Brushfire, just to eleminate one of the variables, could you turn the plane over on it's back and see if you can get the engine adjusted to where the idle is right?

Another thing but this is a long shot is that you should start to make your idle evaluation the moment you put the engine at idle. For example, full throttle then bang to idle.

The engine may slow down from that point and maybe quit? too rich

It may speed back up and maybe quit? too lean


Also, has the engine ever idled correctly? There may be something wrong with the idle needle? It maybe too short? it may not even be sgrewing in and out but just turning?

Good luck,

Jim