"....I Have a 1951 Ford COE on a 76 GM 1 tom dually with a 76 caddy 500 cu in big block....."

Neat would love to see pictures. I had a 51 F6 COE at one point and also a liking for the 500 CI Caddys.....have one in my 83 El Camino.

Kind of hard to diagnose long distance. A couple of quick checks that might or might not help.


".....external mech fuel pump and filter....." Not sure I've ever seen an external mechanical fuel pump. External pumps are usually electric and engine mounted pumps are usually mechanical.

If you are running a factory style engine mounted mechanical pump you might want to check and see if it's using the internal fuel filter. Looking at the bottom of the pump you may find a big "nut" the output line connects to. Remove the line and fitting and you may find a long paper filter (like the ones used in some Quadrajets). Yeah I personally found out about that one the hard way.

Cadillac HEI distributors also occasionally would have intermittent spark issues due to the wires that connect to the module breaking inside the wire insulation (caused by the wires being flexed back and forth as the breaker plate moves due to the vacuum advance). A quick check is to disconnect and plug the vacuum line to the distributor then due a test drive) If it clears the problem chances are that is your problem.

Hope this helps.


.