1973 JH running well after rebuilding carbs and installing new timing gears and belt, until today. Suddenly misfiring on #4. Strobe light fine on 12 and 3, but intermittent or not at all on 4. Plugs cleaned and checked. Strobe light up and good spark when plug is removed and laying on cam cover. Any ideas would be helpful.
The problem could be a plug wire not plugged into the distributor cap..it is so tight under there that when you try to put the cap back on the distributor it is easy to pull a wire from the cap...personal experience...also you can check the firing with a strobe light..just connect the pickup from plug wire 1 on through four..each time just pulling the trigger on the strobe light..If it is firing, it will flash at each plug wire..if it doesn't, it is unplugged...
Also if it seem to run and back fires..check to see that you haven't cross mounted 1 and 3...or 2 and 4 wires in the distributor cap...again,personal experience.
Thanks for the ideas, guys. I have removed and rechecked all connections into the petronix cap. Also replaced #4 wire with longer wire that is not under strain. Strobe light fires when plug is not in cylinder, but is erratic or dead when plug is installed. Plug set is almost new, but I will change plug, just to be sure.