Hi, Can anyone tell me the acceptable compression readings for a xs850G (UK
model, 1984). I'm getting - Cyl 1,- 108 psi, Cyl 2,- 100, Cyl 3, 100. Does
anyone know what they SHOULD be ?
I wouldn't open up an engine and re-ring it if it had 135 psi compression in
all cylinders. You can get some idea of how much compression an engine should
produce by multiplying the claimed compression ratio by 14.7 (if you live
anywhere near sea level). So a 9:1 engine should crank 14.7 X 9 = 132.3 PSI...
I seem to recall that somebody else here mentioned that there were some Yamaha
triples that only had about 8:1 compression though. That would only make 118
psi cranking pressure...
And, I would be sure I was doing the compression test correctly before I ever
started wrenching on the engine...
Have you squirted a few drops of oil into each cylinder to see if the oil helps
the compression reading? Don't over do it, just a few drops and turn the engine
over several times to distribute the oil and then attach your tester and crank
the warmed-up engine over with thr throttle wide open. That way, the engine
gets enough air to compress.
Some CV carbs won't allow enough air to pass the vacuum slides, so it helps to
block the slides up, too.
You may have noticed that the hose on the end of the compression tester has a
little tire valve in the end. The combustion chamber and the hose may have
approximately the same internal volume, so measuring just one compression
stroke won't raise the pressure in the gauge to the manual's specifications.
The compression should rise in equal steps over about
four or five compression strokes. If the pressure doesn't come up with oil
added to the cylinders, it's probably a valve seating problem...
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