My buddies 450 stumbles and sputters at 1/4-1/2 throttle not sure why, We have taken parts off of my bike and tried them on his and still no change Here is a list of what we have done

changed CDI,TPS,COIL,STATOR,
NEW INTAKE VALVES 2 WEEKS AGO and we did a leak down test. Als checked the head pipe for cracks its ok. I put his carb on my bike it ran fine we have checked all the wiring connections they look good. The bike runs ok when cold after doing 5 laps on the track it starts to act up and just gets to the point were you can't ride it safely. It runs fine wide open let off and try to accelerate and it stummbles mid range is awfull tried riding it at half throttle and it skips badly. Any ideas?
Thanks for any help

The only thing's we have not done is change out the fly wheel and put my carb on his bike

hers some more things to check:

fouled spark plug
dirty air filter
clogged crank case breather
kinked fuel line
crap clogging the petcock
bad gasoline

I put my tank on his bike same thing runs like crap, changed plug still runs like crap and the air filter is clean. I will check the crank case breather tonite.
Thanks for the help.

Pull the carb apart and clean it. Also check for plugged or kinked carb breather lines. I had my 250 run like crap and there turned out to be water in the float bowl. I would remove the cap from the bowl and drain it.

I pulled the carb off and cleaned it twice still runs like crap after running for a short time.
Thanks for the help

Are you sure you're not off 1 tooth on the timing?

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I checked it 3 times it is good thanks for the help

Okay 1 more. There are 4 plugs for the electrical system under the tank. Have you checked those?

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Yes, we are going to check the continuity of the wires now Thanks for the suggestions

I had one of those plugs come loose and it was missing and popping.

If you got a god carb on your bike, then put it on his and see if it behaves the same. SImple cleaning acarb won't necessarily correct malfunctions. COuld be the accellerater pump, weak inlet needle valve, float level, etc. If you stick a known good carb on it, you can possibly eliminate a lot of things at once.

We put my carb on his bike and it runs fine now. What could have wron out to make this run like crap? The carb has been cleaned out twice now

You may try replacing the needle seat, I think its called. The thing the main jet screws into, its rare but they can wear out. My brother has a wr250 that is a trouble shooters nightmare.

Is your buddies carb the same year as yours? Each year CRF carb is different due to jet sizes and needles. Try adjusting the fuel screw. If that doesn't help, try using a 40 pilot screw. I had the same problem and with the 40 size pilot jet and a few adjustments of the fuel screw, my bike no longer stumbles from 1/4 -3/8 throttle. If you figure it out, please post what needed to be done to help others.

Thanks for all the help we are ordering carb parts today i will let everyone know what happens again thanks for all the input

Ok case solved we replaced the the needle and nozzle and the floating side of the slide. BIKE RUNS GREAT I also found that the top cover was not sealing due the the locating tabs were to long so dirt was getting ucked into the carb. So I filed down the tabs and it seals mint.. Again Thanks For all the help....