This is from Eric Gorrs web page
10) When you initially start the engine after a rebuild, manipulate the choke to keep the engine rpm relatively low. Once the engine is warm enough to take it off choke, drive the vehicle around on flat hard ground. Keep it under 2/3 throttle for the first 30 minutes. Two common myths for proper engine break-in are; 1) Set the engine at a fast idle, stationary on a stand. 2) Add extra pre-mix oil to the fuel. When the engine is on a stand it doesn't have any air passing through the radiator and it is in danger of running too hot. When you add extra oil to the fuel you are effectively leaning the carb jetting. This can make the engine run hotter and seize.