After owning it for 5 months, I'm ready for more power from my 400ex. What I would like to do is get a UNI filter, a 10.8:1 piston, a XR400 cam, and, if possible, derestrict the stock exhause in some way. With this setup, would I still be able to run premium pump gas, or would I have to go to race gas? Also, what kind of gains could I expect, and at what area in the powerband would the gains be? Also, any information on derestricting the stock exhaust would be greatly appriciated.

 

Although it really depends on many things you should be able to run premium pump gas with that.
I really dont have any specifics on where in the power band you'll get the power, high compression pistons usually add alot of power down low (or so i've heard).
I guess for the exhaust you could probably just just the center part of the exhaust out or something like that but since i dont have a stock exhaust here it's hard to say for sure. I read on exriders.com general forum that somebody modified their exhaust so go do a search there.
If you are really tight on money and just want an exhast there are a ton of people around selling old crappy race pipes for next to nothing. Do a search on ebay.

 

You're on the right track...

If you want some extra power thru the mid and on top, I'd get the GT Thunder cam instead of the XR. It will still work with the stock rockers, but has quite a bit more high lift duration.

You should be able to run premium with this setup, but there are no guarantee's. I'd also look into an FCR 39 mm carb.

My bike is setup as a "sleeper". I run a stock, modified pipe, an XR cam (for woods racing), an 87mm lightened JE 10.8:1 piston, mild head porting and chamber polishing, an FCR 39 carb, and airbox mods.

With this setup the bike is SIGNIFICANTLY stronger than stock, but both looks and sounds stock (except it revs much faster).

I am considering changing to the GT Thunder cam, but haven't decided yet. I'd run it in a heartbeat if I wasn't worried about my bottom end for woods racing and wasn't so happy with the way it runs.

e-mail me if you want any more details

 

a k&n is a much better filter than a uni. i think if you are already buying a piston buy an 87mm piston (416). it doesn't take any boring or anything like that, just more power.

 

doesn't require boring? The stock piston has a bore of 85mm, and it's in there snug. if you don't bore for the 87mm, it won't fit in the bore. although, the 416cc engine i hear is faster then stock, but still reliable.

 

I think what bigred meant was that it doesnt require re-sleeving the motor.

 

Thanks to everyone for the advice. If I do this and start to get early detonation, could I run a 50/50 premium/race gas mix? Even though aquiring race gas isn't a problem (my dad drag races a '69 Hemi Coronet R/T) I have to buy my own gas, and I don't like the idea of spending 3 bucks a gallon on Hi-Octane gas. Also, I would like to keep the stock bore, and save the oversizes for when the cylinder is worn. Where can I get a stock size, 10.8:1 piston? What brand should I get? I already modified my silencer (see post "Internally Derestricting Exhaust" for more details). How is the FCR? Good Gain? Easy Installation? Sorry for asking so many questions, and thanks for everyones time.

 

I think the FCR39 is an excellent mod. It fits really well like it was made for the 400EX. I have heard some other carbs require some extra effort to get to fit not the FCR.

Adam Smith

99 400EX