is there a certain time when you use a smaller venturi, besides when wanting better fuel mileage?

i take it you would have to reset the carb to factory then retune with the venturi in. right now i run almost 7 mins at 230 degrees and that is with no venturi

I use a smaller insert when going to indoor tracks and yes you need to re-tune but no need to go back to factory settings . Running no insert is pointless IMO as it doesn`t gain performance and get bad gas mileage .

I run a 6mm Venturi (smallest available) in my RB. FWIW.

i just put a 6mm in my rb. what kinda run times you getting per tank?

depending obviously on lots of things, but as high as 11minutes. I haven't checked it in a while with the Gen2 Fuel.

Normally a smaller venture is better for tight tracks and bigger for larger tracks that are not so tight. But like most things there are variables.