how to build the fastest 250cc MX motor?

[ZSNOW]

in the class that im looking to enter for next year i need to be 250cc's and under. can i build a 250cc 250r motor that is fast enough to keep up with 400ex's and LTZ's that will be in the class?

[trx250rider]

that doesnt make sense to me but im also not a racer...if its a 250cc or less class,then why are you worried about 400ex's and ltz...last i knew they are over 250cc

[leager-n-ky]

he can run a 250 two stroke, and should be a 440 four stroke

[All250R]

Originally posted by trx250rider
that doesnt make sense to me but im also not a racer...if its a 250cc or less class,then why are you worried about 400ex's and ltz...last i knew they are over 250cc
Because 4strokes are power handicapped.

[2-330s]

what are the class limits? 400 4-stroke 265 2-stroke? or 400 4-stroke 250 2-stroke? does it state stock bore or stroke,or is it a displacement class?

[250Renvy]

You should be able to spank 400ex's and Z400's especially if they are also limited to stock bore.

400ex's especially start to fade due to the air cooling.

[2-330s]

i agree but if they are just running quads together. watch out for any sparks 400ex's i have ran against his 416-480 motors and they are fast!!! i run the open class flat track and curtis can put some big HP down i own one of his 330r motors i know! it's amazing what can be done with any of these motors. it's just a matter of how much money you want to spend and if you can get the motor builder to understand how you like to ride. you might have to send your motor back 2 or 3 times at your expense to get it the way you want it. but it's all worth it when you are the one everyone wants to beat!!!

[ZSNOW]

yes sorry i wasn't more specific. this is a new class this year called sport 15+ A. it is a displacement class 250cc 2 stroke 400cc 4 stroke. i had a 265 motor in my 250r but im looking at racing it in the 250cc class so i was wondering how hard it would be to build a motor that would whip 400's all day long..

[C-LEIGH RACING]

Only way you'll get a 250cc engine to pull like your wanting, is with real high compression.
You will need a complete rebuild to get the bottom end tight. New OEM crank & 8 ball main bearings & seals.
With a high compression build, you dont want to take a chance on a used crank failing.
If you dont have a cool head, go ahead & get one of those so you can get the proper dome size. A dome somewhere around 18 to 16cc's should do the trick. You'll probably as well need to have larger head studs installed & the cylinder tapped to the right size. A step stud size 8mm to 10mm should do the job with the 10mm end tapped into the cylinder casting.
RUN the head stay as well to keep from blowing the cylinder off the cases.

Heres the tricky part, RACE GAS, you'll need to be sure without a dought its at least 116 octain or else you'll melt the engine down.
Dont take, my buddys down the street word for it. You read it on the can or bottle that the gas is 116 for sure.

Carb jets, big jets because with the compression you can burn some fuel.
This wont be a high rpm engine, keep them low & learn how to shift gears at the right point.
Get a CT midrange pipe, it is the best bottom end pipe you can buy other than a custom made one.
Neil

[2-330s]

what kind of racing is it? can you afford a a profesional build?

[ZSNOW]

yes i can afford just about anything. and its motocross.


i don't think i would want a low end motor. for the tracks we race it wouldn't be "feasable"