Compression check...

[Tor-King]

I just did a compression test on the '88 I just acquired (bike was warm and throttle was cranked wide open).  The bike has 60 000km (about 40 000 miles).  It all appears to be in fairly good condition and runs well.  The compression readings are:

cylinder 1: 117
cylinder 2: 130
cylinder 3: 120
cylinder 4: 120

According to Clymers and other manuals the compression should be 128-142. They are all on the lower side but are fairly close across the board... the question is do I leave it and run it the way it is?  Are these typical readings for a bike with 60 k?

Thanks,
Dean

[aviationfred]

Those are similar numbers that I had when I purchased mine. I rode for three years (about 25k miles) before there was significant oil consumption. Leading me to do the cylinder/piston upgrade last year.



Fred

[FJmonkey]

The 128 and your low of 117 is only 91%, or 9 % low. I say ride it. Get more miles on it before you need to rebuild.

[weymouth399]

There's a few way to make cranking compression. I would check valve adj, what kind of oil, before I worried to much about those #s. If bike hasn't run much lately I would put non synthetic oil in it run for a 1k minor so get valves adj and do again. Or do a leak down test that will tell you what's wrong on motor. My bike pumps 150-160 with how my valves are adjusted.
HTH

Bob w

[Pat Conlon]

Bob, are those compression #'s with stock valve timing or have you changed your overlap?

[weymouth399]

Stk 86 1200 valves "not" set to Yamaha specs. Dyno oil not syn better ring seal for a stk motor. When the #s go down enough, I'll slot the intake sprocket. Or I'll build to the 1349 kit I got, xjr output shaft and wide wheel kit to it. 

Bob w

[CutterBill]

Compression tests are notoriously unreliable as a means of assessing the condition of a piston engine.  There are so many variables that can come into play that the numbers you get are almost useless.  That's why we don't do compression tests on aircraft engines.  Instead we do a leak-down test, which involves connecting a hose to the spark plug opening and injecting 80psi air (piston at TDC), then measuring the leakage against a calibrated orifice.  And even this test is highly suspect.

Now, if you do a compression test and one cylinder comes up 0, or 10... yeah, you have a problem.  But if the engine is running well, don't waste your time.

"It all appears to be in fairly good condition and runs well..."  And there ya go...
Bill 

[Tor-King]

Thanks for everyone who chimed in.  I think things are best left alone.  I was not planning on tearing into this one, I was just wondering if those numbers look "normal" for a bike with 60 000 km...

Dean