Tested Cylinder Compression

[dbelch]

I borrowed a buddy's compression gauge and these are the results for the '86 FJ   #1 - 100 lbs
                                                                                                                       #2-  120 lbs
                                                                                                                       #3 -   75 lbs
                                                                                                                       #4 - 115 lbs

What do these numbers tell me?  Remember to keep it simple, I'm a Carpenter by trade and understand a whole lot more about lumber and drywall than I do motorcycles.  The seller tells me that the bike has well over 100,000 miles on it, but went through a recent engine rebuild.  The Odometer reads 15,000 or so miles and I haven't had a chance to run it yet as I have just finished fixing up the electrical end of things.  Thanks.

[Dan Filetti]

I borrowed a buddy's compression gauge and these are the results for the '86 FJ   #1 - 100 lbs
                                                                                                                       #2-  120 lbs
                                                                                                                       #3 -   75 lbs
                                                                                                                       #4 - 115 lbs

What do these numbers tell me?  Remember to keep it simple, I'm a Carpenter by trade and understand a whole lot more about lumber and drywall than I do motorcycles.  The seller tells me that the bike has well over 100,000 miles on it, but went through a recent engine rebuild.  The Odometer reads 15,000 or so miles and I haven't had a chance to run it yet as I have just finished fixing up the electrical end of things.  Thanks.

These numbers look low.  You need to do the test correctly though, so they may simply be inaccurate. 

Others with more knowledge will chime in with greater detail but here are some tips for "doing it right" :

1) make sure the motor is warm (someone please check me on this).
2) Hold the throttle Wide Open while doing the test
3) Squirt a small amount (teaspoon) of oil in the spark plug hole before doing the test.

It may well simply be that you did the test incorrectly.  Alternatively, it could be the values are in need of adjustment.  Do this/ get it done and re-test.

Dan

[Pat Conlon]

Adjust the valves, run the bike, and after 100 miles of good hard riding, retest

[moparman70]

If you want the best measurement - perform a leak-down test.  This way you can tell if you are losing compression around the valves.  I had a mechanic tell that -- so dont ask me too many more questions

Compression testing is a good benchmark but I don't think you can tell where you are losing compression.

PER Dan's message above -- someone told me also that you don't really add oil to do the test but if you have a cylinder that is low -- add oil and if the compression goes up you probably have worn rings -- if stays the same -- probably valve issue

[dbelch]

Thanks for the input!  I did forget to test with the throttle open..warm engine will be an issue..I have the carbs empty after a good cleaning..didn't want to put the tank back on till I had a chance to see where the compressions were at...I'll try again tho, with throttle open..