ive got a 77 f250 with a 351. when i turn the key it turns over and revs really high. when i let off the key it shuts off. then i turn the key again and it happens again. sometimes it grinds, sometimes not. if i try to rev it with my gas pedal nothing happens. then after a couple minutes of this i can rev it with my foot and it stays on. what is wrong. ive never herd of this happening anywhere and it doesnt make much sense. can anybody help me?

 

I've had an Ignition module only run when the key was in the start position. If you let off the starter it would die. But I don't know if that explains the high rev's?
Ofcourse be sure to check the linkage or cable to the carb.

 

Could be a choke or choke pull off problem. Does it run only when you are cranking the starter? Then die when you let key go back to run position? If it runs with key in run position and then dies could be a choke pull off problem. The more you try to start it the choke thermostat is electric and is heating up each time you try to start. Finally getting warm enough to open up some and let the truck keep running.

 

it only runs when the key is in the start position then eventually it will run fine what would i be looking for to see whats wrong?

 

I would replace the Ignition module. Shooting from the hip. should be $25-30.

If you have a way of checking spark while it is running, then have someone start it and let the keys go to the run position to see if it loses spark in the run position. If so, then it is either the Ignition module or a faulty Ignition switch.
There is a tool for checking spark. An inductive spark tester can be placed next to the wire or use a timing light.

You can narrow it down between the switch and module. There are two wires (white & red) between the switch and module - run & start. Both are powered in the start position and only Run in the Run position. (there's a chance only start is powered in start position - But that seems to work). But i can tell you I have replaced 10 modules to 1 switch.

Again, Not sure about the Rev's