sounds like it may be overheating. how much area do you have cut out behind the cylinder head to let air get out of the cowl? you need plenty of cooling air, and you need to have lots of area behind the engine for the hot air to get out of the cowl.

also- tigers need ti run rich so they leave bit of a smoke trail in the air. whenyou set the engine point the plane straight up to make sure it doesn't slow down.

ed