I have a 91 Ford f150 with a 5.8 351. When I go to start my truck after it has been sitting awhile it starts fine but after it gets to about 1300rpm it drops to 500rpm and then dies. It starts up fine after that. Sometimes ill go out and it will do the same thing but when it gets to about 500 or a little more and then jump back up to 1300 until it warms up. I have no idea what this could be other than something with the carb?? The motor runs fine down the road and dosn't die any time else. Can anyone help???

 

To me,it sounds like you would need a good tune up ,including the fuel filter,PCV valve and plug wires.You might as well have the throttle body and air bypass valve cleaned also.In the worst case it could be something like a bad dist. module or even the ignition switch.Do the basics first because if you don't,the repair shop will before moving further into the problem if the basics doesn't fix it first.
Phillip

 

You might want to check for vacuum leaks. Check the hoses to see if they're cracked and replace as necessary. Make sure you have a good vacuum seal aroung your intake manifold.

Slik