No, you're not under carbed.
Can you make a video and upload it? What you call rough and what I call rough may be two different things.
Some carbs run rough at part throttle. I know you said it was a gas motor, so it's not the typical alky roughness at part throttle. You didn't say, however, whether the motor has power valves in it, or if the power valves are plugged off. If there are no power valves, it will run rough at part throttle.
If, however, you can back off the motor 2 degrees, and the 'roughness' goes away, then you may want to try closing up plug gaps to like .018 (just to try it), then up the timing.
If the roughness still occurs, it's time to start changing out electronic parts until you find the culprit.
It could be:
1. cap, rotor, or both.
2. Old plug wires? Have you checked the resistance in each plug wire with an ohm meter?
3. Coil going bad
4. Points in the dist? If points, have you checked the point gap?
5. 6AL box going bad.
6. Excessive plug gap
7. Bad ground... The coil should be grounded to the battery and heads (both heads) via dedicated leads.
Those are all things off the top of my head that can cause 'roughness' due to ignition issues. This is assuming that there's nothing wrong with the carb.
A simpler thing to try would be changing the carb out...