2001 YZ426 wont start.....ANY IDEAS?

I recently just installed new top end, including new piston,rings,SS valves, guides,seals and SS springs. While apart I also removed and cleaned the carburetor.

It doesn't even attempt to start while using the choke. It will try to start using the hot start knob, but only for about 2 seconds then it dies. After trying to start for a while, I pulled the plug to check the spark and also the condition of the plug. I have spark and I also noticed the spark plug is dry as a bone. The AP seems to be working ok as it squirts fuel through the carb when the throttle is turned.

The timing is correct as stated in the manual and the valve clearances, after installing new shims are close to nominal on both intake and exhaust. The engine cranks over like normal and the decomp also seems to be functioning as it's supposed to be.

Any help would be greatly appreciated.


You say the valve clearances are "close to nominal".

nominal |ˈnäminəl|


3 (of a quantity or dimension) stated or expressed but not necessarily corresponding exactly to the real value :

I hate to be picky, but I have no idea what you mean by that, since what you said was that they are close to some number which may or may not be the specified clearance. If you meant "close to minimum specs", then you're OK on that score.

It sounds like a fuel related issue, and most staring problems with a 426 are, in one way or other. Just for giggles, try using both the hot start and choke together, or if that seems too weird, hold the throttle open very slightly (basically does the same thing) and see if it responds any better. May turn the idle speed up a little, too. Once you get it running and dial the idle mixture in, it should be better.

There is a possibility that the idle mixture screw may be set differently than it was before, and even if it's exactly as it was, that may not be what the engine wants now. Then too, depending on how long the carb set with old fuel in it before you cleaned it, it could still have a pilot jet compromised by a film of dried varnish. Wouldn't the first one.

As odd as this may seem...Are you sure the gas is on??? (trust me,I've seen it a ton of times)

Yeah try holding the throttle open a little, that's how I have to start mine lately. Why is that? What is wrong that I have to crack the throttle to start?

I did adjust the idle screw up a little bit. I was able to get it to start twice, however as soon as I turned the choke off the engine quit running. Then I was unable to get it started again. I'm thinking that one of the jets may be clogged. Any thoughts?

I did adjust the idle screw up a little bit. I was able to get it to start twice, however as soon as I turned the choke off the engine quit running. Then I was unable to get it started again. I'm thinking that one of the jets may be clogged. Any thoughts?
Pilot jet.


I disassembled the carb again and cleaned all of the jets and reassembled. Doesn't even remotely seem like it wants to start. Idle mixture screw adjusted in and out with no affect.

Any other suggestions?

I had a similar issue when I was fussing with my yz400 trying to get the popping and backfiring to stop. I'm not entirely sure the carbs are the same in relation to fuel / air screw but my bike would not start at all because when I pulled out the air screw the spring, and little washer that go on the tip and shaft of the pilot set screw fell out and I didn't notice it. Once I looked at the parts diagram and saw that I was missing a washer and spring I hit the dealer, bought new spring, oring and washer, put it back together, set it 2 turns out and wahlah! It started on the second or third kick.

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