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CRF450 Carb surging after fuel crew adjustment Need Help

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Hey Guys,

Im working on my buddies 04 CRF450. We wanted to adjust the fuel screw to help with back firing under deceleration. It seems to be hanging on the revs. Below id what iv done.

1. Back the fuel screw out. We read on TT and other good sources to adjust the fuel screw to idle up the bike and tune the fuel screw to the best running sound. We idled up the bike but we had trouble adjusting the screw so we ordered an adjustable one.

2. Before we took the OEM fuel screw out i tightened it all the way to count how many turns out it was it. It was at 2 full turns out.

3. I replaced it with a new MSR fuel screw 2 turns out. I put a dab of grease around the second o-ring to be sure it was sealed properly. The bike started hanging up on the revs when i started it.

4. Took the carb out of the bike. Cleaned it very thoroughly and checked the throttle cables to make sure they were not hanging up.

5. Checked the boot on the cylinder head to carb for cracks (none and made sure the boot was on correctly.

6. Reassembled everything and triple checked to make sure everything. Still hanging on the revs a little.

7. Sprayed carb cleaner around the the carb for leaks and nothing.

So thats where i am. Bike ran good before we adjust the fuel screw and idle. We ordered a JD Jet kit for the bike. Im pretty sure it wont fix the problem but the bike needs to be properly jetted anyways. Any first hand knowledge is greatly appreciated. My gut is saying we have the idle and fuel screw out of whack or an i missing some thing.

Thanks Guys,

Shawn

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So thats where i am. Bike ran good before we adjust the fuel screw and idle. We ordered a JD Jet kit for the bike. Im pretty sure it wont fix the problem but the bike needs to be properly jetted anyways. Any first hand knowledge is greatly appreciated. My gut is saying we have the idle and fuel screw out of whack or an i missing some thing.

Thanks Guys,

Shawn

Here is a good article that helped me quite a bit with setting the fuel screw.

http://www.thumperfaq.com/fuel_screw.htm

If you keep making adjustments with both the idle and fuel screw together, you can feel like you are chasing your tail! :busted: Start over, and get the idle set properly. Might even help to put the stock fuel screw back in it it's original 2 turn setting to do this. Then try the after market fuel screw again. The bike should begin to sputter and then stall when the fuel screw is turned all the way or almost all the way, then count your turns out from there.

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You could adjust the fuel screw to perfection for the exact conditions that you have on that day. Then, the temp or humidity or elevation could change, and it would be off a little.

If you take a 45 pilot, and turn the fuel screw out 2 turns, it will be so close for 95% of your riding conditions that you could rule it out as the driveability problem.

Any chance that the fuel screw o-ring from the OEM part is stuck up in there, and you have two of them wedged in there? It has happened plenty of times.

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Its possible you have your idle set too high. You cant set the idle until the engine is completely warmed up. Mine did the same thing had a hanging idle and surging. I turned the idle down and the problem went away.

http://www.thumpertalk.com/forum/showthread.php?t=1014420

Read William1's 4th post and follow those directions exactly. Will most likely fix your problem. Especially since you said it was running well before you messed with it.

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Thanks mike. I feel like im chasing my tail. I actually used that link to set the fuel screw.

Leardriver, I thought the same thing. i checked that when i tore the carb down.

I just went an did some more adjusting. I feel like i got it better but not like to was. I have a feeling like i have to keep adjusting till its right and no one can tell me how to "do" it via forum. Ill report back. I feel like i needed to take a step back for a sec.

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Ok so i went back and started from scratch. I think i understand how to "tune" it much better with reading all of the links and doing it. The bike dosent surge anymore or hangs the idle. The bikes dosent run perfect but i dont really remember if it did before. It has a full Ti FMF and Supermoto (K&N) filter with stock jets. So my guess is iv got it pretty good and im being picky. He ordered a JD jet kit for it so i think she should run top. Thanks guys for all of your help. I think we adjusted the idle and fuel so much back and fourth that it got way out of whack.

Thanks again!

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It has a full Ti FMF and Supermoto (K&N) filter with stock jets.

Rejetting for aftermarket full exhaust and KandN filter should definately help. Glad to hear you got it running better! :busted:

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Thanks again for your help.

No problem! :busted:

It's nice to be the helper, instead of the helpee sometimes! :busted:

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You have to keep in mind, the fuel screw and pilot jet are for idle only. You set it up for a perfect hot idle and then leave it alone.

If you have running issues above idle, then it is either the needle, float height or AP. Can also be a choke issue, a hot start problem or other vacuum leak. You need to clearly identify exactly what the issue is to aid in the diagnosis.

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