I recently purchased a new carb for my 86 Bayou 300. The original carb would start up and after a few minutes it would rev up and stay there. I installed the new carb and was only able to start it by covering the intake on the carb with my hand
Once started I had to use partial choke until it warmed up at which time I could throttle up with no problems and it would return to a normal idle. What do I need to do to fix this start problem

 

Do you have the air filter fitted, you won't get the correct air/fuel ratio if it isn't? Check the choke is working properly, if you loose half the travel due to the cable being set wrong, the excess fuel valve isn't going to open much. After getting the engine warm and giving it a run, let it cool off and check plug colour, my guess would be the nose will be white and clean, this is a sign the carb is set lean, and screwing the idle mixture screw out a little (1/4 turn) will richen the idle, take note of how much you do move the screw, so you can move it back if you make things worse. It may also be a good idea to set valve clearances, tight valves can drop compression and lead to bad starting.