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  #1  
Old 06-25-2007, 07:35 PM
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Default YZ250F backfire

My cousins YZ250F backfires when it gets warmed up and on deceleration. It isn't just 1 or 2 backfires, but it will do it almost constantly until you get back on the gas or shut it off. As we ride in tight twisty trails, you would think this is very annoying. Help!!!
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Old 06-25-2007, 08:20 PM
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Default Re: YZ250F backfire

Could be few things but I would start looking at some of the basics. Backfiring is a clear sign of a lean condition on the bottom. Did you do any pipe or intake mods lately? I would look into a larger pilot jet and see if that helps. Good luck

-Kevin-
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Old 06-25-2007, 11:16 PM
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Default Re: YZ250F backfire

That carburetor has a few rear facing air passages. If you have crud or air cleaner oil in them you can get that same symptom. Anything clogging the fuel passages as well. It is a pretty complicated carburetor so have a full manual or diagram handy when you start disassembly.

Did it get harder to start too?
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Old 06-27-2007, 09:02 AM
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Default Re: YZ250F backfire

pay $20 for an adjustable fuel screw and start there and adjust the idle up a little. Once it sounds good it should idle on it's own and you can blip the throttle and check the response from the stand to get close. then ride it and go from braking to accellerating a few times to be sure it's good.

Turn it in to make it leaner in hotter weather, out to make it richer for cooler weather. If that does not do it, I'd look at jetting which is more complicated and time consuming.

My experience has been that my 07 YZF450 varies noticeably if it's 65 or 90 degrees. A 1/4 turn of the fuel screw on the bottom of the carb makes a significant difference. Note the screw is recessed so the easiest way to get to it is losen the clamps on the carb and rotate it out so the bottom is facing you more.

zip ty racing [Only registered and activated users can see links. ] and many others have these fuel screws with knobs on the end. they are much more easy to adjust than the stock screw.

here is part of the info on the zip ty site

If you own one of the modern four-stokes with the Keihin FCR carb, then you know their off-idle acceleration is tempermental when ambient conditions like temperature, humidity, and altitude change. And, you know what a pain it is to adjust the stock mixture screw on the FCR carburetor. The solution is adjustment when conditions change, and the Zip-Ty Fuel Screw allows fast and easy adjustment. The new FMS01s are carved from 7075 T7 Aluminum for improved tensile strength and have a redesigned tip taper and seating shoulder to accomodate the newest (3rd generation) Keihins. They feature a knurled forend for easy grip, anodizing for longevity, and laser etching for adjustment accuracy. The Zip-Ty Racing screw allows you to fine tune your bike easily and quickly with just your fingers - trail or track side. Specific ZTR Screw available for both models of FCR carburetor.

good luck, joe

Last edited by joe346CBMX; 06-27-2007 at 09:10 AM.
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  #5  
Old 06-27-2007, 03:26 PM
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Default Re: YZ250F backfire

Quote:
Originally Posted by ossagp1 View Post
Did it get harder to start too?


Yes
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  #6  
Old 06-27-2007, 03:27 PM
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Default Re: YZ250F backfire

Ty for the help
I will tell him
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  #7  
Old 06-27-2007, 10:42 PM
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Default Re: YZ250F backfire

If it got harder to start I am betting on something being plugged. The easily adjusted needle is a nice item to have, for sure. Most of the time when a good running bike begins to run poorly as this one is it isnt a matter of an adjustment changing. I think this carburetor needs to come apart.
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Old 07-02-2007, 08:31 AM
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Default Re: YZ250F backfire

It would be a good idea to check the valves too.
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  #9  
Old 07-24-2007, 03:53 PM
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Default Re: YZ250F backfire

It turned out he got water in his gas
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  #10  
Old 07-25-2007, 11:19 AM
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Default Re: YZ250F backfire

I'd recommend Madman's decel popping advice. After my eng rebuild with added compression and performance, I had the same situation happen. I upped the pilot jet one size up, adjusted the fuel screw accordingly, switched to 100 octane gas and immediately noticed minimal popping thereafter.

Brian
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  #11  
Old 07-25-2007, 01:51 PM
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Default Re: YZ250F backfire

Hotstart not adjusted properly??
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  #12  
Old 07-25-2007, 01:56 PM
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Default Re: YZ250F backfire

Lol, he lives!!
easy to see lots of things can make something "pop".
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  #13  
Old 07-25-2007, 02:17 PM
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Default Re: YZ250F backfire

My CRF150F is doing to same. Should i try the same thing because it pops constantly on deceleration. Another thing is that it idles high when it gets warm. What should I try?
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Old 07-25-2007, 05:15 PM
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Default Re: YZ250F backfire

2smokeyz:

You should adjust your idle speed to idle smoothly at operating (warm) temperature. It won't idle very well when you first start it, but that's why you ride it a bit to warm it up
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  #15  
Old 07-26-2007, 11:51 AM
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Default Re: YZ250F backfire

Yeah, i forgot to say that it goes up and down when its hot. The idle will jump i will ride it around a little and then it will go down. It just goes up and down randomly.
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