At 500 miles that engine is barely broken in. Could be still tight, which means you may need to set the idle speed higher for a while.
Hmmm? At what rpm does the engine smooth out. If the engine has a modified cam shaft it will not idle smoothly at 500 rpm. I have a pretty mild cam in 59A/B and the idle is set just over 600 rpm to idle smooth and not stumble.
Ignition? if you have a good spark at the plugs the only other aspect is timing. Pretty unusual for timing to be far enough off to mess with the idle. But, to be sure, check where the timing is set on the distributor. The big screw on the side of the distributor near the top on the driver's side. The scribe on the distributor should be near the center scribe on the sliding plate under the screw. Loosen the screw and tap the sliding plate down (retard) two lines, about 4 degrees. That is a lot, but should tell you if your spark is too far advanced for a smooth idle. If that does not help, or makes it worse, go the other way - up to advance. If it takes a big adjustment either way, something is messed up and your distributor is a candidate for overhaul.
