back fireing on 99 yz 400

hi hope someone can help i have a 99 yz400 & it back fires alot when you let off the gas it's fine when you open the throttle i've clean the carb & check the valve clearanses & timing & repacked the exhaust but to no avail any help would be appressiated

Try bumping the pilot up one size. What are your jetting specs now? I have a 99', and it's jetted like nobody's business (it runs cleaner than my '01 WR).

What you are describing is a lean condition on the pilot circuit. I would try backing out the fuel screw first. If that helps and you get to 2 1/2 turns out on the fuel screw you might want to try one richer on the pilot jet and go back in on the fuel screw. This should get you headed in the right direction.


UK, huh? I'm thinking your weather mirrors our Seattle winters. As the temp drops, the air becomes denser, and your mixture leaner- which leads to popping on decel. Fiddle with the fuel screw, 1/2 turn at a time (counterclockwise=more gas. I'm running mine out at about 3 1/2 turns with the stock exhaust in the winter.

My 99 YZ400 was popping alot on deceleration. The higher the RPM's when I chopped the throttle, the more it backfired. I went out 2 1/2 turns on the fuel screw and it still popped so I went up one size from the stock 45 to the 48 pilot jet. I run it 1 1/4 - 1 1/2 turns out now and it cleaned it up. I had read that if you go more than 2 1/2 turns out, you need to get the next size larger pilot jet.


Before you go and mess with jetting try something very simple. Replace the exhaust donut on the pipe/muffler junction. I was having the same problem with my bike and tried all different setting on the carb with no avail. I was talking to a fellow riding buddy and he mentioned replacing it. I did and it cured my issue. Cured my problem.

You can get the donut from Yamaha for about 5$-$10.

Part Number#: 3GW-14714-00-00.

Just my $.02 worth.

Look in your owner's manual and find out how to richen the setting on your fuel screw. A half turn richer is a good starting point. I had the same problem on my '00 426 and this helped clear her right up.

That's a good point too, any air leak in the exhaust will also cause the popping. Feel around the header/pipe junction for an exhaust leak while the bike is running, if there is one, it may also be the culprit.

Happy hunting,


Im kind of surprised no one brought up the point of and exhaust leak sooner. The poping is an exhaust leak at the header pipe junction to the head itself. You wont usually get the backfiring with a leak at the header/muffler connection. Either tigthen the header or replace the donut gasket as previously suggested and your backfire will disappear. A lean condition will cause it to spit back through the carb. :)

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