My 426 had something similar. When I was up and down on the throttle a lot, It would become real easy to stall. Weird thing was, when I stalled it it was always hot, in the tight stuff, when I kicked it, it would light easy but stall again when I tried to take off. It would idle fine. I had real good luck with the Boysen accelerator pump cover. It helps prevent air in the Accel
pump reservoir. I called them. They told me that stall prevention wasn't their main focus, but if I bought one and was unhappy, they would refund my money no questions. This and raising my idle slightly cured my problem. Is it hard for you to relight? Taken the bowl off and cleaned out the pilot? Do you ever foul plugs? Do you have mud gobbed up in the vent hoses? Motoman 383 has fuel screw/accel pump mods on his site. He decreases accel
pump timing (he's working on a 426 though). You might take carb off open the throttle and time how long the squirt is. I think it's supposed to be 1.7 seconds. Do you have to use hotstart to relight if it's not dumped? If so, I bet your rich. I also called Zip Ty about a flywheel weight. They said if you aren't jetted right, a weight will make stalling worse, if you are, a mid size (10-12oz) would help. I love the YZ hit though. Good luck and let us know.