The "compression seal" is the seal made when the piston ring makes good contact with the cylinder liner and the piston, and also involves how well the cylinder head seals to the top of the liner, and how well the valves seal when compression loads are imposed.

If an engine won't idle well, you must first make sure it's well broken in. If the engine can be run at full throttle when leaned to just rich of peak RPM, and hold that setting "all day", then you can get the idle adjusted. Many times, the idle mixture's not adjusted properly, and then the modeler adjusts the high-speed needle. This gets the two out of balance.

You must first start out with a too-rich idle mixture. Lean the top end to just rich of peak RPM, and then throttle down and work on the idle. If the engine's not had sufficient breakin, then it may not idle well. If it has bad compression or leaking valves, it won't idle well.

If the engine can hold 2,500 RPM or lower, with a good transition, then it's OK.