Howdy 66inlinestang:
In addition to the good general info that Slade has already given you, some more info about your engine would be helpful. For instance, if you have a rebuilt Holley on a '66 200, it is not the original carb. The original Autolite 1100 carb was designed to work in concert with the '66 Load-a-matic, vacuum advance only distributor. If the Holley carb is the replacement #1940, it was design to be a universal replacement for the no longer produced Autolite 1100 OEM carbs. However, it could be an early side bowled carb or a Holley one barrel made for another vehicle. It would be helpful to know what Holley carb you are working with. A shop manual is invaluable when working with OEM stock parts, but less so once you start to modify and/or start swapping parts.
Is the distributor OEM? Before you start the timing adventure, do a complete survey and evaluation of the inside of the distributor. Check point gap, gap should be between .024" & .026". If the points have been in there awhile, the gap will likely be less. As points close down, due to the nylon block wearing against the distributor cam, you'll notice a degraded idle first. At higher rpms than idle, centrufugal force helps to open the points slightly. To remedy reset the point gap as per the shop manual. Be sure to relube the distributor cam surface.
Check the quality of the point contact surface. If the surface is anything other than flat and true, replace and reset, as per the shop manual.
Check the contacts on the inside of the distributor cap and the contact of the rotor. Again, anything other than clean and shiney, replace. White, chalky powder or build up and any other dust must be cleaned out. Also look for cracks or chips that could cause less than a tight seal.
Check the point plate. The points, condenser and two springs sit on a plate that is rotated on the distributor shaft by the vacuum cannister. This plate centers on a bushing on the distributor shaft. Check the plate to bushing for excess play. The plate should swivel easily with resistance from the vacuum cannister only. It should not be excessively loose or tippy. The springs and spring posts should be clean and work smoothly. The posts that the spring attach to come up from the base of the distributor through slotted holes in the point plate. The slots should be clean and clear. these slots determine the amount of advance to vacuum cannister can add.
Changing plugs, points, condenser, rotor and cap are normal maintainance. Any other concerns might indicate it's time for an upgrade.
Check to verify that the vacuum cannister is working, is connected to a "Spark control Valve" port on the Carb, assuming it has one. Once you've done all this it's time for the timing light adventure. This maintanance stuff isn't near as glamorous as a timing light, but it will provide you with a good foundation and may solve your rough idle problem. Adjusting the timing is almost never the cause or solution to a rough idle. Timing can alter idle rpm. But timing refinements can be the source of additional power and economy.
Enjoy the journey.
Adios, David
