FCR 39mm on 400ex

[kaptainbutler]

My friend recently got a FCR 39mm carb used off of a 400ex. It is going back on a 400ex. We started with a 170 main and have come all the way down to a 162. We even adjusted the air screw multiple times both in and out. From just over 1/4 throttle to 3/4 throttle it wants to sputter. Any suggestions? Or does anyone know what the default setting is on the air mixture screw?

[kaptainbutler]

Also I need a 39mm Carb myself so if anyone has one. Let me know!!!!!!!!!!!!!!!!!!!!

[drew416ex]

if its between 1/4-3/4 adjust the needle clip position.

[NacsMXer]

Originally posted by kaptainbutler
My friend recently got a FCR 39mm carb used off of a 400ex. It is going back on a 400ex. We started with a 170 main and have come all the way down to a 162. We even adjusted the air screw multiple times both in and out. From just over 1/4 throttle to 3/4 throttle it wants to sputter. Any suggestions? Or does anyone know what the default setting is on the air mixture screw?

Main jet will only affect 3/4 to full throttle. The range you are describing is affected by the needle. It sounds like it is rich on the needle, so I would suggest raising the clip on the needle one notch at a time until the sputter clears up. This will lean it out. Remove the two allen head bolts fastening the top cover to the carb (says "Flat CR"). Hold the throttle wide open to bring the slide to the top. Remove the brass colored allen head screw in the center of the slide, and pull out the needle with some thin pliers.

Also, if you are adjusting the screw on the bottom of the carb, it is a fuel screw, not an air screw. So you adjust it out for more fuel, and in for less. To get to the air screw on the FCR 39, you need to remove the two allen head bolts holding the intake bell to the carb and remove it. You will then see the air screw and main air jet. That's just a side note though, you shouldn't have to mess with those.

To adjust the fuel screw on the FCR it is preferable to have an adjustable aftermarket screw that sticks out the bottom of the carb. That way you can fine tune the pilot circuit with the motor running. Mine is set at just under 2 turns out, but every set up will be different. You can read about the correct way to set it up here: http://www.exriders.com/vbb/showthread.php?threadid=229923

[kaptainbutler]

Thanks fellas, Ill get back with you on the results.