02 426 backfiring, stalling and making no power.

Hi guys,

New to the forum here. Just looking for some help.

I recently bought an 02 426 from a friend. It is completely stock with the exception of hot cams stage 1 cams in it. When I got it, it ran great and all was well. While riding, I landed a jump and something changed. I noticed a change in the exhaust note immediately. It started to pop and backfire. I rode it around the track slowly until it just died. I have been able to re-start it at will, but it back fires a lot and is un-ridable.

Since then, I have checked the carb (all seems to be normal), the timing (normal), replaced the spark plug, checked for leaks in the intake boots (none), valve clearance (normal) and done pretty much everything else I can think of. As far as I can tell everything is normal, but it still runs like crap.

Here is a video of what it sounds like. Any suggestions would be much appreciated!


Could have knocked some of the packing loose in your silencer causeing a blockage.

My 07 yz is doing the same thing...was running fine then I crapped out on me while going around a dune. Tried trouble shooting everything I could, still popping and stuttering. Not the packing so if anyone can think of anything else let me know. Thanks

sounds like carb problems to me

If it was jetting then why all of the sudden? I would assume that having run perfectly fine then not, would not be jetting. Other then a clogged one maybe.

Sounds like it might have jumped a tooth on one of the cams. If the chain is old and stiff/ stretched it is possible that a hard jerk on the motor (like a hard landing from a big jump) could cause a moment of slack and allow it to jump a tooth.

This was just my thought from your description.

Not a jumped tooth, all the timing marks line up and there is no slack in the timing chain. I need to get a good set of feeler gauges and verify valve clearance tomorrow. My next step in to just go ahead and repack the muffler and use some high temp silicone to make sure there are 0 gaps in the exhaust by the head.

Anyone else have any suggestions / ideas? Would losing packing happen instantly like this? I'm puzzled so any information is helpful. Really trying to avoid pulling the motor and taking off the head. Thanks

I have checked timing too. Mine is fine. That was my first thought. Will small leaks in the exhaust by the head really cause this type of issue? I haven't checked that on mine, but I have a new gasket for it so I could try that.

Also, I checked valve clearances on mine. They're all within spec.

I just checked my valve clearance today, I've got one intake valve that is out of spec by .05mm but I don't think that would cause a backfire like I've got. Once I verify that it isn't the valve I'll put the bike back together and check for exhaust leaks. I'm seriously stumped on this.

Exhaust backfiring on deceleration is normally the result of either an exhaust leak (which will not affect performance), or a lean condition at idle. Such a condition is most always caused by a wholly or partially clogged pilot (read: http://www.thumpertalk.com/forum/showthread.php?p=8021726#post8021726 ), a leaking or missing pilot screw or pilot screw seal, or by extremely low float level.

^^ Mine is a backfire on the throttle. It's not just a little poppig it sounds like a 12 guage going off.

did you check your decomp lever and cable?If it a tad tight it will keep the exhaust valve open.

Yeah, It had free play. I'll leave it disconnected once the bike is back together just to double check but I'm positive it had enough play.

By the way, sorry Alex for kinda high jacking the thread. Hope fullly we both find the information we are looking for though.

when you try and kick it over without the decomp does it lock solid or can you kick it through?

It wasn't locked solid but it definitely had compression. It seemed a little to easy to kick over after it crapped out. I'm wondering if the engine getting hot caused the decompression valve to open just slightly. Although, why would it continue to backfire after being cooled off?

I'll be able to check tomorrow, I confirmed that the out of spec valve is not the likely problem. I'll have everything back together in the morning and I'll fire it up with out the decompression lever hooked up.

Lol, i just realized you guys are not referring to the hot start lever... The 07 has the auto decompression

when did it change to an 07?I thought this was about an 02 426.

Hahaha, JTT has been talking about his 07 since the third post. It seems that we are having very similar issues.

And JTT, I don't mind at all. Hopefully we can get this figured out.

I managed to ruin a gasket in my carb while cleaning it the other day and apparently you CANNOT buy these gaskets?!?! It's the most ridiculous thing I've ever heard. I am in process of finding a 450 carb to put on the 426. I'll let you know if this changes anything.

I guess taking the carb body apart can be a $200 mistake because of a $0.10 gasket. Beware!

^^ Mine is a backfire on the throttle. It's not just a little poppig it sounds like a 12 guage going off.

That could be either a lean condition brought on by sucking something into the main jet, dropping a needle, etc., or erratic ignition due to a faulty stator or coil. If it's bad ignition, the plug will usually turn sooty and black.

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