Cut It Close,
Remove the fuel solenoid from the bottom of the carb (careful as the float bowl will come with it) and make sure it is moving freely, it may be sticking causing the fuel to not be shut off when the key switch is turned off, it is there to help prevent backfiring. Did you determine which spindles were in your deck? If the blades are too low besides blowout the vacuum of the deck would be lowered if the blades are too low and the caster area will be the first area to struggle. When at the 3" height on the lift system what are the front blade tips measuring?