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racer_18

Poping and backfiring on deceleration

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I have a 98 yz400 with a big gun exhaust system and a probe 15:1 custom piston and i use 112 octane fuel. When i let off the throttle from a hard run it backfires a bunch. i checked the spark plug and its in ideal condition (coffee brown) and there is no leaks in the exhaust. can anybody point me in the right direction?

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I have a 98 yz400 with a big gun exhaust system and a probe 15:1 custom piston and i use 112 octane fuel. When i let off the throttle from a hard run it backfires a bunch. i checked the spark plug and its in ideal condition (coffee brown) and there is no leaks in the exhaust. can anybody point me in the right direction?

I would mess with the fuel screw first before jetting. some popping is good. depending on when the popping happens it could be lean pilot or main. but try your fuel screw first.

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I beg to differ, popping is never good. My last bike and my current bike both have spot on jetting...no popping.

Like racer808 said, try tweaking the F/S first. If you have to go more than 2.5 turns out from seated you'll need to bump up the pilot 1 size. Nice thing about the 400's, 426's, and steel framed 450's is thats about a 3 minute job.

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With all due respect, you can differ if you like, but both of mine pop a little on decel most of the time, and they are both jetted right. It's a part of the nature of the YZ450 to be more prone to this than other bikes, and it's completely harmless at normal, reasonable levels. If you verify the pilot setting according to Eddie Sisneros, most YZ450's will pop a little.

http://www.thumpertalk.com/forum/showthread.php?t=327405

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Popping is normal, backfires/explosions are not.
Yeah, that's another way to say it.

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Not enough back pressure in the exhaust system causes it to backfire a load when not on the throttle. That sounds like your problem. Try checking for crack/splits in the pipe or air leaks (at the joins) and give your packing a quick check. Thats wat happened to mine i had a crack in the header pipe.

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Not enough back pressure in the exhaust system causes it to backfire a load when not on the throttle.
No, it doesn't. If that were true, all open exhaust systems would always backfire.

There are 3 things required to make something go bang. Heat, fuel, and air. Two of these are normally present in the exhaust system. The reason air leaks cause decel backfire is that they add the third ingredient, simple as that.

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Mine woz backfiring a bunch until i fixed the crack in the header pipe. Now barely even backfires. So it DOES make a difference.

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Mine woz backfiring a bunch until i fixed the crack in the header pipe. Now barely even backfires. So it DOES make a difference.

Was, not woz. And an air leak will cause decel backfiring, as I have now said at least 3 times in this thread alone. The problem with your statement was that you said the cause was "Not enough back pressure in the exhaust system", which is completely incorrect.

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I found air leaks on the intake side to cause alot of fartin but even my brand new 450 pops on decel a little

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i took the carburaetor apart and the choke was stuck open a lil bit so i cleaned that up and put it back on and now it doesnt backfire anymore but sadly it leaks a lil bit so now i have to rebuild it.

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