Engine Hesitation

I've had my 2000 450Es into 2 different Honda dealers to remedy a loud backfire problem when letting off from full throttle. The first one (useless!!) gave up after cutting out some of the muffler baffles. The second dealer said the modified muffler would affect the engine performance and sold me a new muffler ($300 installed). He also tore down and cleaned the carb. It appears?? the backfire problem is gone. The issue now is that after I've started a cold engine it hesiatates when I accelerate from an idle. Sometime it stalls when I hit the gas. After the engine is fully warmed up it is not as much of an issue. I plan on using the bike to plow snow and I don't want to sit on it and play with the throttle until it's fully warmed.

Any suggestions on how to get rid of the hesitation? I've lost all faith in the local dealers.