yz426 backfiring, too lean?

i was riding my bike today and noticed that it backfires quite a bit while im decelerating and downshifting. does this mean that the bike is running too lean? thanks for the help.

You can probably cure it with a fuel screw adjustment :thumbsup:

Turn it out, it will introduce more fuel when the slide and needle are down. That should cure it, if not you will need to go up 1 pilot jet size at a time. You can usually tell if the pilot is the right size by turning the fuel screw, if nothing happens such as the idle raising or lowering the pilot is wrong. You do want a small amount of popping to occur for the best throttle response.

which screw are you referring to? the pilot screw?

which screw are you referring to? the pilot screw?


It's the fuel screw. Backfiring is a sign of to much fuel and that usually happens during acceleration.

You have a lean "pop" during deceleration. Turn the fuel screw, located under the carb on the engine side, 1/4 turn per adjustment. If you get more than 2 1/2 turns out from completely bottomed, you need a bigger pilot.

I just got 2 new bikes Sunday and both needed fuel (pilot) screw adjustments before they would even idle. Both bikes had the screw 1/2 turn from bottom out of the crate.

And I'm thinking both are going to need bigger pilot jets too. I put the TTR125 out 2 1/2 turns as suggested in the manual and the YZ450 2 turns. The TTR runs good but is really cold natured when first started while the YZ runs great, but as eager as it is to run down low I think it's just a tad lean still. I ordered a Zip TY fuel screw from the TT store and should have it when I get home today, then I'll fine tune the pilot I have in there and see if it will work for me.

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