I'm wondering the stock carburetor is a keihin cvk34 if you go to keihin's website they say a 37mm carburetor is the match for 250cc to 375cc so the 34mm venturi is actually too small and in efficient, cost effective suppliment. So if you had an actual fcr caburtor on the stock cylinder would you get better power band? is there a carburtor manufacturer that excells over any other and where is the best place to get a caburetor, bang for the buck, except e-bay?

 

Not true. A local race team used the OE CVK carb' to great success in the Australian Thumper Nationals. All 3 of there bikes were putting out 35 hp + with the CVK34 carb' and other "special sauce" mods.
It's not ideal for slow running, but that's not the point when you're racing.

 

What I experienced with the fcr is this. The stock cvk at 34mm seem to have a little more bottom end than the 35mm FCR. Now I'll admit that my fcr is not tuned in 100%, but based on what I know about bigger carbs is you will lose some bottom end with a bigger carb.

Imo I think that a 37mm would be to big unless you have some major engne work done and want to live in the upper rpm range.

Hands down the best carb in america is probably the FCR. But the mikuni is a cost effective option at almost half the cost of the FCR. Give Bill Blue a call if you want a 33mm(bored to 34mm) Mikuni or Jerry at four stroke works if you want an FCR.

You could also get Tremor38 to hook you up with the part number from his carb a then shoot Yoshimura a message. Even though it is only sold in Japan you can get one to anywhere.

 

Now the stock carburetor slide spring, do you think that its too long? I get a lull in the mid rpm range do you think its carburetor related or exhaust realted? I have a 2 bros exhaust
128 main (keihin)
neddle 2nd(n1tc)
used kdx snorkel

 

Try bumping the needle clip from the 2nd to the 3rd position.

 

What he said.

 

Yeah, but I think we're talking about different animals here. That sizing chart deals with the FCR 'MX', doesn't it? That tells me that it's optimzed for a MX bike engine which is designed to deliver more power on the top end, via lighter flywheel, larger valves, different cam profile, ignition timing, etc. They live in the upper RPM range; the KLX isn't very happy in that range for any extended period.

That's not to say you couldn't make the KLX run very well with somthing that size, but having a large bore carb isn't absolutely critical. I've seen people running with an FCR39 on this bike, but I don't think there's an efficiency/inefficiency issue with 33mm on up...when dealing with our DS engine in stock configuration.

Many of the people who I've seen runnig with the FCR39 have lightened their flywheel which would make the bike stall much easier if you wanted to use it off road in the tight technical stuff. The 39 is also a tight squeeze and it's hard to fit an air filter to it the confined space. Most I've seen with that carb have to run it with just a funnel and screen..YIKES!!

I've yet to see an FCR37 fitted to our macine. I've heard comments that the body is too long. Heck, the FCR35 barely clears the E-starter.

On the other hand, there are people on this forum who have fitted the Mikuni TM36 and reported very good results, once they get it properly jetted. Only a minor mod to the cable pulley to make it clear the bottom of the gas tank. It's fully adjustable, including the accel pump, so no worries about fitting that 'Taffie mod' collar to the accel pump rod, ala FCR35. From what I've heard this carb eliminates a 'flat spot' assocated with the TM33/34, which has more to do with the needle jet than venturi diameter.

Anyway, now that I've made everything clear as mud, good luck! I'm sure you'll settle on something that suits you and works just fine.

 

Yeah all these ideas agree with me except the 39 mm carb on a stock 250 engine its just to big and i would have to agree with the reasons. I wouldnt wanna go any smaller than 34 then your gonna be pulling too much gas from the main jet and just give you crap mpg perfect fuel air ratio is 14.7 parts air to 1 part gas plus if the carburetor is too big for the intake port then its wasted size to have a bigger carb on small intake port, IMO

 

I would agree since I am modding a FCR 39 for my ktm 640

 

I don't think that the FCR39 was on a stock engine.