Do a search on keihin fcr 37.

That carb is on many 250f mx bikes. IIRC there was afew that had good results.

 

How about that stock TM33 on a 250? Is it better bored open to 34mm?

 

Feral, since the stock carb is 34mm, I'd say a TM bored to 34 should be well in the ballpark.

Rim, yeah I'd say 40mm was too big. My KLR650 had the CVK40.

Gass, the 250F model race bikes can probably use the 37mm, but I would guess the KLX is not that demanding for fuel delivery.

From a guy who hasn't swapped the CVK34, I tend to think the BB TM33/34 is probably the best fit to the KLX for an aftermarket carb. It seems the best plug-and-play version out there.

I've done some creative carb swapping over the years. Remember the original KLX250 in 1979 based on the old KL250? I had a Mikuni round slide carb on that bike, then tried the innovative EI carb. I've done lots of others over the years too. Most of it was fun...learned a lot...but didn't gain much on any of them. Now...all those swaps involved a slide/needle carb for a slide/needle carb, whereas our modern KLX has a CV carb. I can see how some improvement can be obtained, but for a bike like the KLX and it's intended use, the CV works great for me. I'm going to Colorado for a couple of weeks next month as I do every year. My KLX will work quite well with a CV carb at varying altitudes.

 

What about the cable bracket. I see the XT225 guys use a different bracket altogether. Will the stock Mikuni bracket work on our bikes or can it be modified to work?

 

The only real options for the klx is
mikuni tm33
Keihen FCR35
Mikuni tm36
And the Bill Blue modified tm33(tm34).

Most pumpers off of different bikes are way to big for the klx. And in my opinion the tm33/34 is only good up to 300cc. FCR35 and the tm 36 should be 300cc and up.

 

I think the 250 would run better if they put the smaller 32mm on it like the old 250r model had. I remember them having a little more low rpm response, even though they were CV's too.

Xr's only used put TM32's on the 250s and the response was 98% as good as the pumpers.

 

My 650 Triumph twin ran awesome on 1 30mm carb. I'm thinking we need to be careful not to make the hole too big or you won't get the air moving through fast enough to vaporize the gas at low speeds. I'm using mine to toe-creep around the woods anyway and I don't spend much time at the top end.

So I got me a stock TM33 and I have it all mounted up now. I turned a sleeve up in the lathe to extend the spigot .400 and I made a spacer ring for the airbox side out of some nylon round stock. The cable bracket was a real pain in the *** but I got it done. In retrospect, if you're thinknig about doing this, just buy the one from BB and be done with it. It was a fun tinkering experience but if I do this again, I'm buying one from BB. Today I ground clearance on the frame and bolted it together. I got it to fire up but it really didn't want to and I couldn't get an idle no way no how. I think it came with 127 main and 37.5 pilot. I thought that would be close because my CVK has 128 and a 40 pilot. I took it all back apart and turned the pilot screw all the way in then back out 2 and 1/2 turns. Now it'll idle and put around the yard but I go out on the street and it pops and coughs like mad. I think she needs more gas.

Anyone got a ballpark estimate on what I need for jets? It's stock 250, pipe, air cleaner, KDX snorkel. Also where does a person get one of those t-handle pilot screws?

 

Monkey, sounds like exactly what I'm doing right now too, fitting a generic TM33 to a mostly stock 250. I agree on everything you said, the cable bracket is real pain to modify and the money would be well spent to just go and buy the whole damn thing from BB.

My jetting is so far 130 / 35. New needle and slide, the needle is set at the middle position. Also using a KDX snorkel (for now).

I haven't had the time to work on the jetting but as it is now the bike runs fine.
It starts with the choke, idle is steady and it runs without sputtering or backfiring across the rev range.
I simply used my LM-2 A/F meter to set the fuel screw for ~13.5 A/F at hot idle. But that's all so far.
When I get to the real jetting I'll post updates.


But. Before putting time into finding the final jetting, I'd like to get some data about the stock vs. KDX snorkel issue.
I do not believe bigger is simple better, there's more to it.
Even the earlier KLX300 used the same "small" snorkel as the KLX250S does. Engineers at KHI are not idiots, they know what they are doing.

If the KDX snorkel is better, why did they choose the use a smaller one in the 300?
A 2-stroke has an intake pulse twice as often as a 4-stoke. The airbox intake would be tuned for 2x frequency on the 2-stroke, even if they shoot for maximum intake boost at same RPM.

Someone here on the board did very well controlled top speed tests with KDX vs. Stock snorkel.
The stock gave better top speed (=better peak HP) and better throttle response.

--
Mikko

 

I see there's a bunch of different styles of main jets you can get but none of the parts dealers say which one goes with which carb. Which ones do we need?

 

Large round.

But... I have huge selection of "small round" jets here at and they use same thread so I'm using those. The small round do work but you have to be more careful to make sure you get good seal between the jet and the collar under it.

But really, the correct Mikuni jet type for the TM33 is "large round".

--
Mikko