Classic/Antique Car Repair: missing at any speed, lean mixture, proper mixture

Question
Dear Brad,
   I have a 312 in a 57 t-bird. I have replaced the sparkplugs, sparkplug wires, points, dis.cap, rotor, but I still have a slight miss when cruising at any speed. Under hard acceleration its totally responsive without any problem. I took it to a shop with the latest equipment and the guy said that the plugs and wires were okay.
    I get good gas mileage, 25 mpg and it starts with no problem but it still feels like a bad plug or wire. I'm not sure about the jet size in the Holley 4-barrel carb. but I would think that it would starve out when gently    accelerating.
   Also I'm thinking of putting something like an r.v.cam or equivalent for more economy.any suggestions?

Answer
You are running a 312 V8 getting 25 miles per gallon and think of changing to an RV cam for more economy? What am I missing here. The first thing that I would do is to pull the plugs and check the color of the tip. A lean mixture will turn the firing tip of the plug bright white, the proper mixture will color the plug tan and a rich mixture will turn the tip black. You want tan. Also check the vacuum advance on the distributor. I believe that distributor uses a dual chamber vacuum advance. One side of it is used to retard the timing under certain conditions and one side of the advance may be defective causing a skip or surge. I would also have the distributor check on a Sun machine to make sure that the bushings are not worn and the engine firing unevenly and that the distributor advance curve is correct. There is no mechanical advance on this car it is all controlled by vacuum.  An engine oscilliscope will check the distributor wear using the raster pattern. Check the point closing signal and they should be one on top of the other. If they vary by several degrees or more the distributor needs service.
Brad