500 AVS2 or 650 AVS2 ???

gzig5 said
If you never go over 4500 rpm, the 500 cfm is not going to be a limiting factor of anything in your combination and should return better fuel economy with less fooling around. With that level of engine build, you are picking fly poop out of pepper. The return on any change is going to be very difficult to discern without instrumentation.

It could be, IF the engine needs more on the primary side. If the 500 and 600 have the same throttle bore and venturi size, as eluded to in previous posts, then you're probably right......but I'd have to measure it for myself. I find it difficult to believe they have the same size throttle bores and venturi.