I purchased a 1984 F-150 2 weeks ago (a 351w with California emissions). I've spent the past many days attending to its many issues (replaced all rubber fuel lines/fuel pump/front brake calipers/brake master cylinder/ALL vacuum lines/battery/starter solenoid/ignition switch/smog pump & air diverter valve/throttle cable/ignition control module/accelerator pump/pcv valve/air filter/breather filter/hot air tube/cold air tube/and on and on).

I am now able to start it and let it run at an idle of 750 rpm for more than 45 minutes. I can rev it slowly or quickly just fine. I can shift from park to reverse and drive, and back and forth just fine. I can even put it in gear while holding the brake and press the gas and feel the truck pulling forward or backward.

What I am NOT able to do is drive it uphill. It just lacks power. If I put it in reverse and give it any gas it backfires, only in reverse. On flat road it will accelerate (not NEAR the power it SHOULD have) and if I floor it, it just bogs down.

If I'm just sitting there letting it idle, and I pop the accelerator to the floor for a split second, it backfires, and then runs like crap. It continues to backfire and the idle drops to 500 rpm and lower and lower until it dies.

I've read about an anti-backfire valve or gulp valve, but have no idea as to its location or appearance.

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My first question would be, is there fresh gas in it?
second, have you checked the timing?
It sounds like the timing is retarded, below TDC.

 

When I replaced all the fuel lines the tanks were empty. The truck sat in the desert for seven years with dryrotting rubber. After replacing the lines I put ten gallons of gas in the tanks (5 per tank) and a can of seafoam in each as well. After I got it running halfway decent I took it to the gas station and filled the front tank. I'm only running off of the front tank for now.
As for the timing, I can sit there and rev it up slowly or moderately fast in park and it runs beautifully. If the timing were off I'd think it would act up constantly, not just in gear. If I'm incorrect on this, please fill me in.

 

This truck actually has an electronic timing advance, not vacuum. I've already replaced the ignition control module.

 

Timing could still be out, engine is not under a load at idle, in park.
I would check timing.

What would it hurt?
You know for sure it's not a problem if it is correct. You correct it if it is not.

 

Point made and taken . I will check the timing second thing in the am. First I'm putting in transmission fluid. It's so low it's not even reading on the dipstick.wow, the things I'm finding out of whack!

 

84 was the change over year for TFI. You can find both, depending where the truck was built, and the build date

 

I think the truck has some kind of sensor in or under the seat. When I'm under the hood or standing around the truck it runs great. As soon as my butt hits the seat it a 50/50 shot. It'll run great for a few minutes (idle great, accelerate great) then it'll run like crap for a few minutes (stalling,backfiring, no power)......

I've replaced all rubber hoses except the cruise control, and I've repaired/replaced all vacuum lines except two, and I plugged them. I've followed the vacuum diagram and although it's running WAY better, I can't take the truck out on the road.

 

What happens if you run it in drive with the park brake on?Or have someone sit in it and hold the brake while in gear?

 

If I sit in it holding the brake and press the gas it does pull, but the harder I accelerate, the less it pulls.

I was talking with an old timer at work yesterday, and when I described the symptoms he said that my "power valve" was bad. Here is the old power valve....

Now with the new valve there is just an intermittent idle control problem. When driving, sometimes it will return to correct idle when at a stop sign an sometimes it won't and gets lower and lower until it dies.

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The crud on that ... Do a full rebuild on the carb .
Set your first adjustments at 1 and 1/4 turns out from stop .
Tune from there ......

 

Oh man..............I been down this road before.
If you are going to rebuild the carb, a word of advise.............
From the pic it appears to be a Motorcraft 2150A 2bbl electronic Computer feedback model carb, made to work with gen1 OBD and TFI ign.
I have a litany of experience trying to cope with these damn electronic POS's on my old 84 Ranger with the 2.8 V-6, I had ZERO luck trying to dial it in on a system that all the sensors died on a 1/4 century ago, and no one knows how to tune anymore.
My only advise is to do a Dura-spark ign. swap and use a std. non-computer feedback 2100/2150 2bbl carb from a 70s Ford V-8 and never look back.
Tuning will be as easy as John describes, and performance will be vastly improved.

 

Will do....