compression testing question

I've been trying all day to do a compression test on my son's yz250. I'm holding the throttle wide open, kill switch on. I can kick until my leg falls off but I'm not building pressure at the gage. The needle moves but it won't build pressure. I've checked my gage connections twice and everything seems tight. The motor has about 5 hours since a complete top and bottom rebuild and runs strong. I'm just wanting some baseline numbers so I can keep track of things. I plan on doing a leak down as soon as I can assemble the parts. I've searched the threads and I think I'm doing the procedure right. Any ideas would be appreciated

Has the gauge a check valve....

Double check the gauge and the pressure release button on the gauge. If the little button is sticking the needle will go back to 0 after each kick.

Even a piston with no rings will give you a reading if the gauge is working properly

I wondered if the pressure release valve could be the problem. I don't think it's sticking but it could be leaking past the opening. I'll plug the opening and see if that fixes it. Thanks

Ok here's the next link in the puzzle. Finally got the tester to build and hold pressure. Now after 20+ kicks it tops out at 175 psi. From all the threads I should be getting 200-240 on a motor with 5 hours on a new top and bottom. Should I worry about this number, or let this be my baseline for future reference with this gage? The motor runs strong no pings or knocks. I'm confident the motor is good I'm just a geek for quantifying things. The bike is an 01 with an "A" piston and rings.

200 would be great, 240....HUGE.

!75 is no worries there

I had 180 when i bought my YZ and the cylinder was toast.

It now has 230 with a new cylinder and modded head for lower comp.

Maybe take a peak up in the exhaust port.

Yeah, I have 230 with a freshly replated cylinder and new piston.

Of course, if you were having trouble with your gage, I would think it is suspect. Find a friend with a good gage and see how they compare. Or rent an oreillys gage, but I have had trouble with them in the past.

Unless the pressure gauge is calibrated, no two gauges will read the same. I have three gauges in my toolbox. None are calibrated and all three read differently.

Gauge #1 reads about 200lbs.

Gauge #2 reads about 215lbs.

Gauge #3 reads about 190lbs.

Now, all three gauges are within 10% of the middle reading. Which one is correct? I have no idea. I just know that I always use gauge #1 on my bikes. #2 and 3 are used when I go to the wrecking yard to check other engines.

I would say that since you had trouble with the gauge already, then it may be suspect. Only way to know for sure it to have another gauge to compare your readings with.

But, if you have only the one gauge, note your pressure and go from there.

First issue is your tester needs a schrader valve replaced.

Second, don't get all hung up on static compression numbers, they don't mean squat. If you know what your bike normally does they can be used to help judge condition, but a bike with a raised exhaust port will show less static compression and still crush the stock bike in HP. Anything over about 100 static will run.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now