Hi!
The ASP engines are very fine engines...and powerful too.
It sounds to me that you haven't set the needles correctly. When you hold the nose of the plane straight up you should set the high speed needle at max rpm and then back off a couple of hundred rpm...this you can hear! No tach necessary.

Seeing your picture how the engine is mounted I can understand that you have trouble. The engine seems to be mounted too high compared to the tank. The best mounting position is to mount the engine on its side so that the carb center comes in in line with the tank when the plane sits horizontal. This is vital for good engine performance.

The ASP is a powerful engine and can swing a fairly large prop like a 12x6 APC without trouble . This is in fact the prop size to use even for the ASP .40 in a high wing trainer.

The ASP.40 has 2 head shims (believe the .52 has the same). If you run low nitro contents like 5% you can remove one of the head shims for better performance.
The engine also runs better with the high speed needle mounted close to the carb.
Tanks size: 10oz preferable set up with Uni-flow (Two clunks).
Glow plug: Several ...but Nova-Rossi 4, Rossi 4 OS 8 and Enya 3 and 4 are good ones