compression specs..93 c40 (2cyl)

[sea-nile]

Hi Gang..I've squinted and gone cross eyed lookn thru the e manual for comp. specs...I've read 130 to 90 in other sources..i kno the 3 cyls are 130 ish....mines 90 & 88....wicks up quick...runs strong....

[99yam40]

from what I have heard ,as long as all cylinders are within 10% 0f each other you are good to go
If I remember correctly my 99 C40 was in the 120s
gauges can make a difference, not many are certified accurate

[boscoe99]

6,5/1 compression ratio for that model. Ruff rule of thumb is to multiply the ambient air pressure by 6,5 and then add one ambient air pressure. For instance, 6.5 times 14,7 = 95 plus 14,7 = 110.

This would be for a perfectly good working motor. Yamaha specs for some two strokes having a compression ratio of 6,5/1 is a minimum compression reading of 81 psi.

Then too there is the variance factor. 15% difference amongst the various cylinders is the value that I have used.

Known good compression gauge, motor hot, fully charged battery, throttle wide open, motor cranked until the compression value stops increasing.

If the motor is starting and idling well and making good power the compression pressure is really a moot point.

[sea-nile]

Thanks guys..Sorry it took so long to say thanks once again...U were jst the ones i wanted replies from...lmao...99 was urs the 2 cylinder??..yea...i was just wonderin...it runs to good to be wore out...bein an anal whaco...Id feel alot better if it was 90lbs...so spect wise its at the bottom of the curve.90...from 110/110....bt as good as it runs..i dnt care...lmao...still be nice to kno factory specs....and boscoe forces me to think once again.....i can deal with numbers...lmao...thanks guys

[99yam40]

No, my 99 C40 is a 3 cylinder

Boscoe why multiply by 6.5 and then add 1 more?
why not multiply by 7.5?

[boscoe99]

Multiplying the compression ratio times the ambient air pressure and then adding one ambient air pressure was just a rule of thumb I was given years ago. But that was for four strokes. Could be different for two strokes.

I believe that it is just to take into account some of the effect that the build up of heat due to compression has on the pressure.

I have always wanted to develop a test where I could compress a perfectly sealed cylinder and then let it cool back to ambient temperature to see if the measured pressure was exactly the compression ratio times the ambient air pressure.

[99yam40]

Maybe use a scuba diving tank.
Just pump it up to a certain pressure and then let it cool down to see what the pressure does.
one of those pony tanks ( emergency 5 minuet tanks)would be good for that maybe.

I did not understand that the 6.5 was the compression ratio on that motor,
different motors = different compression ratios.
I get it now

[sea-nile]

Now ur talkin...Sentences with #'s in um....I like it..lol