Compression check on the 03' yz250

DRTYMEX · Sep 28, 2005

Ordered a new service manual for the yz and should be in around next week. WEEE! But in the mean while, was doing a compression check and bad news, it's low (90psi). :(

Just out of curiosity, what is the spec psi as per the yamaha manual?

Looks like several things I need to look for. Is there a history of cdi problems on the yamaha's?? Or flywheel magneto? (forgive me, I don't have my manual yet)

Bike is farting while kick starting and sounds like it's the ignition. Started with cleaning the carbs out, and that seems to be all right.

Buying a used bike sucks butt, especially without the service manual. Oh well, any topend rebuild kits you recommend for the yz 250?

uts · Sep 29, 2005

Don' know the specs but usually with a 250 you would want about 180 psi.

Rcannon · Sep 29, 2005

Altitude makes quite a difference with compression...Here in Utah, 180 is a high number. 90 is not going to cut it in any location. Rebuild the top end before you worry much about the farting.

DRTYMEX · Sep 29, 2005

Yeah, figured the psi was really low.

Rebuild the top end before you worry much about the farting.

A big 10-4 on that. Thanks for the replys.

bedell99 · Sep 29, 2005

At sea level at least 180psi, wow that is low. remember when doing a compression check kickstart the engine at least 5 to 10 times quickly at try to leave the piston on the compression stroke and then read. If you do it right you will see how much leakage is coming from your rings. If it is at top dead center.

Erik

Rcannon · Sep 29, 2005

Here is the funny thing. In the time it took to post the answers we could have had the rings changed out on your bike.

This is about the time your going to start giving yourself big bear hugs for buying a two stroke.

By your third or 4 the top end, you can complete them in slightly over 1 hour (assuming all the parts are in the garage). The process is not worth worrying about.

I did my last cr 85 in way under an hour. Thje little bikes give you a lot of practice. My Yz 250 woudl take under 2 hours.

yzrulz · Oct 3, 2005

After my last rebuild on my 2002 YZ250 I ran a compression test and it was up around 240 psi.

chalie · Oct 3, 2005

After about 6 kicks on my 2001 kx250 i got a max of 220psig
I have more than 55hr on topend too, but all trails, so she's not been beaten up too badly. Hope this helps. :cool:

jimyz · Oct 4, 2005

Just completed my top end on an 05yz250 with 58 hrs. Not to hard, and compression is 240psi.

Compression check on the 03' yz250

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