How long (time) is your run up to the vertical?  If you take a long time, you will be plagued with the problem.  Make sure your pressure line is on the muffler and does not  have a split or hole in it.  Fuel has got to be lifted from  the bottom of the tank all the way to the carb.  That is some distance of achieve.</p>

Cheers,</p>

Chip</p>