Windy hit it right on.. You need a bigger carb. And yes, a bigger motor pulls fuel more efficiently resulting in the smaller main jet. That's why I can run the same size main jet in my 18DM as people do in a 10mm Cub.
The other thing that could be causing you some issue is your pipes. Shearers just don't make the same power and torque up the curve like CPI and Sniper do. Unless you're riding that motor around at 10,000 RPM you're not going to see max performance from them.