I'll give this a shot. So, I bought a used Yamaha 2001 BB400 a month ago. The guy I bought it from said he put in a brand new carb a year before. It does look fairly new compared to everything else.

My son ran it well for the first three days... then (all of a sudden) it started stalling. Here'a what happens. Starts fine,but after 2 to 3 minutes of running it it starts to sputter a bit (maybe a back fire) and as soon as you let off the throttle (even slightly) - it stalls. Then you have to turn it off for 5-10 minutes before it starts up again. This happens every single time.

After a ton of research here is what I know:
1) Sparkplug - black and after reading a bit I guess that means it's running too rich - too much fuel.
2) Idle was low - increased idle speed at start
3) Fuel Valve - I reset fuel mixture valve on the carb (or whatever its called) to factory suggested - 2 1/2 turns
4) There is another valve on the carb facing out toward the bottom - easily accessible... if I turn it then fuel comes out of the carb? Is that supposed to happen?
5) Fuel line from tank to carb is good - fuel getting through
6) Air Filter - outside sponge is pretty clean - a little oily but I guess that's normal? AF itself is pretty clean. Outside is white to off white

Here is my concern... I'm not handy enough to take off a carb.. ain't gonna happen. I've watched videos and don't feel comfortable. If the spark plug is black (and dry) it HAS to mean it's getting too much fuel BUT... How?? I adjusted the mixture valve.. I can't see a leak anywhere...

Any help is very appreciated.. sorry for the long message!