Also, though the books may say the engine is broken in after 4 or 5 tanks, I say the engine is not fully broken in until 8 to 10 tanks. Once you've brken in the engine according to the books, keep the engine on the rich side but slowly lean it for a little more performance each time. If the engine blubbers at full trigger, then richen it; it is blubbering from loss of lubrication and is about to stall from friction.