Thermoquad race tune

Metering tree adjustment or the secondary air door is opening to quick, or both.

Those will give you an off idle stumble. Also I would think it would like a little more initial.

One thing to try would be pull the metering tree up with your fingers and clamp it there with a small pair of vice grips. Leave your air cleaner off and try it. Maybe if you're out at your Dad's just run it down the road. Start and hold for a second, stop and try again and see if the off idle stumble is better or goes away. Bump up your initial to 16* and try it.

Do you have the tool for the air door adjustment? If that door opens to soon it will add to the problem.