Compression test

[R.J Fish]

I'm looking at a Yamaha 200 hp 1989 and wantto do a compression test does anyone know how many pound of compression in good for this motor.Any info would be helpful .Seller says power head wasrebuilt 5 yrs ago .Thanks

[Bob H.]

Don't know specifically for that motor but it's probably between 110-120 new. If it's below 100 I'd stay away. I've run them down to 90 with no real problem but they tend to get finicky when compression gets that low. More importantly is that it shouldn't differ more than 15% from one cylinder to another, 10% if you're picky like me...it depends on your comfort margin.

[R.J Fish]

Thanks Bob for the info I'll be sure its in that 10% range .Thanks again !

[WildeOne]

Check compression with all plugs removed. Only takes a couple of seconds cranking per cylinder. Write down results. Difference between cylinders way more important than absolute values. Mfgs don't specify any values. Generally, as noted < 100 psig not too good.