If it runs correctly at road speed, the problem is likely a partial blockage of the idle jet in the carburetor. There is a quick fix you can try which might prevent haviing to do a rebuild on the carburetor. When the failure to idle showed up on my dads old 1936 truck, he would crosss the two middle sparkplug wires. Then he'd start the engine, rev it up a time or two to make it backfire through the carburetor. The backfiring would clean the idle jet. Saw him do it a number of times.
meansam
