is this a jetting problem??

A freind of mine just picked up a 2006 yz450f. The only addition is a fmf powerbomb header. The bike pops alot, and i mean ALOT... Also when you rev it hard it will sometimes shoot a flame out of the exhaust?? Is this a jetting problem?? How good is the jd jet kit?? Will buying one of those allow us to jet his bike correctly??? Thanks very much.

I agree with gray, an air leak will cause the popping and can cause a severe lean condition which will cause the popping.

An air leak in the exhaust has no effect whatever on the fuel mixture, rich or lean, and cannot cause a lean condition.

What it does do is provide air to the exhaust system, which, since it already has heat and some fuel, is only lacking for air to make fire.

Lean idle circuits will also cause decel backfire, but since we have an aftermarket header, I consider it the more likely culprit. In any case, one needs to start by looking the exhaust over, because if you start with jetting when you have an air leak in the exhaust, you'll end up with a bike that runs dirty rich and still have a backfire.

An air leak in the exhaust has no effect whatever on the fuel mixture, rich or lean, and cannot cause a lean condition.

What it does do is provide air to the exhaust system, which, since it already has heat and some fuel, is only lacking for air to make fire.

Lean idle circuits will also cause decel backfire, but since we have an aftermarket header, I consider it the more likely culprit. In any case, one needs to start by looking the exhaust over, because if you start with jetting when you have an air leak in the exhaust, you'll end up with a bike that runs dirty rich and still have a backfire.

Ok, I understand completely what you are saying... So when i look over the exhaust system what should i be looking for?? The fmf header is mounted to a stock muffler/silencer.. If you can tell me what type of issue i am looking for i can look over it. My freind is not really mechanically inclined at all and i think he bought too much bike for himself. I am just trying to help him out. Thanks for the advice

Normally, the only two places the header can leak is at the joint with the head, or at the joint with the mid pipe. The mid pipe joint is more likely. If he's using the stock muffler, there is a packing gasket that may have failed, or the clamp could just be loose.

Normally, the only two places the header can leak is at the joint with the head, or at the joint with the mid pipe. The mid pipe joint is more likely. If he's using the stock muffler, there is a packing gasket that may have failed, or the clamp could just be loose.

so all this popping and small flames out of the back could simply be a bad gasket or loose clamp??

It seems to pop and what i would describe as back fireing when you start it too. Like every 3-4 kicks it will pop really loud??

What if the problem is where the header meets the head?? Is there some kind of gasket there as well??

Thanks again for all the help..hopefully i can help him get this fixed..

It seems to pop and what i would describe as back fireing when you start it too. Like every 3-4 kicks it will pop really loud??

That's ordinarily because you've loaded the pipe with unburned fuel, then had a weak attempt to start. The flame sent into the exhaust ignites the fuel. Has nothing to do with either an air leak or a lean condition unless the lean condition is making it hard to start.

What if the problem is where the header meets the head?? Is there some kind of gasket there as well??

Yes, there's a gasket there. Or there's supposed to be.

When I was agreeing with you gray I was also pointing towards a possible air leak in the carb boot between the motor and carb thus the lean condition, sorry if that was unclear.

Also bhsballer76 when your trying to start it and/or have it running have someone standing back a bit to look at the exhuast for any possible smoke/flame. At least to me it would seam that if there was a leak somewhere that there could be a possibility for a tiny flame coming out the air leak spot as well as the end of the exhaust. I may be wrong though.

alright, thanks for all the help. Hopefully i can get this figured out soon. I will have an oppurtunity to look at it later tonight.

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