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deleting pcm what is needed for carb motor

Angie

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Hey all, not sure how to word that "title" however, I need to know what, if anything, is needed to keep from the PCM on a swap 1991(ish) to carb.

I will be running electric fuel pump, with separate on/off switch, relay. (and inertia switch)

keeping all these electrical items (as in) staying with the original wire harness as possible.

All lights, turn signals, horn. Keeping the fuel gauge. Heater, wipers ...

Custom placing tach, volt meter, temp. back lights in dash.

Deleting the 4L, and trans. Placing 351C and FMX so deleting all computer for engine/trans functions.

so basically I am asking what wires/circuits do I need to keep from the PCM.

cheers
 


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franklin2

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Best way to do this without thinking too much and looking at diagrams too much, is to start at the computer. Take it out and unplug the large plug. Snake the plug through to under the hood. Anything connected to this large plug and the wiring leaves. This will be mostly sensors, wiring to the dist, the injection system, etc. You will have a couple of wires that feed electric power to the PCM relay and the fuel pump relay. You can just snip those off. You will have to re-wire the original electric fuel pump if you are going to keep it, since the computer turned it on and off originally.
 

Angie

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Interesting way to attack this. thanks for the ideas. the fuel pump won't be used as the in tank has too much pressure for a carb and I don't want to go down the "in tank" route with this. however, that did give me an idea as to wiring in the "new" pump, just use the original key on start system, to a relay. thanks for that.

I will do as you suggest and retrace the harness back.

cheers
 

RonD

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You can remove all the PCM wiring
You keep the dash gauge wires
Temp sender(not sensor)
Oil pressure
Alternator wires including battery light wire to dash(green one)
These can be mixed in with engine wiring harness but don't go to the PCM

Also the START wire, red/blue comes from ignition switch via safety switch
You can use the PCM relay to provide key on power for spark system and fuel pump relay, and other key on power requirements in engine bay, ignition switch activates this relay with key on, but not in ACC

You can add an oil pressure switch to GROUND the fuel pump relay after engine starts and there is oil pressure, safety device, inertia switch is OK, but this is better
Carb has fuel to start the engine in the float bowl, so fuel pump can start AFTER engine does

Oil pressure switches, for this purpose, were very common before EFI and when electric fuel pumps were first being used
EFI needs the pump to build pressure FIRST, for start up
You can still have an on/off switch but not really needed if you have the oil pressure switch, the switch closes(grounds the relay) above 5psi pressure so just cranking the engine will usually start the pump, in case you run the carb dry, lol

You need a vacuum advance Distributor
 

55trucker

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The last time I saw a vacuum distributor he was knocking on the front door wanting to sell us an Electrolux. We didn't bite.
 

alwaysFlOoReD

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Hmmm.... it was about 40 years ago I spent a few days training to be a Filter Queen sales rep. Gave up after a week of no sales. Glad I did.
 

55trucker

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Hmmm.... it was about 40 years ago I spent a few days training to be a Filter Queen sales rep. Gave up after a week of no sales. Glad I did.
74767
:icon_rofl:

Is this guy you?.........
 

alwaysFlOoReD

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Not far off...lol.
 

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You can add an oil pressure switch to GROUND the fuel pump relay after engine starts and there is oil pressure, safety device, inertia switch is OK, but this is better
Carb has fuel to start the engine in the float bowl, so fuel pump can start AFTER engine does

Oil pressure switches, for this purpose, were very common before EFI and when electric fuel pumps were first being used
EFI needs the pump to build pressure FIRST, for start up
You can still have an on/off switch but not really needed if you have the oil pressure switch, the switch closes(grounds the relay) above 5psi pressure so just cranking the engine will usually start the pump, in case you run the carb dry, lol
When Ford used a system like this it was common on the 460 equipped trucks with a carb and was called a "hot fuel handling" option. They used the oil pressure switch thing with a relay AND had the crash switch with it too. They ran a wire from the starter solenoid "i" terminal to power the fuel pumps directly during cranking. Then the oil pressure/relay system took over after that.

If you start out with a dry carburetor, it's going to take a lot of cranking to finally get it going.
 

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if your truck has a return line for hte fuel injection, you can use the intank pump fine with out it over powering the float in your carb. i have done it on bmws and on jeeps. just hook the return line to the carb feed line with a T after the regulator and it won't push past the needle/seat assembly. i ran webers, which are very low pressure, like 2.5 psi and it worked fine on a bmw intank pump for hte inline 6 motor.

when i swapped my 4.5 stroker in the jeep to a four barrel holley, it ran even better since holley can handle more pressure.

a pressure regulator dead headed from the fuel feed line with no return line will not let you run a carb on the stock in tank pump though. it will give you problems nonstop.
 

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superj,

can you draw the fuel circuit you suggested and post?
Thanks in advance.
 

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if your truck has a return line for hte fuel injection, you can use the intank pump fine with out it over powering the float in your carb. i have done it on bmws and on jeeps. just hook the return line to the carb feed line with a T after the regulator and it won't push past the needle/seat assembly. i ran webers, which are very low pressure, like 2.5 psi and it worked fine on a bmw intank pump for hte inline 6 motor.

when i swapped my 4.5 stroker in the jeep to a four barrel holley, it ran even better since holley can handle more pressure.

a pressure regulator dead headed from the fuel feed line with no return line will not let you run a carb on the stock in tank pump though. it will give you problems nonstop.
Not saying that tee-ing in a return line wouldn't work... because apparently it does. I've never even thought of doing that. It seems a bit sketchy. Maybe with a pressure regulator with a return. Did you use any pressure regulator? How did you set fuel pressure?

When I did my carbed 302 swap... I just drew fuel though the in-tank pump with a Holley Blue to a quality regulator then to the carb.
 

superj

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i had a fuel gage plumbed in, originally, so i could see when messing with the regulator. in later versions, i took the regulator off too since the return line worked so well. i had a holley regulator , than a T to the return line, then the gage, then the carb inlet. you can also put the regulator after the T but you will see the pressure is so low. after the T works best on the weber because of how sensitive it is though. but, use a carb regulator that gets down to 0-12psi (whatever the are, the low pressure ones) for the weber. if it doesn't go below 5psi, your weber will run rich because the gas will keep pushing past the needle

final versions were feed line, T, pressure gage, carb inlet. you still might have to keep the regulator on the weber since it runs at such a low pressure. on the holley, it can handle more so no troubles there.

my wrangler had a weber 32/36 on it when i bought it but it always had trouble. turns out the carb was not a real weber so it would always diesel and lack power. i had a real weber in the garage from my 79 bmw that i swapped from fuel injection to carb and had bought up a few webers on ebay as back (back when deals were had on ebay), so once i swapped one on, it was better but the jeep felt under powered. i converted the carb from progressive to synchronous but it still felt weird so i considered going to a weber 34/34 but decided since the engine was so close in size to a ford v8, i would just get a motorcraft. but, since my offenhauser intake was set up to fit a four barrel, the motorcraft carb would hit the bottom of the hood with the adapter (weber was on the stock jeep intake). so while ebaying, i came across a holley from a 68 or 69 mustang 302 and just bought that. it was perfect. the jeep would spin the tires easily and i gained about 6 miles per gallon on the highway and lost the dieseling. luckily, through it all, the old jeep tf999 kickdown was easily able to be adapted to each carb.
 

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On the system Ford used on the 460's, they used the Tee method also. But in their tee they had a orifice in the fitting to control the amount of return.
 

smilinjack

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OR..... just yank the tank, install a fuel cell in the bed, and plumb in a Holley 7 psi pump w/ filter and live happily ever after :cool:
 

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