Originally Posted by Sylvain

A smaller venturi will reduce power and increase fuel mileage. I heard people having runtime issue with smaller venturi but you should be fine with a 6.5mm.

A bigger clutch bell will reduce acceleration and increase top speed. In theory, you should also save on fuel.

I have the exact same engine, glow plug, pipe and buggy with the same runtime.

I switched to a 121VR-ST which is a little more powerful version. It increased my runtime a little bit, mainly because I have to control my heavy finger.

I think the best to increase your fuel mileage, is to try to be smoother on the throttle.