Loss of power when driving

Is it really running rough/ smoking/ etc. While trying to accelerate? Investigate a Stopped up Catalytic Converter.--The Medic

Any other observations? Does your Check Engine light work at start-up. Is it/ Has it been on constantly?--The Medic

Attempting a solution for your problem. Trying to get you one soon, as to get you ridin' again. I did leave a message for another Expert to assist, If I cannot come up with a solution soon.---The Medic

For your idle problem

NOTE: A change in idle speed occurred because of a problem elsewhere. Enter this procedure after eliminating the possible causes listed below:

Contamination within the throttle bore and/or idle speed control device.
Throttle sticking or binding.
Engine not reaching operating temperature.
Vacuum leaks (air intake, vacuum hoses, vacuum reservoirs, power brake booster, etc.)
VERIFY:

Transmission is in PARK (A/T) or NEUTRAL (M/T).
Parking brake is APPLIED (automatic brake release is DISCONNECTED where applicable).
Wheels blocked.
Cooling system is filled to proper level.
Engine is at operating temperature.
All accessories (heater, radio, lights, etc.) OFF.
Throttle lever is resting ON the throttle plate stop screw.
Ignition timing is set to specification.
Perform EEC-IV diagnostics and resolve any vehicle malfunctions that are indicated by service codes. (Ignore this step if sent here from an EEC-IV Pinpoint Test step).
Engine OFF, disconnect the negative (-) terminal of the battery for 5 minutes minimum, then reconnect it. NOTE: Memory to radio, phone, and other programmable accessories may be erased when the battery is disconnected.
Idle Speed Setting

PROCEDURES

With engine OFF install specified feeler gauge between throttle plate stop screw and throttle lever. Automatic transmission.030 inches, manual transmission.030 inches.
Unplug SPOUT line and verify that ignition timing is 10°+/- 2°BTDC.
Disconnect idle speed control-air bypass solenoid.
With transmission in NEUTRAL (M/T) or PARK (A/T):
Allow engine to idle for 2 minutes.

Check/adjust idle rpm: Turn the throttle plate stop screw to 780 +25 rpm automatic transmission, 730 +25 rpm manual transmission.
NOTE: If you must turn the throttle stop screw IN, turn engine OFF, make estimated adjustment. START engine and repeat Steps 4, 5, and 6 before continuing.

Remove feeler gauge from throttle plate stop screw and throttle lever pad.
Reconnect SPOUT line.
Turn engine off and reconnect idle speed control-air bypass solenoid, verify the throttle is not stuck in the bore and linkage not preventing throttle from closing.
START engine and stabilize for 2 minutes then goose engine and let it return to idle, lightly depress and release the accelerator. Let engine idle, if problem exists, see Diagnosis by Symptom (Diagnostic Routines.)
NOTE: A condition may occur where the engine rpm will oscillate. This can be caused by the throttle plates being open enough to allow purge flow. To verify this condition, disconnect the carbon canister purge line and plug it. If purge is present, the throttle plates must be closed until the purge flow induced idle oscillations stop.

On Automatic Overdrive Transmission (AOD) applications check TV pressure adjustment, See Automatic Transmissions

Won't accelerate over 25mph-check and test the Airflow meter/sensor and throttle position sensor also check he fuel pressure -

**** What vacuum modulator did you replace -is for the tranny? Could very well be the transmission slipping

Check fuel pressure with a gauge auto parts rent it. Also check vacuum at intake if 15" or below at idle then suspect clogged exhaust. Fuel pressure should be 30-45 psi with key ononly

Good luck!

