My 79 f150 started backfiring & now wont start

How many miles on her? And that's the 351 M 400?

It is possible that the timing chain has jumped time or broken. You will need to do a compression test to determine if this is the problem. How many miles on it. Have there been any recent repairs.

What ever you find you need to access if it is even worth fixing. I had a couple of 351 M 400s and took good care of them. Sorry to say they only go 115,000 miles before needing major repair. They are junk. At the time, any one I knew who had one they had problems at the higher mileage. Now a days 100,000 miles is nothing. But were talking the 351 M 400, Not the 351 Cleveland motor.

It is a 351M that was rebuilt at 300k, now only has maybe 20k on it, the OD isn't working. When it was rebuilt I had a double roller timing chain and gears installed, I am hoping its not that.

Well then lets get-err going, listen to Bobs input, very knowledgeable, I hope you have it somewhere so you can work on it. I myself seen this once before, and it had to do with timing. Do you know how to work on it? or have a hand. I would look at timing first, #1 TDC on compression stroke, see if rotor is pointing at #1 plug wire. It should be there. If not I have seen where the pin on the distributor gear shears, so would need to pull distributor out and inspect the gear. Let us know from there, and we'll continue to help.

Well, with a double roller timing chain and gears 20,000 miles ago, that leads me to think something else is causing the problem. Backfiring is usually a symptom of a timing issue, but not always. As Rowefast said maybe something in the distributor. 351M was a great engine that would go many miles if properly serviced. The only thing bad I remember about them was that they used lots of fuel. Keep us posted on what the repair ends up being.