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My Cortina runs thru the gears up to speed without missing a beat, but once doing 65-70 put foot down and under load it starts missing and backfiring. Let out of the gas a little giving it less load and it increases speed. I have checked wires, plugs, put a new distributor in it as the old one had a little movement in the shaft. It has Pertronix LS module with a pertronix coil. Any suggestions.

Thanks

Does it feel like ignition break down or is it possible that the carb is leaning out causing a lean misfire? What distributor? Ford or Lucas?

FoMoCo. I'm not sure as you can rev it up thru the gears and it pulls well. Its only in top gear and only above 65mph. It doesn't do it at lower speeds in top gear when you stand on the gas.

It is a bit difficult to diagnose from afar as there can be a few things that cause that sort of thing. Did it just come on suddenly or had you made some sort of change beforehand? High speed in top gear puts a fair load on when you give it throttle, fuel demand is up and a weak spark can show its face then too. I have heard of the Pertronix stronger spark causing problems arcing through standard rotors so check that and the cap too.
As I said many things can cause it, I once had a situation where my voltage regulator caused my car to miss like hell only when I gave it a high load. I measured the voltage and it kept going up to the point where the points/condenser could not handle it, soon as I disconnected the alternator the problem vanished although it is very unlikely this is what is happening to you. Keep checking.