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Hey guys I've got a 2009 CRF450X with a RS-2 Yoshimura full system on it. I've had it for 2 and a half years now and its great, but since I put the Yoshimura on 8 months ago, it has not stopped backfiring on deceleration. I've played with the fuel screw both ways, and when I finally pulled my plug the last time I had my bike apart I saw white, so I turned the fuel screw out to give it more fuel. Didn't work. Actually seemed to make it worse. I'm beginning to suspect that I'm barking up the wrong tree, that there might be something else going on besides just jetting. The guy who owned it before me put a JD Jetting kit in there with a 165 Main jet with the red needle in the 4th position from the top. 

Maybe I'm overlooking something or maybe it something deeper. Help me Thumper Talk! 

-Kennedy

p.s. I just had my valves adjusted about a month and a half ago

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Normal, you have a pipe with next to no muffling capabilities. I bet if you put the stock muffler on, the backfiring will stop.

Fuel screw is for fine adjusting the idle, nothing more.

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1 minute ago, kgish475 said:

Alright so is this a problem then? Will it lead to bigger problems later down the road? If it is normal then what should the fuel screw be tune to?

You adjust the fuel screw for the highest rpm you can achieve with the fuel screw. It is very subtle.

Get the motor hot, lower the idle with the idle screw, then raise it back up with the fuel screw, if possible. Again, it's very subtle.

 

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1 minute ago, Krannie McKranface said:

You adjust the fuel screw for the highest rpm you can achieve with the fuel screw. It is very subtle.

Get the motor hot, lower the idle with the idle screw, then raise it back up with the fuel screw, if possible. Again, it's very subtle.

 

So by raise it up do you mean turn it out? Run it rich? And this is obviously while the bike is on and running, correct?

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2 hours ago, Krannie McKranface said:

You adjust the fuel screw for the highest rpm you can achieve with the fuel screw. It is very subtle.

Get the motor hot, lower the idle with the idle screw, then raise it back up with the fuel screw, if possible. Again, it's very subtle.

 

Krannie I could kiss you! :wub: That did the trick! The idle seems very quite and low now but I guess that's because I've been running it high. I backed it off as you said and turned the fuel screw out 2 1/4 turns and boom! No more backfiring! You just saved me a severe headache! Thanks man!

-Kennedy

Edited by kgish475
misspelling

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31 minutes ago, kgish475 said:

Krannie I could kiss you! :wub: That did the trick! The idle seems very quite and low now but I guess that's because I've been running it high. I backed it off as you said and turned the fuel screw out 2 1/4 turns and boom! No more backfiring! You just saved me a severe headache! Thanks man!

-Kennedy

It's not magic, it's all in the service manual.... You should be adjusting your fuel screw everytime there is a subtantial change in air density.

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after tuning that fuel screw  repeat the process  of backing off the idle and playing with the screw again, when you feel its great turn the idle screw up a bit again because a low idle is hard on your motor, a bit of speed helps the oil pressure and takes some stress out of just running.

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If your plug is coming out white you may want to go up a size or two on the main jet.

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9 minutes ago, CDNSXV said:

If your plug is coming out white you may want to go up a size or two on the main jet.

This information is incorrect.

You cannot read the tip of a plug using pump gas, and learn anything, anymore.

You should not be reading the tip to begin with. You  read the bottom of the insulator.

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