my 300ex keeps pooring fuel into the air box, has no power on the lower end, and is nearly impossible to crank

 

sounds like a carb problem take it off and clean it good put it back together and try it then.

 

Hmm so is the fuel traveling back the air intake into the filter? That sounds suspicious because there should be suction from the engine so fuel cannot go backwards, i would also advice taking off the carb and check the slides and gates, i suppose if one was stuck shut fuel may be able to dribble backwards into the airbox.

 

the float was stuck

 

the float was stuck

 

that has happened to mine be4, i had to open up the carb and mess w/ the floats/shutoff needle, if the carb runs dry it might do it again.

 

Wrong section.