I have a similar problem with my rossi engine. On a hard pull up it will cut out for a second and then come back to full throttle. It has appr. 3 gallons of fuel ran through it and runs like a champ except for the hard climbs. Fuel clunks ok and no air leaks. I was told by the distributor the "G" forces are pulling the fuel from the carb and maybe a check valve on the exhaust side may help. Gonna try it this weekend and see how it works. I will let you know if it helps my situation. Any other suggestions would be appreciated. BTW these steep climbs are at about 100mph then straight to vertical.