You are trying to run the engine too slowly. With a 12 x7 APC prop, you start the engine with the high-speed needle at about 1-1/4 turns open, and advance to full throttle. Immediately lean until it just stops running in 4-cycle mode. You h ave the non-ringed engine. Running it too rich will ruin it, not allow it to break in. Don't worry about RPM. Just accept whatever RPM you get.

The pipe won't be tuned, of course, because the engine's not broken-in yet. Just make sure the tuned pipe system is too long for the RPM you're running.