I recently performed the mod on this 85 F-150. Let me tell you, it was a learning experience. So much a veteran am I that I just jumped right into the mod without asking about the performance before.
This vehicle has the first gen electronics. A carburetor with a air valve and a TPS. I had ever seen such a thing. So, I just did the mod and reinstalled it. When I restarted the engine, it was spitting and sputtering and shaking like a dog in cold bath water.
I spent three days with this truck.
Here's the scoop: In (partially) solving for this issue, I made a RE-discovery. It turns out the intake manifold gasket was S-H-O-T. When I sprayed it with carb spray, I got only a very slight uptake in idle speed. It was only after going to three shops to ask for advice, an old-timer (like me) got his gas can with a tiny spout and poured a slight dribble in the area.
The engine sprang to life!
I have told many of you time and again that when something isn't right after performing the mod, vac leaks can occur anywhere. If, after you've fixed all the vac line and diaphragm leaks and the engine still doesn't respond correctly, the brake booster AND the intake gasket should be checked. The booster can be pumped down. The intake cannot.
So, how do you check an intake gasket?
You put a ratchet (in this case a wrench) on the bolts and tighten them. When I did this, in the area where the gas was poured (behind the carb), I found two bolts that weren't even hand tight, with those at the ends being perfect.
What happens during normal operation is the gasket decays over time, and part of the material falls away. This leaves a gap between the intake manifold and the head. This gap causes the bolts to appear loose.
Anyway, once the gasket was tightened the engine immediately smoothed out and began purring like a kitten in your lap. The emissions dropped like a rock and when I took it out for a spin, spin I did! It will rip the tires and accelerates like no tomorrow.
And the moral of this story, my friends, is this: No matter how many mods someone has done (I've done more than 1500 now!) you must ALWAYS remember the basics.
VACUUM VACUUM VACUUM is almost ALWAYS the cause of failure to respond appropriately.
Just because you haven't been able to find it doesn't mean it ain't there!
And always remember that if you can't figure it out, there is always someone to turn to for advice. The old-timers are your best resource so make friends with some of the older mechanics in the area.
60+ is the best range. Younger mechanics tend to like being the authority. The older guys tend to be more laid back, and will be happy to pass along some of their knowledge to you, young whippersnapper!
PS: The customer has been advised the intake manifold gasket still needs to be replaced. I also recommended replacing the electronic ignition and carb to an old-style. From there, I will take that truck to more than 25 mpgs.