1953 f100

[NORMFREEMAN]

Machine shop owner/operator sense 1970
rebuilt 239 all new valve train/new exh. Seats.
Mis fire on # 4/#7/ no vac leaks/comp. 110# all, plugs out/12v batt. 6v starter. #7 had 75# idling/ reground 4 valves/seats #4/#7
mis fire continues. Added .030 shim to springs on all.can valve guide seal leakage cause this? New guide seal installed.
Norm

 

[19Fordy]

Replace your spark plug wires. Use solid copper core wires.

 

[40cpe]

When your ignition is functioning, Check your intake manifold underneath for rust, cracks, or such that would cause a vacuum loss. A vacuum gauge should give you a hint of inconsistent vacuum. Do the missing cylinders' spark plugs show to be wet or dry?