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Discussion Starter · #1 ·
Hey guys, I am in the process of putting an 39mm FCR style carb (38.79mm to be exact) originally for an 08 KX250F on my 400 and would like your guys input.
As of right now my jetting is as follows:
Pilot: 42 (down from 48)
Main: 150 (up from 138)
Starter: I will have to look again
Leak: 55 (I will have to look again, I do not remember)
Needle: CR 38, 3rd from bottom
Elevation: 420ft
Mods: Stage 1 Hotcams, FMF Titanium Powerbomb header+Q4 silencer, stock airbox with lid, stock bore, stock compression, stock stroke.
What is weird is it has a hanging idle and when it goes down, it just about stalls out and idles rough and our eyes were burning after it idles for about 30 seconds. And it bogs every time I give it a little blip of throttle, but going down all those sizes seemed to help it idle a little, it threw black smoke and backfired like crazy when it had the 48 pilot. I made sure the throttle cable wasnt binding or pinching, it wasnt no matter where I turned the bars. Any feedback is appreciated
 

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Discussion Starter · #3 ·
Update, I brought up the pilot 2 sizes from stock as well as putting the clip on the needle on the last slot and it comes down quicker now!! Just ordered a RD flex screw for the idle fuel mixture to make things easier but it still has a pretty bad bog when I hit the throttle hard from idle. I am also tempted to buy a Raptor 660 petcock and maybe it will help resolve it
 

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Discussion Starter · #5 ·
Also made an extension for the back of the carb to reach the air box boot. Gonna sleeve lock that into place, shouod be getting a genuine Raptor 660 petcock in the mail as well
 

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Search for timing the pump shot/squirt on a 39. Many people wire things together so it pumps right away. Have you done this?
 

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Discussion Starter · #7 ·
Search for timing the pump shot/squirt on a 39. Many people wire things together so it pumps right away. Have you done this?
Yup I have! Got the idle air sorted out(kinda) it comes down much quicker now but any more than 1/3 throttle its PIG rich. I might have to raise the clip on the needle a couple slots, its on the second from last one now. And I mean its PIG rich from 1/3 throttle onward and bogs and or completley stalls itself out
 

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Discussion Starter · #9 ·
Update: I got sick of dinking with this stupid thing and put my stock BSR back on it. Turned out the throttle shaft had a ton of side to side and up and down play. I didnt have to funds to rebuild it so this is where we are at. Maybe a YFZ FCr39 for the future🤷🏻‍♂️
 

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So who made the FCR style carb you were working on?
 
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