yamaha concern
 

looking at a 1992 yamaha 250 with 98 psi across the board. is this number a little low? thanks

I have had a few and they had that number with out issue. Yamaha's love to get carboned up!! That's why they all say to use carbon free when you fill up.
I used a Sea foam treatment on one of mine, and saw the compression go up 10 PSI across the board. It would not worry me if the rest of the motor checks out.
Good Luck

thank you Mj !!!!!!!!

also understand that a compression test is to be performed on a warm engine at WOT

Sorry Smoke, I have to disagree. How can you do a compression test at WOT? Warm engine yes, ... I totally agree, but I don't recommend trying to do a test on an engine running around 5000 rpm.

First, warm the engine up, You'll also need to disable your ignition system. You're going to need to crank the starter to turn the engine over but you don't want it to actually start. Then unscrew a plug and screw your adapter into the vacant hole, then screw in the hose leading to your gage into the adaptor. Turn the engine over a few revolutions... that's enough to get an accurate reading. Record the reading for that cylinder, then repeat on each of the other cylinders. They should all be within about 10 lbs of each other. That's all there is to it.

i think he means with the throttle in the wide open position, not actually running. that would be a little tricky..

Agreed, and that would make more sense.

check your gauge, I've got three different gauges that read 30 psi difference between them, I've a got a selling gauge:zip:, buying gauge:sly:, and one that reads correctly. Normally if they all read close to each other, thats all you need. I did an engine check out for a customer buying a boat, compresion on both engines came well withing spec around 125 psi. He called me after taking delivery all pissed off because his mechanic said both engines were worn out, he said his gauge read 80 psi. I told him to get another gauge and another mechanic. He called back two days later and apologzed to me, he took it to anotehr mechanic and it checked out OK

Whats wrong with thumbin the hole?? My right thumb is accurate to about 3psi. My left thumb however, is off about 6psi:beer:

it makes your thumb pretty hot if the engine is up to temp, leaves a funny burnt ring on your thumb(don't ask how)