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08 crf250r need help dialing in engine and carb jetting

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Tried everything to get this bike dialed in but cant get it figured out. What I have is a 08 CRF250r it has a new cylinder, JE 13.5 piston, stage 2 Hot cam, ported head, FMF exhaust with mega bomb header and sent the carb to Tokyo Mods. for cleaning and jetting, valve job by CRFS Only with kibble white valves new seats and springs. The bike just wouldn't start so I took it to my local shop and had it checked out and they couldn't figure it out they checked my valve adjustment and all was good you could push start it and it would run awesome but don't shut it off or you will be push starting it. Took it to Tokyo Mods and had them check it out they said it was not the carb, it was my decompression arm needed to be replaced to much slop. I installed a new one and no change. I took it to Dan Crower Engineering and they checked the valves all was good. He took out the 45 pilot jet and installed a #40 and bam it starts cold at about 3 kicks and starts hot really well. I rode it and it run ok but if you were at about 1/8 to 1/4 throttle it will hesitate bad and then take off like a rabbit. Dan had the fuel screw at about 3/4 turns out I tried adjusting it with no real change. I put a #42 pilot in it and same story it will not kick start. I put the 40 back in and it starts but hesitates off idle, need some direction on what to do next, Why will it start with a #40 when I should be running a #45 pilot I am in San Diego at sea level. Need to get this figured out dumping way to much cash into it and getting no where thanks.. The carb also has a smaller leak jet than stock.

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Something is wrong, as you should get great results with this jetting:




45 using brass fuel screw at 1.5 turns

stock starter jet

stock air jet

NCVS needle (450r)

55 leak jet

2008 450r apump upgrade

tokyo mods apump linkage spring upgrade


Your symptoms (requiring a smaller pilot) to start means you have a rich condition.

That can come from a few places:

- leaking choke plunger

- clogged air jet

- worn out needle jet (what the main screws into): it can cause strange rich/lean conditions


I would check the  choke plunger to make sure it is seating, and check the starter jet to make sure it is the stock size.

I would also check to make sure your needle is a NCVS or leaner.

I would also check the air jet in the intake bell for debris (makeing it rich)


You have made no mention of a leak-down test on the head, which is SOP when you have a motor that is hard to start after a valve job.


Have you done a proper fuel screw adjustment?

You start with one turn out, then lower the idle with the idle screw as low as it will go with a hot motor

Then raise the idle with the FUEL SCREW to max idle. If it goes above normal idle speed, lower it with the IDLE SCREW, and repeat with adjusting the fuel screw for highest idle.

You goal is to get the fuel screw to reach you deisred idle speed.

Edited by TheKoolAidMadeMeSick

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I had a leak-down test performed early on and found bad exhaust valve seats, that's why I got the head gone through. I have not performed another leak down after the new top end and valve job. If I have time this weekend  I will tear back into it and go though the items you pointed out and hopefully its one of them, Thanks for the tips. Any idea what size the stock starter jet size is.

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I had the same problem with the 45 pilot. I am at about 1 3/4 turns out on the fuel screw now. I had to turn the idle up about 3/4 of a turn from where it was with the 42 pilot. It wasn't obvious either, but it worked. I typically ran the 42 pilot at something like 2 1/4 to 2 1/2 turns out.


The 45 pilot definitely has a better response off idle, but it is using overlap and loading the carb up some. 

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