Engine Compression Testing
Basics:
- Valve lash adjusted to correct spec first.
- Accurate, screw-in type, no-leak gauge (with built-in Shraeder check valve) correctly in place. Important: only one check valve should be in your entire test rig!
- Proper technique: all 4 spark plugs removed, choke “OFF” (knob “IN”) and throttle wide open.
- Strong, fully charged battery.
- Robust starter that spins at correct rpm (no drag).
- Spin each cylinder until the reading stops rising (usually 4-12 complete crank revolutions).
Leak-down testing usually lets you “hear” the source of any compression problems:
- Escaping air in the muffler means an exhaust valve is not seating properly.
- Escaping air at the carbs indicates an intake valve problem.
- Hissing at the oil filler hole or crankcase breather means the problem is probably stuck or worn rings.
- Air sounds in the adjacent spark plug hole to the one you are testing means a blown head gasket.
Leak-down testing also permits you to make small crank rotations to help isolate actual trouble spots along the cylinder bore surfaces.
More here by Mike Nixon on leak-down testing and a method to make your own leak-down tester.
Remember: a motor than has been idle for an extended number of years will usually have lower than normal compression readings when first returned to service. Often, the values will rise after a few hundred miles of running as the rings “loosen up.”
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[…] compression. Always do a compression test after valve check / adjustment. Good technique with a strong battery on an accurate gauge should yield uniform values of 150+ psi on all 4 […]
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[…] Low compression (piston ring / cylinder wear issue) This can be diagnosed via a compression test / leak down test. […]
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[…] You should do a valve adjust and compression test (in that order!) before doing ANY ignition or carb tuning. GL1000s will run poorly when valve lash […]
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[…] healthy starter and a fully charged healthy battery. More details on compression testing here: https://www.randakksblog.com/engine-compression-testing/ __________________ Randall Washington (Randakk) Randakk's Cycle Shakk, LLC Pinehurst, NC […]
gl1100 low compression all cylinders will timing belt cause this kind of problem
Thanks for a great site.
Should you compression test on a hot or cold engine?
I am perplexed. I have low compression on an 76 GL. About 120, but it runs fine. Just got back from a 5,000 mile trip on her. Poor mileage at about 32 mpg running at 55-70 mph. I replaced the timing belts when I got back and noticed that the right side, the belt that slips, was about a tooth retarded. I hand cranked it around and there was no interference, so out of curiosity I ran a compression check and as anticipated the compression dropped to around 100. So, I purposely set the belt one tooth advanced and again hand cranked for interference and all was well. Now this is the odd part. My compression went up to 130. And yes it was clearly one tooth advanced. I set it correctly after that and the bike runs well with its old 120 compression. But what is up? Why was the compression better when the valve timing was advanced from where it was supposed to be by one tooth?
thanks
That fuel mileage is acceptable. Belts should be installed EXACTLY on the marks. Compression readings are corrupted otherwise.
my comment, is a question. my used 1200 has 160 psi on 3 of 4. The other one is about 70. what is the most, to the least, likely cause of problem? valves are sealing when closed. how likely is a stuck ring that loosens up after running, to be the problem? not very, I’m afraid.
How do you know the valves are sealing?
You need to do a “wet” test to eliminate the rings, then a leak-down test. That will tell you the source of the problem.
Bikes that have been “sitting” for a long time usually loosen up with a few hundred miles of running. Here’s my method for resurrecting a “sitter”:
https://www.randakksblog.com/starting-a-gl1000-after-a-long-lay-up/