I really don't know what size the carb is, as it is a bit of Frankenstein. It is basically an 850 Brawler with a Quick Fuel 950 annular main body and some older Holley 2 emulsion hole metering blocks. On the list it is C4. In my testing last year C3 seemed to work best with the RPM and 2" 4 hole spacer. but I hadn't run the 850 version with the RPM intake and 2" spacer, so that is what I used for my first test and tune. It's not pulling the front tires any more but the 60ft was better.