Hello Beeza, Yes there is a difference in using the tap on the front part of the case (by the crankshaft) as compared to a backplate tap.

Pressure in the case varies. When the piston is decending then positive pressure builds in the case. When the piston is accending you get negative pressure. The negative pressure allows the engine to take in air through the intake and the positive pressure helps force the air/fuel mixture in the case into the cylinder.

With a backplate tap you end up with relatively low pressure to the tank because you are mixing both (alternating) levels of pressure. But with the tap by the shaft you only get "timed pressure" that occurs when the intake in the shaft lines up with the pressure tap and when the piston is decending and is higher pressure.

The first VA .049 RCs I got came with a backplate tap. I could not get them to idle satisfactorily. Switching to muffler pressure solved the problem.

I'm thinking there is little reason to use case pressure on a sport RC engine, unless muffler pressure is not available.