If an engine won't run well much past low throttle, it's likely that the idle mixture's not set correctly. During the breakin running, we usually leave the glow ignitor on the engine and drive it around. We accelerate up and down the parking lot, getting up to full and back down. We gradually work the high-speed needle towards peak RPM. After a few tanks' worth of running, we then work on the idle mixture. After a few tanks, the engine's run-in well enough to accept being set to a good idle.