1989 150 PROV compression

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Working on this and compression is 100 (warm) and even across the board. Runs great. I know PROV is lower compression but this seems on low side. I usually see 115-125 even 130 on 200s.
Is this normal? Motor has very low hours.

[walleye1]

I'd say it is good.

[walleye1]

I have two compression testers and they reed different.

[99yam40]

yea, I was going to say have you checked the accuracy of the gauge lately

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Yes, all plugs out but throttle not wide open just fast idle lever all the way up.

[99yam40]

2 strokes do not need the throttle to be opened to get a compression reading.
they will scavenge enough air thru the exhaust to get a reading.
may take a couple of extra revolutions to peak but it will work just fine.

try it both ways next time yourself you do a compression test.
I too was use to getting readings on 4 strokes and did not believe it would work, but it does

[99yam40]

I'm pull starting MANY machines (mostly high end chain saws) and want the MOST ACCURATE # possible.

Even with the throttle wide open, I'm pulling a starter cord at least 6 times (hard and fast), to get to the max #.

I'm NOT pulling anymore than I need for an accurate #.

I don't know why NOT opening the throttle is a big deal.

Just my 2cts.

Just saying try it once .
leave the throttle closed and pull a couple of times .
then open the throttle and pull the same amount of times to see what difference you get

[rodbolt17]

on a two stroke it simply does not matter.
two strokes do not gradually lose compression.
it is always sudden and catastrophic.
In 30 yrs I have only seen two two strokes simply wear out.
they were both running when I pulled them apart but all the moving parts were so worn it was not a viable project.
both were early 80"s 70 jonny/rudes.
both had seen commercial use for about 20 yrs.
as long as a two stroke compression is even across all cyl, move along nothing to see.

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I'm thinking if you crank it long enough it doesn't matter if you have throttle open or not. I tend to to do 4 or 5 revolutions on each cylinder. Probably wouldn't hurt to to a little longer but only would yield higher number on each. Being within 10% is the key.....with decently high numbers.

[99yam40]

I am guessing no one has any specs for compression ratios for this and other motors.
That Is what the OP was looking for

You would think the Service manual would have something, but not the PSI

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According to Yamaha brochure PROV has 5.6:1 ratio and the 200hp has 5.9:1. Just wondering how that translates to compression numbers.

Excel has 6.4:1 and my Excel has shown 130 on this same gauge.

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Compression ratio

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Just tested my 1986 90 and it was 120-125 with same gauge. It is listed as 5.9:1 in brochure.