Jump to content

1994 yz250 compression

Recommended Posts

I've been workin on the bike trying to figure out why it wont run right. Finally took it to a guy to have him look at it. He said he couldn't get it to run so he checked the compression on it and said it was around 40. He said i need a whole new cylinder. Could it be piston ring or maybe cylinder head? Just wanting second opinions before putting this much money down.

Share this post


Link to post
Share on other sites

Could be the wrong piston in it. 40 is awfully low. I once had a CR250 in my shop with a flat head piston and that gave it a 80 psi compression, to low to start. With a domed piston it rose to 210PSI which is is more like normal for a 250 2 stroke.

A new cylinder you need once the plating is gone. Other wise you need a new piston, most likely D sized with new rings.   

  • Like 1

Share this post


Link to post
Share on other sites
12 hours ago, Hudson Charlton said:

What's the history of it?

Not entirely sure, got it early summer for $250. The guy i got it from got it from some people that didnt take good care of it. And the guy i got it from didnt ever ride it. Its been sittin for 10 years maybe. The mechanic did say the carb needs all new parts, like the floats sticks most of the time.

Share this post


Link to post
Share on other sites
13 hours ago, arnego2 said:

Could be the wrong piston in it. 40 is awfully low. I once had a CR250 in my shop with a flat head piston and that gave it a 80 psi compression, to low to start. With a domed piston it rose to 210PSI which is is more like normal for a 250 2 stroke.

A new cylinder you need once the plating is gone. Other wise you need a new piston, most likely D sized with new rings.   

The bike would run and idle good, but when it got to 3/4 throttle it was all mushy and wouldnt rev higher. Although when i turn the petcock closed and some of the gas got out of the carb it would open up like there wasnt any problem.

Share this post


Link to post
Share on other sites

Starting and running a 2 different issues.

Once running it could still idele well and run "strong" but with a higher or near perfect compression you'd be surprised how much power your bike develops. 

Share this post


Link to post
Share on other sites
7 minutes ago, Hudson Charlton said:

You plan on keeping it? If so I would do everything: crank, jug, piston maybe the head. If you skip something than you risk it blowing right up again.

I would like to keep it, i can probubly spend less rebuilding it than trying to find something decent anyplace else. Do you know a about price to replace all of it?

Share this post


Link to post
Share on other sites
I would like to keep it, i can probubly spend less rebuilding it than trying to find something decent anyplace else. Do you know a about price to replace all of it?

350 for a crank 200 for a piston probably 300 for a jug. Close to 1000 by the time your all said and done.

Share this post


Link to post
Share on other sites

New crank? Why not rebuilding the crank, new roller bearings and a needle bearing (OEM $20) is all what is needed. A hotrod crank kit, a piston oem is around 100, replate the cylinder @ powersealsusa for 175 plus shipping (last time I looked)  

  • Like 1

Share this post


Link to post
Share on other sites
New crank? Why not rebuilding the crank, new roller bearings and a needle bearing (OEM $20) is all what is needed. A hotrod crank kit, a piston oem is around 100, replate the cylinder @ powersealsusa for 175 plus shipping (last time I looked)  

20 bucks plus the installation which is the big cost. I've heard it works out to be approximately the same. Replateing the cylinder might be worth it.

Share this post


Link to post
Share on other sites
6 minutes ago, Hudson Charlton said:


20 bucks plus the installation which is the big cost. I've heard it works out to be approximately the same. Replateing the cylinder might be worth it.

Most of the stuff you can do yourself, putting the needle bearing in I haven't got a clue how much a machine shop would charge 

Share this post


Link to post
Share on other sites
8 hours ago, arnego2 said:

Most of the stuff you can do yourself, putting the needle bearing in I haven't got a clue how much a machine shop would charge 

Usually a lot and you have to send it away because most machine shops will not even attempt to play around with such a no tolerance crank.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

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

Reply with:


×