Up sizing a carb

[250xTrailRider]

Hmm kind of sounds like super sizing your friies.. Anyway I see that some people put a 400ex carb on thier 300ex machines and see an improvement in performance. Would a 250x engine respond in the same way even though it has a few less cc's? How about getting a 300ex carb and putting on the 250x? Would a performance gain be seen/felt? Or are the 250x/300ex carbs the same. Maybe updated internals but the same size in mm? I understand that a performance exhaust will help as well but lets just deal with the carb by itself.

[300ex-boy]

The 400EX carb would be best for the 250x. It wont do it any good unless you have an aftermarket exhaust. If you do, make sure you jet the carb properly for optimum performance.

[44oEX]

Originally posted by 300ex-boy
The 400EX carb would be best for the 250x. It wont do it any good unless you have an aftermarket exhaust.

oh yes it will, it's the best mod to a stock 300ex, I would think a 250x would be the same, there is only like 30ish cc diffrence

[250xTrailRider]

Do either one of you know where to get specks on the 400ex carb? And then maybe on the 300ex/250x carb? Throttle bore size, etc.

[44oEX]

honda.com maybe, what specs are you looking for just the size?

[250xTrailRider]

Yea it might be nice to know. I would assume that the 400 carb is larger at least the bore so then the outside diameter on the cylinder side would be larger as well. I would also need the carb boot in order to get it bolted up? Thanks for the .com I'll look there.

[44oEX]

the 400 carb is a 36mm and the 300 is 32mm I think. the outside of the 400 carb is bigger but you don't need the boot, juste take yours off and use a dremel, or a drill with a grinding rock and make the hole a little bigger for the carb to fit.

if you do it, for jetting start with stock 400ex size ( 148 ) and go from there on my buddys stock 300 it was prefect