Kero, on 20 June 2012 - 02:56 AM, said:

I already own it and I'm not dropping $500 into the bike.  I'll put the stock carb back on if I can't sort it out... again.

Paid $200 for mine, second hand off a 2001 WR 250f, delivered to Australia from Texas. Made an adapter from 50mm pvc drain pipe. Starts first kick, runs beautifully, snaps the front up in fourth and I run a 40 tooth tear sprocket. Better fuel economy too, with 42 pilot and 145 main. I LOVE THIS CARBY!

 Kero, on 20 June 2012 - 06:52 PM, said:

You should start a new thread with a complete set of specs for getting the FCR 37mm to work on an XR400.  
My friend put a FCR 39 on his and it was a real pain.

Try 2.5 out on the fuel screw.   1 out on the fuel screw is probably much harder to start than necessary also.

The stock Keihin is easy to sort out compared to the Mikuni TM36.

I am pretty sure I tried a 12.5 pilot.  Know I have tried a new 15, 17.5, and 20 pilots. Still popping on deceleration.  I am guessing either the float is low, causing a lean condition, there is a leaking o-ring, or Clogged passage.   There have been reports of bikes running great, then start popping.  Mikuni states some popping is normal.

Looking at reports of what works, the variables include:
Trail muffler vs open exhaust
P8 with original needle
P4 (leaner) with newer needle, (may require richer pilot per Mikuni)
Pilots from 12.5 to 25

Edited by Kev_XR, 24 June 2012 - 08:49 AM.

I think I fixed the problem after hours of trying and searching the internet.


The problem was the XR400 would start popping  like a NASCAR racer going into a corner.
Not a bop-bop-bop, but BAM! BAM! BAM!   It was annoying.
I tried a 15.0, 17.5. and 20 pilot.  Still lots of popping.f

Now the bike fires up, runs a little stuffy (mid range), Bat out of hell at full throttle, and almost no popping on decel!
I feel that I am now in the ballpark.

(Today's fix.  
Yes, multiple changes.)
New o-ring for the intake manifold (tried before)
Needle to 2nd from top notch  (recommended here) with 1 nylon washer.
15.0 pilot  (down from 20)
New fuel screw.

Broken fuel screw? -   I think the tip of the fuel screw was broken off.


A common problem I found on the internet was "the fuel screw has no effect".   I checked my fuel screw and it looks like the tapered end was much shorter than it should be.   I took it to a local motorcycle tuning shop and they thought it was snapped too.   I bought the best fuel screw available (wrong size).  

Should there be a pin hole opening from the fuel screw to the carb passage?
There is a small hole in the carb right above the fuel screw.   I shot "air in a can" through it and it was plugged.   I don't know if it should be plugged or not.  I guessed the fuel screw broke off and was blocking the opening  If the tip of the fuel screw is snapped off, since it is tapered, it should put out from the carb opening towards where the fuel screw threads.   I was not so lucky and had to drill it out, which is a last resort.   I made sure to not drill through, only until I connected with the narrow passage.   This may have ruined the fit of the tapered fuel screw, but I was ready to toss the carb at this point.

I put the new fuel screw into the carb and closed it all up.  The fuel screw is too narrow (rich), so I need to order the correct one.   Maybe do some repairs to the carb.  I'll see how it runs with another new fuel screw that is the correct size.

Edited by Kev_XR, 29 June 2012 - 02:50 PM.