[jfbetz]

I am considering pulling the trigger on a pair of 1999 Yamaha 250's. I've read a lot about compression and gathered that consistency throughout cylinders is most important. The motors are reported to have 110# throughout all 12 cylinders. I have seen other motors with higher compression (115-125). What is the difference between consistent 120# compression and 110# compression? Any performance issues? Is lower compression necessarily a sign of remaining engine life?
Thanks,

[mpsyamaha]

often times there is fluctuation between gauges so the same motor will read 20psi different with two different gauges. also it depends if the test is being done with the throttles open or not. and if you put a little oil in the cylinders it will come up. so the point is it may not be low on compression.

in my opinion, if all 6 cylinders are extremely close to the same, its good to go.


i dont like to take peoples word for it with compression. see it for yourself if you can.

[rodbolt]

on most 3.1L yamahas you will see a 5 PSI variation as you go down the bank. compare 1-2,3-4 and 5-6 all should be within 10 PSI bank to bnk.
I have seen a few replacement powerheads with 80 psi on one engine and 120 on the other with no performance issue.
2 strokes dont die like 4 strokes, usually your first indication its failing is when it does.

[jfbetz]

Thanks for your reply's mspyahama and rodbolt.