Never idle an engine for a gallon, ever. Preheat the engine with a heat gun or hair dryer. Fire it up, gently clear it out and get a stable idle. Throw the car down and drive it easy at a slightly rich setting for a tank. Shut it down, set the piston to the bottom of the stroke. Let it cool completely. Repeat this process about 6-8 times, each time letting the rpms get a little higher. Try to keep the temp above 180F, 200-240F is ideal for breakin. Some engines don't run that warm though, that's why I say 180F at a minimum.
After about 8 tanks, lean the engine just enough to perform good and leave it there until the mechanical pinch relaxes. Then give her af ull race tune. Running too rich is just as bad as running too lean.
Did you just put a new piston, sleeve and rod?
If so you need to replace the engine bearings if they have not been replaced recently.
A lot of people replace pistons and sleeves when all they needed in the first place was a set of bearings.