I have a HPI .12R SS engine that I'm getting ready to start using. The rotary carb that comes with it is old, and the throat is noticeably narrower than the inlet and the outlet. I was thinking of switching to the carb that's used on the Nitro Star 3.0 engine (yes, I've checked and it does fit); the Nitro Star 3.0 carb has a 6.5mm throat, which almost exactly matches the diameter of the inlet and outlet. Additionally, I've used this carb before and I know it holds a tune well, so it would be nice to have that reliability.

If I understand the venturi tuning guides I've read, switching to a carb with a larger diameter will increase max RPMs at the expense of low-end torque, but will the difference I specified (.6mm) actually have a noticeable effect on an engine with a .12ci displacement? If so, is there any way to compensate by tuning other aspects of the engine to regain the lost torque?

EDIT: I should mention, what originally gave me the idea of using the Nitro Star 3.0 carb on this engine is, the carb that came on this engine is also used on the old .18SS Outlaw engine. The .18SS and .12R SS share a lot of parts. So I figured, if this carb is suitable for use on a .18 engine, maybe the carb from a .18 engine can also be used on this engine.

It will probably work just fine. The only problem you could run into is that if you go too big, the velocity of the air going over the spray bar gets so slow that you don't get a good, consistent fuel metering which makes tuning nearly impossible. I doubt 6.5mm would do that though, you would probably have to get over 7mm before that became a problem.

Cool. I guess I'll give it a try. The newer carb also has better sealing, and a composite body to keep the fuel feed cooler, all features that I appreciate. I'm not too worried about fuel consumption, because the same car with a .18 engine runs until I get bored. If the engine turns out to be a pig at low RPM and lowering the shim stack doesn't help, I'll have my answer and I'll swap the stock carb back in.