Poulan Mod. 2550 will not start. Approx 8 yr old. Occasional use?

The magneto produces a nice blue spark on a new plug when grounded to the outside of cylinder. The mag field coil has proper air gap and no sign of rubbing the flywheel magnet. The carb has ben properly cleaned with new overhaul kit installed. With the filter removed and trying to crank the engine I notice a small amount of blow back thru the carb.