friends bike backfiring then he decelerates?

what would be the cause of a 09 yz450f backfiring when he lets off the throttle? it is a very new bike with under 5 hours on it.

All yamaha's will do it some. If it is excessive, he has it too lean...

popping on decel is a lean condition. richen it up.

ok, would it be his air/fuel screw or his main jet?

thanks

+1 hes most likely lean lean... had the same problem with mine when I got it home, richened it up a little and no more popping.

I went up one step on the pilot and it cured my problem...

If nothing has been modded then leave the main jet, for now. First try a larger slow jet and/or richen up the air-fuel screw setting.

slow jet = pilot jet?

why dont they include pilot jets in jet kits? they only include the main jet and needle :thumbsup:

thanks guys.

Not quite sure what a "slow" jet is, my buddies mechanic just told me to go up one on the pilot. So thats what I did, and my bike doesnt backfire anymore. Go to your dealer and they should know what your talking about.

im pretty sure slow jet means pilot jet but just want it confirmed.

then theres the main jet.

then theres the needle as well as the clip setting.

Yes slow jet = pilot jet. Sorry, I'm a bit old-school.

ok and one more tiny question.

ive never fooled around with a pilot jet before, but ive done several main jets. the main jet has a number marked into it to tell you what size it is... how do I tell the size of my pilot jet? is there a number carved into it like the main jet?

thanks

Theres supposed to be lol, should be stamped the same way.

oh ok perfect.

that will be all then.

thanks for all the quick and accurate help guys, + gas to you all.

Backfiring? Really? All the way back into the air-filter?

Ooooh, you mean Misfiring or popping.

Yeah, that's normal on a YZ. Too much = lean mix.

1) Turn the fuel-screw down (in) a 1/4 to 1/2 turn (which is actually a air screw).

2) if that doesn't do it, go up one in the pilot.

3) last but not least, move the needle clip down one position.

Backfiring? Really? All the way back into the air-filter?

Ooooh, you mean Misfiring or popping.

Yeah, that's normal on a YZ. Too much = lean mix.

1) Turn the fuel-screw down (in) a 1/4 to 1/2 turn (which is actually a air screw).

2) if that doesn't do it, go up one in the pilot.

3) last but not least, move the needle clip down one position.

yes my bad, its popping.

he said he turned the fuel screw in 1/4 turn like I told him (like TT told me) and it fixed the problem within 5 minutes. no wonder everyone comes to me for help with their bikes!

thanks for the help, + gas for you too!

i got a 06 yz250f 50th ann... and when i give it a really quick shot of gas from idle rev it bogs out. and when i decelerate it also pops no mater how i turn the f/m screw. any help?

Not sure about the Yami's but sometimes if there is a poor seal in the exhaust system(i.e. where header meets motor) a bike will pop on decel. Permatex makes a exhaust sealant that works well.

It worked on my KTM. Hope this helps.:thumbsup:

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