[BRENT in 10-uh-C]

Adding some thoughts to this as I have never been a fan of a used Tillotson because of their casting issues. The one key thing I read was that adjusting the GAV at the lower speeds made no difference. To me, that is an indicator that you have a vacuum leak somewhere. Is it possible the throttle shaft is sucking air which affects the signal at low RPMs? I think to a certain extent, the Idle circuit does play a role as the engine's RPMs are between 500 - 600 in the 12 - 15 mile an hour range. None-the-less, when the throttle is snapped-open at that RPM range, it should begin accelerating smoothly, ...especially with a Tillotson since it was an economy carburetor with a smaller venturi.

I would adjust the carburetor where it will rev smoothly and very quickly from an idle all the way to 2k RPMs. Then richen the mixture just a tad and go test-drive. If you are receiving any type of hesitation or stumble during the snap-throttle test, then find where your problem is with that first. Maybe a carburetor change??