Just recently did a top end on my once tired 01 YZ 426. It was my first top end and I am a still a bit of a novice mechanic. I did a lot of research and followed the manual as much as I possibly could. Overall, I changed the piston/rings, flex honed the cylinder, cleaned/removed/inspected the valves, replaced the valve springs, changed the stock exhaust cam to a hotcams auto decompression, changed the timing chain, cleaned the carburetor and some other smaller things.
That being said the bike starts up cold or hot very easily, but when I rode the bike today it didn't run smoothly between about 1/8-1/4 throttle. The bike runs awesome between 1/4-full throttle, but I noticed that the more I opened the throttle the worse the bike seemed to get "loaded up" when I backed off the throttle and would stumble if I tried to blip the throttle. Also, after the bike was warm if you were to abruptly open the throttle from idle it would stumble and die.
I have checked and rechecked the valve clearances and the timing. My other thought was the idle mixture adjustment, but I couldn't seem to find a sweet spot to make it much better. I've had the carb off this bike many times and have never had the bike run like this. I'm stumped....any ideas??