Two Barrel to Four Barrel Upgrade?

It is not the carb size. You have tuning problem. If you are unfamiliar with tuning, get some help.

Basics first:
- Loosen met rod piston covers & swivel slightly. Push down on pistons & make sure they move up/down freely.
- Check that the velocity valve in the secondaries moves freely when you flick them
- Make sure the choke is fully off [ blade is vertical ] with engine hot
- Make sure the linkage goes to WOT!!!!!!
- Remove the airhorn to check float level, should be 7/16". Unlike brand H garbage [ & clones ] np gaskets needed if you are careful