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Popping/backfiring when letting off throttle

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I'm willing to bet this has been covered here somewhere and I've looked but can't get a good read on it. My boy's bike is an 05 YZ250F. After running terrible and figuring it down to a carb issue, we pulled it apart and did a thorough clean up. This seems to have been the culprit. Carb was filthy and after install the bike runs 100% better from idle to throttle response and smooth running. Problem is now when he let's off the throttle, the thing pops/backfires through the exhaust something terrible. I've also noticed that the header pipe gets VERY hot. after reading around here the general concensus is that this is pretty common. I just don't ever remembering it getting this hot(this was my bike from new until two seasons ago).

Anyway, I have seen something about a possible mid pipe gasket leak causing this popping. Just wan't to get all the bases covered, from what i gather around here it seems an air problem is causing this?

Any help? Thanks folks!

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Stock pipe? You boy races it or just trail rides?

Some popping is normal, though you are right about the typical culprt, an exhaust leak.

Hot header is typical, esp. if the bike is not moving.

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Both....ok we'll start at the pipe gasket then.

Might be ready for a muffler repack as well. It didn't make a huge difference for me but it helped a little. Re-sealing my header to slip-on with exhaust sealer did more to help.

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Yep, I figure we might as well go from the engine all the way back as far as new exhaust gaskets are concerned, repack as well. Five seasons on the bike, least I could do. Parts availability here may be my only challenge.

Running lean? Possible I suppose but the setup has not changed in five seasons and this is the first time I've had this issue this bad, Yes, the bike has had some popping before, but now, whoa. Anyway, aside from rejetting, what adjustments? Fine tuning?

Oh, one quick thing. I know it's off topic, my apologies, but if anyone knows......what is the nitrogen pressure charge in the rear shock? Can't seem to find that spec. anywhere.

Thanks again folks.

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Had a problem like this similar to yours after removing/reinstalling exhaust. It turned out to be an exhaust leak. New gaskets fixed it for me. Also, make sure that you are getting it off the stand, and running it under load during testing and fine tuning.

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On the nitrogen pressure - 150 psi.

Running lean - when you close the throttle, the pilot jet is pretty much taking over. Set this up by the correct procedure but don't be tempted to fatten up your idle circuit to cure a decel pop.

Like mikewrf says, ride it and see.....:smirk:

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I would check for an exhaust leak first, if there isn't one, I would try turning out the fuel screw 1/2 to 1 turn and riding it to see if that helped. If you make it too rich, you'll have a hard time hot starting it.

William has a good method for adjusting the fuel screw, but I couldn't get it dialed in just right... Maybe I'm just dumb and couldn't hear when it was just right...

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All great advice, thanks folks. Yes, after every adjustment or "check" we made after the carb reinstall, he got on it and rode up the street (thankful for dirt roads in that respect). We also did a thorough check of valves and cam movement, looks good. Quite a bit of options here, all good places to start, thanks again.

Thanks for the nitrogen pressure as well.

Anything else that may come to mind, please, shoot away!

P.S. Just to check off one more possibility. Spark plug? New plug before this season I could have sworn I always used a 9, but noticed the book calls for an 8 (actually the boy caught that). Would the different part number plug cause any of this?

Going to get some parts tomorrow.

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All great advice, thanks folks. Yes, after every adjustment or "check" we made after the carb reinstall, he got on it and rode up the street (thankful for dirt roads in that respect). We also did a thorough check of valves and cam movement, looks good. Quite a bit of options here, all good places to start, thanks again.

Thanks for the nitrogen pressure as well.

Anything else that may come to mind, please, shoot away!

P.S. Just to check off one more possibility. Spark plug? New plug before this season I could have sworn I always used a 9, but noticed the book calls for an 8 (actually the boy caught that). Would the different part number plug cause any of this?

Going to get some parts tomorrow.

I'm a newb, but popping after deccel is 100% normal. Tell your kid to get worried when it pops on acceleration.

:smirk:

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i have a 09 yz 250f and had to go up two main jet sizes and move clip on needle down one, and up one piliot jet. for same reason,decel popping. bike runs like fi...now

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Might be ready for a muffler repack as well.

DING DING DING....that would be my take too.

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Thanks for all the info folks. He's back in the dirt and the bike's runnin like a raped ape!

For those experiencing the same symptoms....We started by thoroughly cleaning the carb, replaced a bad fuel screw O-ring, replaced exhaust seals from the head back, and finally replaced the carb to intake rubber. A few small adjustments to get it dialed back in....back on the track runnin like new.

Thanks again!

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