FCR40 Carb Jetting

[firstturn]

I have a Keihin FCR40 carb that I plan to use in place of the stock carb on a 2003 KFX400. With the exception of the pipe (yosh ti full system) and ignition, the motor is stock. Has anyone installed one of these and if so, what jetting has worked the best?

[hang1p]

with airbox lid open around a 180 mj middle clip on slide and i think its a 45 pilot or 42 that at 1000 feet above sea level or below in maine

[Sprinterx]

This carb set-up would work best with performance camshafts and filter with the airbox lid off. That's more in line with my set-up (full pro circuit exhaust, ported head) and I use a 195 mj @ 3000 ft.
It works fine/good but where the big carb will shine is when you get the BB.

[rrguy]

Wow. I think 180 - 190 is a little rich.


Running pump gas at 1200 ft with a 435cc aftermarket cams motor, with full Yoshi TRS exhaust etc.. I run 42 pilot, 3rd clip, 160 main. Acording to the dyno thats right on for a/f mixture. I bump both those up for cold weather.

[Sprinterx]

Originally posted by rrguy
Wow. I think 180 - 190 is a little rich.

I agree, it sounds fat for sure, but don't know what to tell you. Even at a 195 the plug stills look'n a little lean.

[rrguy]

Well your altitude is probaly the main factor.


I am at about 1200ft. with a fcr 40 & 435cc / cams / full yoshi exhaust etc.. 45 pilot 160main. came out good on the dyno? Of course when it gets cold I bump that up.

[hang1p]

it's funny how it all works when i took my air box lid off with a stock motor i ran a 155 and that was lean on the tear down i could see it on the valve train ( white deposits on valve stems and surrounding areas) so when you add 4mm of area for the air to go thru .160 of a inch you would think that you would have to run a bigger jet to accumidate but like they say every machine is different but it is all relavant i run a 38mm carb on my 330r and i run a 182mj buy a rack off jets and decide for yourself start big and come down until you get it right jets cost $2.50 to $5.00 a piece but spot on performance is priceless my 2 cents, p/ out