[Purdy Swoft]

Wick makes a good point in post # 8 . I usually run four ounces of marvel mystery oil for every ten gallons of gas , more in severe cases . Marvel oil and sea foam work in a similar manner . slow sticky valves can cause enough compression loss to cause a miss

It could be ignition . Check the gaps between the rotor tip and the contacts in the distributor cap body ? Ford recommended a gap of twenty five thousands at the rotor tip . More gap at the rotor tip will give hotter spark at the spark plug . If you've heard of putting a jump spark in the plug wires on old stove bolt chevys and flat head Fords after they started pumping oil, the added gap at the rotor tip will have the same effect with no change to the plug wires . I run mine with a gap of thirty five thousands, not because I need to but because I want hotter spark . A lot of distributor caps will have gaps that are all over the place . The cylinder with the least gap will have the coldest spark . I file the contacts in the cap body to increase gaps . It is always best to have the same gap at each contact . Another possibility is that some of the older and reproduction distributor caps will short between number four and number three cylinder and cause a miss . I had this problem once and eventually could see a spark in the dark where the number four spark plug wire strip passed over the edge of the cap heading for number four spark plug . The model A firing order is 1243 . If you make a close study of the distributor cap you may see a slightly melted spot on the insulation of the distributor cap where the spark was jumping through the insulation of the distributor cap and partially shorting out the spark to number 4 . It would be good if you had some used distributor caps to check and experiment with