WR426 Backfire

[smj]

Looking for some help... I bought a 2001 WR426, starts well and runs great from idle to full on, however, tends to backfire after hard cranking when the throttle is shut down. The shop, a non-Yamaha shop, says these all do this. True or false? If false, where to start? Many thanks!

[MiniMadMan50]

I'm not sure what you are talking about.. is it a TRUE back fire? These are VERY loud KA-BOOMS.

Or are you talking about some semi-loud popping when you let off the throttle? It sounds to me the 2nd one is what you mean. I have an '01 WR426 also (that's currently doing the same thing), and I've been adjusting the jetting, it sounds to me like yours is running too lean. Mines goes away when I richen up the pilot screw (on the very bottom of the carb.) ?

[tctrailrider]

The shop, a non-Yamaha shop, says these all do this. True or false?

False. The loud bang on decel is caused by a lean pilot circuit. Turn the fuel screw out or go a size larger on the PJ.

[Hick]

The shop, a non-Yamaha shop, says these all do this. True or false? If false, where to start? Many thanks!

A good way to determine jetting on this circuit (throttle closed or nearly so) is the "cruise" method. Put it in third or fourth and cruise for a mile or so at 1/8 throttle. If it surges, bingo, too lean. Blubbers, then it is too rich. Cruise control? On the money.

If you jet it to completely eliminate all decel popping you usually end up too rich, but of course this depends on the exhaust you are running.

If you have no airbox lid and an open exhaust I highly recommend trying a YZ needle. Easily the best twelve bucks you can spend on any uncorked WR 426.

Hope this helps.