Spacer-spacer-spacer, This is more often a summer problem or when it gets above say 80*, It normally happens after shutdown & it sits a few minutes, Its a cronic problem with mech. fuel pumps, they tend to hold pressure against the seats after you shut off (and running if the carb gets too hot also), The heat from the intake transfers & starts boiling the fuel, then its blows past the seats & flushes the rings clean

An engine will increase a good 20-25* after shut down & when the coolant stops flowing the heat goes up. Get it fixed soon, your engine will thank you for it. All you really need is a 1/2" Phonolic spacer.