'03 RM 250 milled cylinder

Hi folks,


I just picked up an '03 RM 250 in great condition. The previous owner had used it until it needed a re-sleeve, and then put in a 275cc big-bore kit. After some time, he handed it down to his kid and had the engine rebuilt with a stock sleeve and piston kit. No big deal here. 


During the rebuilt process - performed by a reputable local shop (CEF in Santa Clarita,CA if you are familiar) - milled the cylinder (not the head) down to increase compression. This, of course, necessitates the use of race fuel to prevent pinging. 


Now I'm all for a few extra ponies from the engine, but my concern is two-fold: First, this thing is a bitch to kick start. I've owned a KX 500 that wasn't as difficult to kick, and this thing is close to being as bad as my WR 450. This makes starting this thing no fun, but also scares me with longevity concerns about the internals. 


Second, I don't want to run race gas (at $80/5 gallons) it is a bit expensive for the play riding we do out in Mojave. I can do this for now, if the thing holds up, but ultimately want to bring this engine back to it's original design/powerband.


So, my questions are: Does anyone have any familiarity with this method of increasing compression and is it safe? And also, short of buying another cylinder is there any way to reduce the compression that won't screw up the port timing of the motor?


Thanks in advance!



more compression equals more pressure on the piston down to the bottom, so yes it is more stressful on her.


might be possible to run a thicker base gasket if they are available.


I really think a new jug is the best option though, from there reset the jetting and adjust to your area.

Stack some base gaskets together. Or get the head opened up by a shop.

I have used the double base more than once.  To be reliable, you need a shim (shim + gaskets -20% gasket squish = thickness desired) for all the surfaces to seal against (this works).  Two paper gaskets stacked on top of each other will blow and then suck air.  Not knowing how much was milled is the problem, so you don't really know how much to raise it back.  You need to tear it down and put the pencil and paper to so you can get close to figuring out what is the best and practical way to fix the problem.  The 03 in stock form was on the border of using race fuel or mixing pump and race together.

Edited by Padgett

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