Running a new ABC (or ABN in the case of the TT) excessively rich during the break-in period can damage the engine, particularly during cold weather. New ABC/N motors should be run at full throttle, just at the lean side of the 2-4 break-point, for periods of 2-3 min, then shut down to cool & the process repeated until ~ 1/2 hr of run-time is achieved.