MT-A4 fuel supply problem...
 

The fuel tubing supplied with the Thunder Tiger MT-A4 (and presumably MGT) is of a different diameter (internally and externally) to regular fuel tubing.

My question is, does this make a difference? Does the reduced diameter of "regular tubing" affect the back-pressure from the tank, and therefore the fuel supply going into the engine? The engine was performing great with the stock tubing, but when i replaced it with clear tubing of a different diameter the engine seemed inconsistent, and had little/no top end, suggesting that it was lean (although was not excessively hot). I richened it 1/8 but this made little difference. When i checked the plug afterwards it showed signs of a lean run (grey/black & slight distortion).
I cannot replace the stock tubing as this has been cut and used for other applications (before i realised what may have been the cause of the problem ).

So do i need to get some more tubing with a greater internal diameter, or can the problem simply be rectified by "richening" the mixture further? Do .21 engines and larger require different fuel tubing?

Any help would be greatly appreciated.