Concentrate your efforts on the manifold
I have a 1975 F250 Ford pickup with a 300-6 and a Carter 1-barrel YF carburetor. Being that I use the truck for daily transportation, a sound motor is important to me. This past summer I had the opportunity to have the tired motor completely rebuilt. Actually, I exchanged the tired motor for a rebuilt one and had nearly all new components installed from the starter motor to the distributor.
However, I did not replace the rebuilt carburetor since it is only 2-3 years old. When I picked up the truck it ran exactly the same as before. Upon light throttle application at any road speed the motor kicks and bucks like a cold motor with partial choke on. In all other conditions she runs smooth and starts easily.
My mechanic could not fix it but he did say that the manifold is warped and it leaks. I resealed the manifold and I am pulling a steady 20 inches of vacuum at a high idle, with 0 inches at idle. But it still runs the same, bucking upon light throttle application.
I went to a second mechanic and had a scope job on the ignition system and it is just fine. I installed another rebuilt carburetor and it actually ran worse. Mechanic number two said that the replacement carburetor was “garbage” so I sent it back.
I rebuilt the already rebuilt carburetor and all of the adjustments are accurate including the accelerator pump. Incidentally, I found some aluminum shavings under the pump diaphragm. Now, after all that I’ve done, the engine still runs the same with the same problems.
I am getting 12-16 mpg and I have been dealing with this problem for a couple of years.
I am seriously considering the installation of a set of Clifford racing manifolds and a new two-barrel carburetor to avoid these “rebuilt” carbs.
Still, I would still like to know what the heck is going on.
I don’t understand why you did all that you’ve done and invested all of the money you’ve spent when you know there’s a problem with the intake manifold.
If you have zero inches of vacuum at a normal idle and you know that the manifold is warped why wouldn’t you fix that problem before you do anything else?
Installation of a new Clifford manifold and carburetor will likely solve your problem. An Offenhauser aluminum 4-barrel intake will also do the trick.
If you have the ambition to tackle the project, a great alternative is to scrap the carburetor altogether and convert to electronic fuel injection. Ford began putting EFI on its inline six around 1987 and installing one of the OEM systems in your truck is a fairly straightforward proposition. You can also buy a complete, bolton aftermarket TBI conversion system from Affordable Fuel Injection (affordable-fuel-injection.com).
If you want to stick with stock parts, have your original manifold milled by a qualified machinist if it’s not warped too severely. If it’s not salvageable, buy a good used one and have that checked and milled, if needed, before installation.
You state that original Ford 300 cid intakes are “very difficult to obtain” but I disagree. A very large number of these engines were made so the parts are still floating around. A quick look at the Internet shows more than a dozen available at the moment. Chances are good that you’ll readily find examples at salvage yards near you as well.