ok, i recently picked up an 01' yz426f. Its pretty much stock(except for the suspension and some other things on the chasis) and back fires when you let off the throttle. I cleaned the carb, moved the clip on the needle down 2 positions(its a kiehn FLAT CR carb), seems to rev a bit stronger after i did that, but still backfires when its warmed up. Before i did anything, i checked the plug, and it was black with white speckles/spots(detonation obviously) I also put fresh 93 octane in the bike. There is no exaust leak, and the intake manifold and rubber boot that connects the airbox to the carb are tight. should i move the clip down one more slot? The bike also sat for 2 years(which shouldn't factor in, because i just cleaned the carb, and put new gas in it) Any ideas?
I also amd having some freinds come to my place to ride tommorrow, so i would like to figure this out asap, I know you guys know your stuff, and you've helped me before, and i do greatly appreciate it.
thanks
rich

 

OK, so your saying that the bike does not have an aftermarket exhaust system on it?

 

i'm not sure what your thinking of choose, but i know that when i pulled my baffle out of my kfx it popped all the damn time when i let off the gas(stock). i asked a guy who works on atv's and he said it was the internal baffles from the back pressure. i got my yoshi and it stopped. i thought because it was backfiring that i was running lean ,but i knew that i was already running rich[img]i/expressions/face-icon-small-confused.gif[/img]. not sure if that will help but if it's running good exept the pop it may just be the pipe. ????? hope you figure it out.

 

Kawi, thats exactly what I was going to say. Good job man. Now I dont have to type all of that. Thanks, [img]i/expressions/face-icon-small-wink.gif[/img]

 

Also, check your valve clearances. Could have a Very tight valve. A tight valve could cause what you are saying, and make a plug look black. Valve service might be in order. I know that it's particularly easy on that bike.

Good luck and let us know what you find.

 

choose have you talked to bigbird(pete)? he was trying to get a trip together for us to come and play at your little sand box ,i think he said 2 weeks. the pics of your bike are sick!!!!!!!!

 

Kawi, you have a pm!

 

i'm not sure if this matters, but the previous owner put a flywheel weight on it(he rode in trails alot), could it be possible that something came loose and maybe the timing is off?

 

Are you reading a 2-yr old plug?

If I was going to change the needle clip at all I'ld suggest no more than one clip position change difference.

I take it the needle clip change didn't do anything to change your popping?

Don't the YZ dirtbikes run lean by nature? How much poppin' you hearin'?

Stupid question, is the hot-start button out?

How's it idle?

How's the idle with the choke on? Choke off?

 

my KLX 300 did that when i removed the stock baffle,I rejetted and now it only does it when i dump the throttle at high RPM (drag race)