426 backfiring in cold weather

[patty]

I fired up my 2000 YZ426F in cold weather (-5c, 20f) and even after warming up it still continued to backfire if I gave it more than about 1/8 throttle. If I rolled the throttle it was OK but not when i cracked it open. I have stock the jetting and pipe on my bike.

Any one else noticed this before? This is the first time i've tried running it this cold, so I'm just trying to see if thats normal.

 

[alphado]

That happened to me as well, try backing out the pilot screw 1/2 of a turn. That took care of it for me.

 

[Trailryder42]

I agree with the pilot adjustment. Cold air is more dense so it's making your jetting a little lean.

 

[vetwfo'er]

If it was a pilot screw, it would induce air and intensify your problem by leaning out the mixture.

By opening the "FUEL" screw, your richening the mixture, eliminating your lean condition (popping). If you get to 2.75 turns out from bottom. You should go up one on your pilot jet.

vetwfo'er

 

[deagle]

Look at some of the obvious things as well. On my KTM400 I need to periodically re-do the high temp silicone around the exhaust connections. If it sucks and air it will want to backfire. My buddy noticed the backfire on his yzf250 and resealed his exhaust and it works fine now. Mine usually breaks the seal after one of my patented crashes but the cold may make the silicone more brittle which would make it pull away from the other surface.

 

[Corey]

I have noticed some sputtering every now and then recently on my '02 426 as well, but nothing that I would screw around with the setup for. Does anyone know if it is 'bad' for the engine if no adjustments are made I continue to ride in the cold weather?

 

[JasonJ]

If your lean, yeah its bad, lean motors run HOT. My 99 WR 400 started sputtering when it got colder, I adjusted the air screw in 1/4 increments till it stopped. I tink I did it 2 or 3 times, I alredy have a large PJ so I was able to get it right with out rejetting the PJ. If its just a few pops when say your downshifting and running high RPMs for a while with the throttle closed, you can run your idel a bit higher and that helps, but the issue is like was said, colder air is more dense, you need more fule at idel. The stock jetting is usally to lean when the baffel is removed and the airbox lid is off. For example, my WR will NOT start if I have the baffel in. I have to start it then, put the baffel in and not go past 1/4 throttle, I only use the baffel at home when working on the bike so the neighbors dont get THUMPED out of bed :)
Also, remember that the ignition of the bike is retarted when the tranny is in neutral, were you smacking the throttle with the bike in neutral?