How did it run with the mechanical pump? If not good, your carb may need a new accelerator pump. This gives a shot of fuel for take off. Which carb is on it and I will see if I can locate a diagram.
Here are the specs and testing info for the fuel pump. Mechanical, you electric should be set to deliver this pressure.
FUEL PUMP
NOTE: 5.0L MPFI models use electric fuel pumps and 5.8L 4-Bbl. Models use
mechanical fuel pumps. On 7.5L 4-Bbl. Models, the Hot Fuel Handling
application uses in-tank mounted electric fuel pumps. All other
applications use mechanical fuel pumps.
Mechanical Fuel Pump (Volume Test)
On 5.8L and 7.5L 4-Bbl. Models, remove air cleaner. Slowly disconnect fuel line at filter. Using
a suitable container (one pint minimum), place at end of disconnected fuel line. With high
tension wire disconnected from coil, crank engine 10 seconds. If fuel flow is within
specification, proceed to pressure test. If not, proceed to next step.
1.
If fuel flow is low, repeat test using remote vented can of gasoline. Remove fuel pump inlet
hose. Connect a length of fuel line hose to fuel pump inlet and insert other end into remote can
of gasoline. If fuel flow is within specification, problem is plugged intake fuel filter or a kinked,
leaking or plugged fuel line or hose.
2.
Mechanical Fuel Pump (Pressure Test)
Remove air cleaner. Connect suitable pressure tester to end of fuel line. Start engine and let run for at
least thirty seconds. Read pressure and if not within specification, replace fuel pump. Reinstall air
cleaner.
Electric Fuel Pump
On 5.0L MPFI models, disconnect fuel return line at fuel rail. Connect hose to container. Connect
pressure gauge at diagnostic valve on fuel rail. Disconnect wiring connector at fuel tank and apply 12
volts to pump with jumper wire.
On 7.5L 4-Bbl. Models, disconnect fuel line just before the vapor separator unit. Connect a hose from
fuel line to a suitable container. If fuel flow is not within specification, electrically check fuel pump.
If still not within specification, replace pump-sender assembly.
FUEL PUMP SPECIFICATIONS
Application Specification
Electric Fuel Pumps
5.0L MPFI
Pressure (1) 39 psi (2.7 kg/cm2 )
Volume (1) 1 pint (.5L) in 30 seconds
7.5L 4-Bbl.
Pressure (1) 4.3 psi (.30 kg/cm2 )
Volume (1) 1 pint (.5L) in 20 seconds
Mechanical Fuel Pumps
Pressure 6.0-8.0 psi (.42-.56 kg/cm2 )
TUNE-UP - V8 -1986 Ford Pickup F250 Page 1 of 2
4/28/2010
Application Specification
Volume 1 pint (.5L) in 20 seconds
(1) Delivery capability averages.

TUNE-UP - V8 -1986 Ford Pickup F250 Page 2 of 2
4/28/2010

That was my first thought, but it might also need a mixture adjustment, fully warm motor should have a fully open choke. How old is the choke thermostat?

Yes an aftermarket brain is available for this truck. Also, the fuel filter should be along the frame rail. Also, there is only one fuel filter.

Based on your discription, it sounds like the catylatic converter may be plugging. Have you checked it?

