My Yamaha 400 runs fine on the flat ground and sometimes fine when trying to climb.  But when I try to start out on a incline it loads up and will not build revs and thus lacks the power to head up the hill but will climb just fine if I can start out on the flat and get onto full throttle and then head up a hill with no problem.  I had a problem of it running very rich so I tore it down and rejetted for higher altitude but discovered that the boot on top of the carb had a small rip in it causing the air gate not to lift allowing enough air thus sucking to much gas.  Replaced boot and ran great until the problem I am having now of not being able to start on uphill slope.  Can anyone suggest what is possible problem.  I think float is fine and carb is clean.  Like I say it seems fine sometimes and other times just won't rev up to make it climb.  HELP PLEASE...  

Im having the same issue with my 400 did you ever find out what was going on?