YZ400/426 suddenly not running right

Ok some background on the bike first. It's a 99 YZ400f that I rebuilt with a 426 crank, piston and cylinder about 2500miles ago. It's a street ridden supermoto, but previous to this it hadn't been ridden in a month or two.

This weekend I took it up to Monterey to watch the RedBull USGP. Thursday and Friday it was running great, just like normal. My buddy and I rode from the track, down hwy 1 to Big Sur and back with no problems at all (about 85 miles total). Friday night we went to ride into town for dinner and about 5 miles into the ride I noticed that it was idling really high. When I stopped it was idling around 5-6k rpm. I decided to shut it off to try and find the problem and that's the last time the bike started. Now when you try to kick it over the kicker just stops when you hit the compression stroke, you cant kick through it. It will sometimes do that when it's hot and I shut it down and it cooks for a bit, but never when it's fully cold.

What could possibly cause this? Stuck choke? Some seal or o-ring come apart? Idle screw vibrate out? Anything I should look for?

High idle is caused by a lean condition, which is usually caused by an air leak, a blocked jet, tight intake valves, etc. Tight valves are contraindicated by the amount of compression resistance you have, however. It could possibly be a timing issue, although those don't usually pan out this way.

Yeah, I checked all the valves when I did the rebuild and they were all dead on. Since it's been less than 3k miles I cant see how they would have changed, especially since this bike was raced for years and years and never even had them checked as far as the PO knew.

I guess I'll just pull the carb and start cleaning. Since it sat for so long I suppose it would be logical that some crud had built up and managed to float into someplace bad. Quick question. The idle mixture screw on the bottom of the carb, is that an air or fuel bleed?

When you pull the carb off check the plate on the slide to see if it is broke. My 400 did the high idle when a corner of the slide plate broke off. I could still turn mine over but had no compression due to a small piece holding a valve open.

Just pulled the carb and that's exactly what happened. A small piece broke off one side of the slide, so now it looks like I get to pull the motor to make sure it didn't do any other damage. Do you know what that slide is made out of? it almost looks like some kind of phenolic plastic, but I'm not sure. Also can I just get another slide or am I screwed and need to buy a whole new carb? Is that something the dealer would have?

The slide plate is available separately. There's no need to pull the whole engine; the only place it could have gone was into the combustion chamber, so the head would be all that would have to come off. If it ran for any length of time like that, it's probably just gone.

well I pulled the head and found the piece sitting on top of the piston. From the look of things it didn't fall in until after the motor stopped running because there was no damage at all except a few marks in the carbon from me trying to kick start it. Now I just need to order that plate. Best price I've found so far is around $150 :worthy:

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