Shoestring is right on the money.
I make glow fuel and run engines for a living and the one thing that always baffles me is why the break in instructions for most these car engines are written incorrectly. You must get it up to operating temp (about 230F in an ABC type engine) as rapidly as possible and heat cycle the engine with complete cool downs like shoestring mentions. I have even gone so far as wrapping some foil around the cooling fins when its cold outside on some of these engines just to retain enough of the heat so the top of the sleeve will expand to design fit. Never think you're breaking in an engine by letting it sit and idle for a tank or two blubbery rich, you are just destroying the piston / sleeve fit and setting up the manufacturer for another parts sale.
If you break them in like Shoestring suggests and tune them correctly, you can get 10 or more gallons of fuel through an engine. I have had as many as 20 gallons through a cheap buggy engine with carefull tuning.