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I have a 2005 Yamaha YFZ 450 that has been having some carb issues. It idles fine, but after about half an hour or an hour of riding, it backfires and stalls. But it starts right up after sitting for half a minute. My YFZ is pretty much stock except for removing the baffle. I’ve done a ton of carb work, from a new mid body for the carb, R&D adjustable air fuel screw, and recently a 165 main jet and 45 pilot jet. Right now, the carb is set at two turns out with the 165 main and 45 pilot, and stock needle setting. I don’t have the air box lid off due to the fact that my area is very dusty. Any help, suggestions, and ideas are welcomed. Please help me out!!!
 

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I have a similar problem with a 2005. Started happening first ride after a rebuild. The bike will start and idle fine. As soon as it gets up to high rpms for a few moments it will backfire and lose power and die. If I immediately try to restart it won't, just backfires. But if shut off the ignition for 30 seconds it will start right back up. I tried replacing the cdi with 2 known good aftermarket ones for an 2005. Neither one will allow it to start, just an occasional backfire. Replacing with a new stock cdi shows the exact same issue with high rpm load.
 

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I have a similar problem with a 2005. Started happening first ride after a rebuild. The bike will start and idle fine. As soon as it gets up to high rpms for a few moments it will backfire and lose power and die. If I immediately try to restart it won't, just backfires. But if shut off the ignition for 30 seconds it will start right back up. I tried replacing the cdi with 2 known good aftermarket ones for an 2005. Neither one will allow it to start, just an occasional backfire. Replacing with a new stock cdi shows the exact same issue with high rpm load.
Exact same thing happens to me, starts fine and idles good. But as soon as i get into the higher rpms, it backfires and stalls. If i let it sit for 30 seconds or so it starts right back up like nothing happened. Maybe its getting to much fuel? Maybe not enough air?
 

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For Tanner, that could be the case. Might have a piece of dirt or sand getting stuck in the main jet, then falling back down after killing it. Doesn't explain why I can't start my bike on an aftermarket CDI since the engine rebuild. I have tried a new stator and regulator, and swapped known good oem flywheel. Tried new battery. Only thing I have not ruled out is the possibility the timing was never reset correctly. I don't know what I'm doing when it comes to that. Is there anything electrical I'm missing? The rebuilder is not standing behind his work and just keeps throwing parts at it and charging me. He was reputable.
 

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Spark, fuel and air are whats needed. Check that you have a good spark and fuel.

"The bike will start and idle fine. As soon as it gets up to high rpms for a few moments it will backfire and lose power and die. If I immediately try to restart it won't, just backfires. But if shut off the ignition for 30 seconds it will start right back up."

Sounds like the same as his. fuel starving issue. One other thing for both of you to check is to make sure your carb vents and tank vents are clear and functioning. If they are clogged up, you will get a fuel starving problem as well.
 
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