, but I’m having problems with the compression test, but this is on a four-cylinder motorcycle. I didn’t know where else to ask, but I thought you guys could be a big help.

I bought a compression test kit at Harbor Freight and hooked it up as I should in a while. Turning the engine over with the throttle open. It only reads about 65 pounds for each of the four cylinders. I thought the gauge was faulty and I exchange it Harbor Freight for a new one does exactly the same. How can I only be getting 65 pounds. The engine would never run at that level. The engine does run fine, but I’m just trying to take a test to determine how the cylinders are. Thanks for any help you can give me.

This conversation still provided me with some good info. Thanks all!

 

[bud&dusty]

I hope I don’t get thrown off for this, but I’m having problems with the compression test, but this is on a four-cylinder motorcycle. I didn’t know where else to ask, but I thought you guys could be a big help.

I bought a compression test kit at Harbor Freight and hooked it up as I should in a while. Turning the engine over with the throttle open. It only reads about 65 pounds for each of the four cylinders. I thought the gauge was faulty and I exchange it Harbor Freight for a new one does exactly the same. How can I only be getting 65 pounds. The engine would never run at that level. The engine does run fine, but I’m just trying to take a test to determine how the cylinders are. Thanks for any help you can give me.

 

[sheenist]

cdestuck: Run a compression test on all four cylinders and record the numbers. Then squirt a couple of shots of oil in one cylinder and try it. If the compression is noteably higher, then yon may be leaking some compression past the rings. If the reading isn't much higher, you are probably leaking past the valves. Good luck.
Sheenist

 

[Gord Baker]

This is getting like the 'winter storage' episode! Most important is that the Compression is near equal in all Cyl's.
It appears to be so just 'run it'.