I've got a 05 Yfz 450, with mods as follows:

WR Crank, WR piston, New Head from East Coast ATV, ported and polished and all new springs and valves. Full Duncan Fatboy 4 exhaust. Open airbox with aftermarket filters. Stock carb, with 190 main, stock pilot, stock needle, stock leak jet. Stock cam, with cam mod.

When I picked it up, the thing ran like a rapped ape. Even after I got it home, it was running amazing. I took the plastics apart to reassemble the lights and change the battery. After that it wouldn't start, period. I ran over all the wiring and redid any splices, and now it starts and runs. Idles perfect. But if I feather the throttle in neutral slowly giving it gas it shuts off. While riding, i must drop the clutch and thumb the throttle hard to keep it running, at mid throttle it runs fine. But if i try to take off slow or putt around it shuts off. I'm not too sure if its a bog      down shut off, or shutting off for being too lean. I've been messing with the air/fuel for some time now to no real avail. I'm assuming its running too lean, since it must have a heavy throttle to stay running. Also since  pulling the choke shuts the bike off immediately. I haven't taken the carb apart yet to mess with the needle or any other jetting, and I am hoping I wont have to, since it did run great with the carb set up as it is.  Any help thrown my way will be very much appreciated. Maybe I'm overlooking something, or skipping a step. Thanks folks.  

So I talked to the guy I got the thing from. He thinks its something electrical. The bike has a Honda throttle, and I've read you need to bypass the throttle switch, which he says he did. But I cant find where the switch is bypassed to check the connections. Any help? The previous owner is positive thats the problem from the way the bike is acting. But I haven't heard back from him.