Running to rich?

My yz426f stalls out if you give it to much throttle, too fast, off of idle. It is worst when coming idling in neutral, but will also stall out when in gear if the rpms are low and i'm not thinking about rolling the throttle on slowly. Occasionally it will bog for a split second, and then take off, other times it just quit's. Am I correct in assuming this is just running too rich and can be fixed by adjusting the air/fuel screw on the bottom of the carb? Or am I going to have to get into changing the jetting?


Has it always ran like this or is it a new development?

When was the last time the carb was cleaned really well?

I always start with that first.

Otherwise, could be the mixture screw or possibly the AP squirt duration.

I would check all those before messing with jets.

Ditto the carb clean, start there.

I just purchased a 04' YZ450F yesterday (06-23-04) and i am experiencing the same thing. If i don't ease on the gas very slowly my bike stalls. This also happens when i come to a stop...

Mine runs great right out of the box however all the Honda atv's and Xr's I've had did the same thing. Turns out they were running too lean. The fix for those bikes was to raise the needle (lower thr clip) one notch.

I'm not up on the jetting screws on the Yamaha, but raising the needle isn't that big a deal.

Something to try.

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