Another question on Compression

[grumpyone]

Hello and thanks for the previous answers on compression.
One more if you dont mind. We have and r80 1986 the compression is running 96 and 100 so both are real close to my way of thinking but both are 40 psi low also?? Since they are so close to even would the problem most likely be in the valve train or the rings?? I also do not hear well anymore but dont hear any valve tick in this machine but it could be there I have not used a stet scope yet.
Thanks you all have been very helpful
Jim and Esther Spradlin

[Bobmws]

I would check and set your valve clearance first, then check compression again.
One way to differentiate between ring loss or valvetrain loss is to squirt a bit of oil in the cylinder before measuring compression. The oil will act as a temporary seal for the rings. If compression goes up, you can figure the rings are worn. If not, the problem may be elsewhere. Note, I've never tried this on an airhead, only auto motors. You will want to squirt the oil toward the top of the bore so it coats the entire cylinder to make the temporary seal.

[20774]

We've had some discussions recently on the proper technique for checking compression. The best approach is to temporarily removed the carbs from the intake stub. This does not restrict the inrushing air when creating the compression. Alternatively, remove the air intake tubes to each carb and prop open the carb slides.

Definitely be sure the valves are set properly and do the check on a warm engine. My thoughts about the slug of oil is that it might be inconclusive...even on a good engine, oil in the cylinder is going to help seal...what will you have learned? Best thing to do is to use a leakdown tester. This will definitely tell you where the problem is. I bought one at Harbor Freight for around $50. They go on sale once in a while. Seems like a decently constructed device.

[dsenn]

when using oil to check compression, squirt some in and roll the engine over a couple turns before taking the readings. Yes oil will increase compression on good rings but not very much, if the rings are worn you will get a dramatic jump in compression.

[AnnapolisAirhead]

Just curious, when you ran your compression test, how many times did the engine turn over to get those numbers? Also, did you have the throttle cracked open a bit or not at all?

I ask out of curiosity and I may be way off, but the first time I ran my test I wasn't happy that I'd run it properly so I re-ran it after these suggestions from a friend. On my older bike (1976 with 112k on it), it took 5-6 times to get 110-115. When I cracked the throttle open just a bit, it took 4 and yielded 120 and 119. This bike has the standard single plugged heads.

My newer bike (1983 with 60k on it) took just 3 turns and yielded 125 and 127. For whatever its worth, this bike is dual plugged and I used the lower plugs for the test (pulled both sides).

Both bikes have new batteries.