YZ426F wont idle

Ok guys. 


I went to Haspin Acres today. Up until now, the bike would idle perfectly. I rode it yesterday to make sure everything was ok. It idled and ran perfectly. I get to Haspin, fire it up, it idles perfectly. I put around the track one time, then hit the drag strip. It ran through the gears good, but as I started to decel, it popped and backfired pretty bad then died. It normally would only pop lightly once in a rare while. 


It would start back up, but in order to make it idle, I had to pull the choke out a little, or open the throttle slightly. When I say open the throttle, I mean just a slight, slight crack. If I closed the throttle, she would idle rough, then die.


I tried to raise the idle, but that didn't really help.


I was able to ride the rest of the day, as 99% of the rest of the throttle was "normal", it would run strong and pull great. But if I closed the throttle all the way, boom, she would die. 


On decel at the track, if the throttle was closed all the way, it would pop something fierce.


If I decel with the throttle open 0.5%, it would not pop.


So, one moment it idled great. Very next moment it would not. Other than that the bike felt normal.


Is there an idle circuit? I would think if the pilot got plugged it would be more than idle.


What else could cause an instant idle problem? Again, I did not notice any, any other difference other than the idle.


Thanks guys!

Start by checking to see if your pilot screw is still there.  If the O-ring fails or is missing, they can drop out.  If it's there, try setting up the idle again from scratch. 


It is also possible that you slurped some dirt or water up into the pilot jet and lodged it there, so that would be the second thing to look at. 

I had the same problem with mine, I cleaned the carb and it ran great. The pilot jet was clogged.

Ok thanks. Frustrating part is that this will be the third time I have had to do this in the last 6 months. I even bought a new gas can, cleaned the tank, carb etc..


Why are there not fuel filters on these bikes? If they are that picky.. do people run filters?


Is there a way to tell what year carb I have. Supposedly I have a 450 carb, or at least that is what the last owner told me. The only reason I believe him is that I have the hot start lever, not pull knob like a 2000 yz426 should have. But if I was to order a rebuild kit, how can I tell what year I have?


Thanks guys

There is, or is supposed to be, a filter screen built into you petcock.  You'd need to remove the petcock to inspect it. 


If you have the number that is etched/stamped into the left side of the carb along the float bowl joint, I should be able to find the year, but if has the hot start set up where the cable runs in from the top of the carb instead of the side next to the choke, it's an '03 or later for certain.  Yamaha doesn't market "rebuild kits" for the carbs.  Sudco does, though, and so does Honda.

Sudco ok? 


5TAI 00RJ21



Also, with Indiana climate and altitude, and a stock exhaust/air filter combo, would stock jetting be sufficient?



The 5TA1 was used on the '03 and the '04 US model YZ450.   The stock jets should be a good starting point, yes.  Probably quite close to right, although you might find it happier with a pilot one size larger.


Sudco is a great place.  Mostly all genuine Keihin stuff.  Any aftermarket parts you get there are top notch.

Great will do! Thanks for the help.

Ok got carb off. Air screw was missing, must of fell out. No wonder idle was so bad!


So while I had everything apart, I checked the valves. My smallest feeler is 0.006". On the exhaust side, they would accept 0.010 but not 0.012". So that is good.


On the intake side, it would not take, or might start a pinch, the 0.006". 


So the bike was running fine, I am going to assume the valves are still well within spec.


Next question is how tight the cam chain is!! Holy smokes! Maybe I am a newbie to motocross bike guts, but I've never dealt with a chain that you could play a note on! Are they supposed to be this tight? There is no give or movement!



They do run tight, yes.  The tensioner is stepless, completely automatic, and can't be reversed by pushing back on the plunger.



got the rebuild kit. All the new jets appear to be the same as the stock ones except for the pilot.


Again, my bike is a 2000 yz426f. Stock exhaust. It seemed to run well before I lost my air jet.


The carb is an 03-04.


The pilot I removed was a 165. The one that came in the kit was a 162. 


You mentioned the bike might be happier with one size larger. Is this the 165? SHould I just keep it 165? Or should I put the new 162 back it?



Edited by 5point9

sorry i meant main jet

But I said "pilot".

yes but it appeared to have a "stock" 42 pilot in it now. The kit came with a 42. I don't doubt that a larger pilot may help, but I don't have one handy.


Should I put the larger main back in?

You can use a 42 but your air fuel mixture screw will have to be way out, like 2.5 or 3 turns out

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