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Discussion Starter · #1 ·
Somebody please point me into the right direction. I have tried a lot to get my '99 400ex to stop back firing on deceleration. Carb is super clean, all jets are perfect. Back fires only after you gas it good and then let off.

Upgrades: Hmf Slip on, FMF powerbomb, no airbox lid, and k&n filter. Yes I know k&n isn't the best. Everything on the quad is clean.

Jet sizes: 172 main, 45 pilot. Don't know about the needle position. Air fuel screw is 3 1/4 turns out.

Could this be an air leak? I mean it does not back fire when you are in like 4th-5th gear and let off slowly. Not super slow, just not an immediate let off.

Also, exhaust packing. Would this cause/worsen back firing? I can see some black packing coming through the core. I'm guessing it needs a change.

Air cut valve: I don't know if this has gone bad, and don't know how to check if it is working properly.

Header crush gaskets: Don't know if the previous owner changed out the crush gaskets when he installed the after market header (fmf powerbomb.)

I'm close to sea level: 550 ft.

I'm at a loss, please lead me to the right direction.
 

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Put on an exhaust that increases back pressure and the backfiring will go away. Increasing the idle speed will decrease backfiring. Go up another size on the pilot jet and if you can adjust the fuel screw for a smooth idle, that will help also.
 

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Discussion Starter · #4 ·
I bought an HMF quiet core, it still back fires. And I am now currently running a 48 pilot jet, a/f screw 3 and a quarter turns out, and idle speed all the way up. All this and it still does it. Thanks though. Need other suggestions.
 

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You can just use a piece of hose and suck on it - if it is leaking, you will feel the loss of vacuum - like sucking on a straw - it's either that or invest in a vacuum pump

The purpose of the air cut valve is to cut air to the idle circuit during deceleration to enrichen the mixture to prevent backfiring in the exhaust. This is with the stock exhaust. Any change to the exhaust will cause backfiring - it doesn't hurt anything, it's just annoying.
 

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Discussion Starter · #7 ·
Thanks, and put a piece of hose on the part where the black plastic piece goes into the carb and check? And yeah, my friends have after market exhausts and they don't back fire like at all.
 

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Change out the copper crush gaskets. Back firing on decel is usually caused by leaks in the exhaust.Also causes the famous 400ex tick most times. Might be jetted a little fat causing unburnt fuel in the exhaust to detonate from the heat. Dont overpack your silencer, in some instances it will cause performance issues and believe it or not might make your exhaust louder.
 
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