I have been running a 400EX carb on a built 250X engine for the past couple of months. Last night I setup my 39mm FCR with the same settings and installed it. To my surprise, the motor didn't run at all. I'm looking for some info from those of you that went from a 400EX carb to a FCR, and some general rules of thumb that you observed when doing so. Looking at the design differances between the two carbs, I'm guessing that everything on the FCR would require a bigger/richer adjustment.
What are some jetting tips?

specs on the motor:
87' 250X motor
365cc bore and stroke
race cam
race porting
larger intake and exhaust valves
350X breather tube
no airbox lid
unlimited cdi

Thanks for any advice that can be given.

Chris

 

Hmm...

I might be inclined to help if I got an apology from the person (who will remain unnamed at this time) that was insinuating that my "bir air" pics are fabricated, a while back. I'm a bit above fabricating pics to impress the kids on the board....

Wouldn't run at all, eh? Sounds like things are really off. Is the FCR new or used?

If the FCR is larger than your current carb, it will likely take larger jetting as the vacuum and velocity will be lower within the carb itself.

What settings are you currently using? The jetting of the FCR typically follows that of the 400EX stock carb fairly closely on the main & needle - just go about one or two steps more rich. Add about 10 to the pilot - (i.e. if you were running a 42, try a 52 or a 50).

Gabe

 

Over the weekend, I had some time to play with the jetting. I purchased the carb new, and it came with a small array of jets. I am currently running a #50 on the pilot, screw 1 3/4 turns out, clip in the fourth position from the bottom on the needle, and a #170 main. The motor runs fairly well. But has a hickup off idle, and skips on the top end. The plug reading is extremely lean. That's where I'm at until I get some larger jets. My plan is to go up 1-2 sizes on the pilot, and buy main jets from #175-185. Once I get it away from running so lean, then I'll try to do the fine tuning.

I am very impressed with the FCR. It bolts directly onto a 250X/300EX engine with no interferance to the frame or gas tank. I am using a 350X breather tube and a Motion Pro throttle cable to complete the setup.

 

gabe im having problems getting my fcr lined back up after ive re done my motor it is sputtering or missing on the bottom end and a small amount in mid range ive tried to go smaller on pilot and i moved the arm on my accelerator pump im runing a 168 main but its still lean on top i think any suggestions would be helpfull thanks shawn ps also are you going to be at thayer this weekend im thinking of going just to see whats its like i wont be rideing though my bikes set up for mx shawn

 

The 400 is really sensitive to exhaust if you're running an aggressive cam. This results some jetting problems that can't be solved without derestricting the exhaust.

Just remember you're jetting for throttle position, not RPM. What throttle positions are your problems occurring at? If at WOT, you may be experiencing the problems I talked about above. Main jetting becomes non-linear with RPM when a wild cam is used with a restrictive exhaust.

I'm probably going to be at Thayer for pit support. My quad's torn down at the moment for repair/refreshing...

Hopefully I'll be ready for the Marshfield race on the 30th.

Gabe