Limitations of Stock Carb

[JOEX]

Just wondering when the stock 400ex carb begins showing its weakness when building the rest of the motor, beyond the basic jetting.

How much can you build the motor before it becomes necessary and cost effective to look at carb changes? Starting from boring the carb to replacing it with an aftermarket carb and where do the larger carbs show thier strengths.

[Rico]

Cams and ported heads demand larger carbs more than any other mod..

[Doibugu2]

I've had two FCR's on mine. I have a 416 with WB track cam. The FCR definatly made a difference. However, I could never get it jetted right to take full advantage of the carb.

I don't ride enough to justify spending a weekend trying to get the carb working right. So I just went back to the stocker.

I was reading the specs on Brandon Summers quad that Laz just sold. His was a 450 stroker, and I didn't see anywhere that it had an FCR on it. He may have taken it off, but I was suprised to see that. I was going to ask him but never got around to it.

[khen]

At one point Laz said that he was running the 450R carb on Brandons EX. Not sure if that was just a temporary thing or not though.

[Colby@C&DRacing]

The FCR will make more peak HPand torgue than the stock carb even on a stock motor but I have used the stock carb on alot of bulit 416 motors without any problems, would the FCR run better? sure but the stock carb does a pretty good job for what it is.:)

[44oEX]

well you don'T have to change the carb at any point if you don't want to, I had the stock carb on my 500ex that is ported and has a bigger cam and all the good stuff. And it ran just find. The only thing is that you won't be taking full advantage of your mods. I have a 450R carb know and it made a big diffrence. But you don't have to. You can put a bigger carb on a stock motor if you want and you will see a power increase.

So basicly you don't have to, but you can at any point if you want and have the money.

[JOEX]

Thanks for the replies, they are very helpful:)

[cals400ex]

here are my opinions. i have run stock, fcr 39 and fcr40mm carbs. the throw on the fcr 39mm is the shortest, thus you get much quicker throttle response. the lever seems a tad easier to push than stock too. the fcr 40mm carb had better throttle response too and seemed easier to push also. i have run all of these carbs on my built motors along with my buddies. my bike when dragging is just as quick with the stock carb on, even with the ported head. the throttle response was quicker so on a track where i was getting on it then letting off a lot, the fcr's seem to help. i am betting on a dyno the fcr's will show a gain but in dragging if there was a gain, it was so small that i couldn't notice it. if does feel like the fcr's will take the bike to a higher rpm, but most of the time my motor doesnt' want to go much higher. if it was me, i would get the carb as a last mod. i honestly didn't feel like i had a gain from the ported head either. the cam and piston do help a ton. with that, a good pipe, and proper jetting a 400ex can run really well. my buddy noticed huge gains on his 406 from running a WB slip on with a modded header to a full HMF. it really shocked me. the carbs do make the bike feel like they run better because of the awesome throttle response but i dont' think the actual gains are HUGE.

[K_Fulk]

With my experience if you have a fully ported head and a full pipe the carb would be your biggest restriction. Bore size also plays a role.

Getting a bigger carb without those mods will still give you a good gain though. Except say if you put a 39mm on a stock motor with a full pipe. Kinda seemed like overkill, but it did rev quicker.