Joined: Nov 2016
The fact that it does this at a stop 'after a few minutes' is making me wonder about heat soak. Are you using a phenolic spacer between your carb and manifold? If things get so hot the gas in your carb begins to boil, it will barf extra fuel into your engine, and at idle, that would definitely cause some hiccups.
A phenolic spacer will help insulate your carburetor from the manifold heat, and usually prevents this problem.
Additionally, YES, vacuum advance is a critical part for any street driven car. Mechanical advance only sets your timing for wide-open-throttle. When you are at idle, your cylinders are getting so little air and fuel, even at full compression, there's not much pressure. When it's ignited, it burns much more slowly than it does at WOT, with peak cylinder pressure chasing the falling piston on the power stroke. Without vacuum advance to light the mixture earlier, based on manifold vacuum, your engine is going to be spitting a lot of still-burning gas and air into the exhaust manifolds, and running rough. At idle, your exhaust manifolds will get HOT. The same holds true at cruise. Worse gas mileage, poor idle quality, high under-hood temps, and even a tendency to bog a little when you step on the gas are all to be expected without vac advance.
If you advance your initial mechanical timing to the point where you get a reasonable idle, then when you step on the gas you've got way too much timing, resulting in detonation and excessive cylinder temps. (that would lead to dieseling on shutoff, because parts inside the combustion chamber are glowing, giving your engine a new source of ignition even without the spark)
Adding vac advance will allow you to idle at a much more reasonable RPM too, so you can shift into gear (with an automatic?) and not bark the tires.
I smile a lot. It makes people wonder what I'm up to...