YZ 426 Backfire Problem???

I have a YZ426 with an aftermarket pipe that backfires on deceleration badley. I have checked the valve clearance and it is right on spec. Cam gears are aligned according to the manual. I checked for obvious exhaust leakage too - none. Any ideas out there?

it sounds like it's running lean. Try putting in a larger main jet.

It's all about the Jetting. The main jet should be a 165 for elevations near sea level. The leaner the motor runs the more it back fires. Try the main jet first to 165 then a 168. The parts will cost you about 5.00 each.

Have fun.

The riverbum

ya definetly sounds like its a little lean

Welcome to TT. Changing the main jet is a good idea. But, it probably could use a fuel screw adjustment also. Get an extended fuel screw so you can adjust it while its running.

Stick with 642MX's advice first, backfiring on decel is not always an indication of lean overall, it is sometimes just the fuel screw or pilot circuit. I second the extended fuel screw, though a small screwdriver bit will work alone. Richen the fuel screw by turning it out, the popping will lessen but not vanish entirely. A little popping seems to have the best throttle response, too fat and it slows the response. Tinker.

What 642MX and Fastest1 said :thumbsup:

A heads up! I know just about everyone will be telling you to play with the pilot screw and they would be right most of the time. However, after many hours of tinkering with the pilot screw and going one step richer, my problem persisted as you described. I too knew everything was within specs and the timing was spot on. I replaced the gasket at the head. and hand tightened the flange at the muffler and header pipe. All was well. On a hunch, I un- bolted the muffler from its two mounting points on the frame, and to my amazement, I was able to slide the muffler off. This meant, that dispite my efforts to tighten up that flange, the gasget there was so spent, a closed seal was impossible without damaging the pipe. If the bike has had its muffler swapped back and forth from closed course to trail riding you might want to install a new gasget there. But try the test by unbolting the muffler from the frame and see if there is any play. All the time I spent with the carb was a waste. It turned out to be a 5 minute fix. If I would only had known...

Any ideas out there?

Is the gas old? Mine gets cranky if it's been sitting around or I put the "cheap stuff" in there.

Is the gas old? Mine gets cranky if it's been sitting around or I put the "cheap stuff" in there.

Bingo!!! Mine backfires with cheap gas...no big deal. Don't break the top end down, or take apart the carb or rip your bike apart. Just drain the float bowl and tank.

Done.

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