Any black smoke from the exhaust? Could be a vacuum leak or mixture adjustment. Is the carb a carter? holley?
Post a photo if you can. Also check the mounting gasket and hold down nuts, if the base gasket is cracked or loose, you will NEVER get it adjusted right... Snug the nuts down evenly, but not excessively. Here's a few simple things to check, Float is important and can be adjusted on Holley carbs without taking it off or disassembling it...
FLOAT LEVEL - WET 1. Start engine until it reaches normal operating temperature. Place vehicle on a flat surface. Remove air cleaner, if installed. Run engine at 1000 RPM for 30 seconds to stabilize fuel in carburetor. 2. Stop engine and remove sight plug from primary side fuel bowl. Check fuel level. Fuel level should be at bottom of sight plug hole. If fuel spills when plug is removed, lower fuel level by turning adjustment nut clockwise. 3. If fuel level is below sight plug hole, raise fuel level by turning adjustment nut counterclockwise. Turning adjustment nut 5/32" (4 mm) will change fuel level about 1/32" (.79 mm). 4. Tighten lock screw and install sight plug. Start and run engine at 1000 RPM for 30 seconds. Stop engine, remove sight plug and check fuel level. If necessary, adjust fuel level. 5. If fuel level is at bottom of sight plug, install sight plug using a new plug gasket. Repeat procedure for secondary fuel bowl. Secondary throttle MUST be used to stabilize fuel level in secondary fuel bowl. ACCELERATOR PUMP LEVER 1. Place throttle valves in wide open position. Using a feeler gauge, measure clearance between lever adjustment screw head and pump arm with pump arm manually depressed. See Fig. 3 . 2. To adjust, loosen adjustment screw lock nut. Turn adjustment screw in to increase clearance, and out to decrease clearance. Each 1/2 turn of adjustment screw equals .015" (.38 mm). Tighten lock nut. Fig. 3: Accelerator Pump Lever Adjustment
NOTE: Each 1/2 turn of adjustment screw equals .015" (.38 mm). ACCELERATOR PUMP STROKE Accelerator pump stroke has been preset at factory. Setting should not be changed. If original setting has been changed, adjust as follows: Check that plastic accelerator pump cam is aligned with correct hole (top or bottom) in throttle lever. Plastic accelerator pump cam is located behind throttle lever. If not aligned with correct hole, remove screw. Reposition in correct hole. Install and tighten screw. SECONDARY THROTTLE VALVES Hold secondary throttle valves closed. Turn secondary throttle shaft stop screw out until secondary throttle valves seat in throttle bores. Turn screw in until it just contacts secondary throttle valve lever. Turn screw in an additional 3/8" (10 mm) turn. CHOKE PULL-DOWN 1. Place carburetor on a stand which will allow access to pull-down diaphragm vacuum passage on underside of throttle body. See Fig. 4 . Mark choke cap and choke housing for adjustment reference. 2. Using a hacksaw, carefully cut a slot in head of choke cap breakaway screws. Using a screwdriver, remove breakaway screws and conventional screw. Remove retainer, choke cap, and gasket. 3. Temporarily install choke cap and gasket. Line up marks made prior to disassembly and rotate cap 90 ° counterclockwise from this position. Secure choke cap with one screw. 4. With choke plate fully closed, actuate choke pull-down diaphragm by applying at least 17 in. Hg of vacuum. Using a drill or gauge, check clearance between lower edge of choke plate and air horn wall. 5. If adjustment is required, carefully remove diaphragm adjustment screw cap with a small punch or screwdriver. See Fig. 4 . Using a 5/16" Allen wrench, turn screw clockwise to decrease clearance and counterclockwise to increase clearance. 6. Maintain vacuum applied to diaphragm during adjustment. Cycle vacuum from 0 to 17 in. Hg to verify adjustment. When adjustment is correct, apply RTV sealant to adjustment screw cavity and check fast idle cam position. Fig. 4: Choke Pull-Down Adjustment FAST IDLE CAM POSITION 1. Using a hacksaw, carefully cut a slot in head of choke cap breakaway screws. Using a screwdriver, remove breakaway screws and conventional screw. Remove retainer, choke cap, and gasket. 2. Temporarily install choke cap and gasket. Line up marks made prior to disassembly and rotate cap 90 ° counterclockwise from this position. Secure choke cap with one screw. 3. Apply vacuum to choke pull-down diaphragm and cycle throttle. Fast idle speed screw should rest on top step of fast idle cam with throttle closed. See Fig. 5 . 4. If adjustment is required, turn adjustment screw clockwise to position fast idle screw higher on second step. Turn counterclockwise to position screw lower on second step. 5. Remove temporary choke cap and gasket. Install original locking gasket, choke cap, and retainer. Secure choke assembly with 2 breakaway screws and conventional screw. Fig. 5: Adjusting Fast Idle Cam Position CHOKE UNLOADER Hold throttle valves wide open. Apply light closing pressure on choke valve. See Fig. 5 . Measure choke unloader clearance between lower edge of choke valve and air horn wall. To adjust, bend pawl on fast idle cam lever.