A buddy of mine has a 83 f-150 with the 300 and manual. His problem is when he tries to accelerate past half throttle, the motor starts bucking, popping and loosing power. At first he thought it was an exhaust leak, but after going through and changing the gaskets and now even the manifold, the problem still exists.

I kinda suspect a sticking valve, but I don't know much about the 300s and how they behave. It really bothers him cuz he can't drive over 45 and it is his main transportation.

What could it be?

 

Fuel filter

 

Dave is probably right, as always. But, I've had bad plug wires do that. You might be able to tell by opening the hood and running the engine on a dark night. Really bad wires will arc under no load and you can see it. But you might get lucky and spot a problem anyway. (If it was an auto you could put in gear, hold the brake, and throttle up to simulate the load.).

 

That makes sense, I'll have him check.

 

This was my first thought too.

 

The 300/4.9L engine HATES vacuum leaks. If the fuel filter is not the culprit, look for vacuum leaks. All vacuum lines are suspect [rubber and plastic get old and brittle and crack], but what can happen is the carburetor will loosen at the base. It has two bolts holding it down. If they loosen, one gets eratic running.

With the engine off and hood opened, gently grab the air cleaner and see if you can rock it from side to side. If it wobbles, get out a 1/2" wrench and gently snug the two nuts at the base of the carburetor.

Sometimes, the top half of the carburetor will loosen from the bottom [throttle body]. There are either 3 or 4 screws holding the two halves together. You must remove the carburetor, turning it upside down, to tighten these. If it comes to this, be sure to replace the gaskets between the intake manifold and the EGR adapter and between the EGR adapter and the base of the carburetor.

Good luck, this is not difficult!