Spitting and Sputtering?

Hey guys. I have been having trouble with my 426. It is getting harder to start and when I finally do get it started it spits and sputters and runs really crappy. It also runs crappy throughout the powerband, when I open the throttle wide open on a straight stretch it also backfires and sputters. I have checked the bowl on the bottom of the carburetor and there is no water in the bottom what-so-ever. There was a few of dirt flakes in the bottom so I cleaned the dirt out and tried it. It did absolutely nothing when I cleaned it out. So I unscrewed the main jet out to check if it was clean and it was spotless. I decided to run a 162 from a 158 to try and lean it out. It didn't really do anything so I am about ready to call the dealer. Thanks for your help!


sounds like a valve train problem, you should check valve specs and fix yours, or take it to the dealer.

First things first,You should probably check for moisture around the plug cap and pull the plug to check for fouling. I fouled a plug on mine when I fist got it and it ran as u described. New plug, problem gone.

I agree with Kasper, CHANGE THAT PLUG!

Going from a #158 to a #162 is 2 steps richer...not leaner.

Well guys, mine started doing the same thing today, 20 miles into a ride it started acting like it was starving for fuel, popping and backfiring, wouldnt take the gas, pulled the plug on the bottom of the bowl, plenty of gas poured out. Loaded up, came home, completly took apart and cleaned carb, changed plug, dumped whole tank of gas and went to a different station and filled up, no change. The bike was running GREAT before this, Ive raced it for a year and it has a lot of hard hours on it but has been very well maintained. Any ideas?

Norman K


Norman, Blake,

What year is y'all's bikes?

In the past I have seen other four strokes act like this when the valve clearences get out of spec (on the tight side). Once it gets too tight the performance drops off pretty quickly time wise. I'm not guaranteeing that this is the problem, but I think it's worth giving the clearances a check. I would find out what it is before you ride it much more because valves that hang open a little bit can be burned, along with the valve seat.

Is your plug looking black and sooty? Mine would go into this "twilight zone" where it would pop and backfire and either come out of or foul a plug. If this does sound like it do a search on the yz400/426 forum under "foul mouth" that should give you some good background to this plug fouling problem. Good luck!

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