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Discussion Starter · #1 ·
When converting to a carb is there any harm in cutting the fuel wires off at the harness? If so what would the recommended procedure be? Any other suggestions in regards to wiring, such as for the distributor would be appreciated. Thank you.
 

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Right now the ECM turns on the fuel pump. You will need to wire it so that it comes on with the ignition. The ECM sends a negative signal to the relay now, so you need to cut the wire and ground it. This will make the relay stay on all of the time, so you need to cut the hot side and wire that side to the ignition. Actually, the signal from the ECM goes through the oil pressure sending unit to the relay. This is incase you lose oil pressure it would shut off the fuel pump to save the engine. I doubt it matters much with a carb because there is enough fuel in the float bowls that the engine will already be toast by the time you run out. Obviously with fuel injection once the fuel pressure stops the engine will immediately die.

You will need a new distributor. The one you have is computer controlled, and obviously you won't be using the computer anymore. Just a standard HEI will work just fine. I went with a Mallory because it is billet and far cheaper than a billet MSD. Take a look around on the internet. The MSD HEI distributors are identical to several other brands anyway except they say MSD. I didn't quite believe it until I saw with my own eyes. I'm sure their boxes are nice, but don't waste your money for the MSD name on an HEI distributor.

When I converted mine, I disassembled the whole ECM harness and traced all of the sensor and actuator wires from their connectors to the ECM. This told me what I didn't need anymore and I knew it was safe to cut them. There are two connectors at the ECM. Almost every wire you will get rid of but there are a few than you will keep or at least need to know what it goes to, like the fuel pump relay. There wasn't many, but I think I also had to rewire something for the A/C.

You will need to run an igntion wire for the new distributor, and your carb choke. DO NOT use the same wire for both. You will lose the temp sensor on the passenger side because that is the ECM temp sensor. The one for your gauge is on the drivers side and you won't have to do anything at all to keep it working. I belive the only other wires I kept for the engine was for the alternator and the A/C clutch.

You will also need to regulate your fuel pressure. The pump you have is regulated through the TBI and without the TBI it is capable of pressures well beyond what your carb will need. I already had an Aeromotive bypass regulator so that is what I used. I think there are cheaper methods out there that will work like a dead head regulator but you might want to do some research just to be sure. Cut the fuel lines just after the fuel filter and loose the connecting pipes that used to go to the TBI. They get in the way of the distributor. Run the lines to the inner fender and then a line over the engine to the carb.

I think I have some pics of what mine looks like on this forum somewhere. Just do a search. My project was (and still is) a '91 C1500 truck.
 

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I changed a TBI to carb once
I used a regular fuel pump on the block and pulled the fuse out for the fuel pump ,
The fuel pump on the block will pull fuel through the electric pump with no issues ,
I got rid of all the fuel injection stuff on top of the motor and cut the fuel line going too or coming from the fuel tank and made it go too the fuel pump on the block .
Used a stock G.M. electronic distributor and ran a wire into the fuse box into something that only runs with the key on .
I worked just fine till the van got so rough it had to come off the road .

I am not sure why you are are changing the system but I already had a rebuilt 69 four bolt main motor sitting here .
 

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406cu.in. of tire smokin' fun
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That will only work if you change the block. The TBI block is not cut for mechanical fuel pump, though for some reason they left the mounting provisions there. There just isn't a hole to the camshaft.
 

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Not sure as to when they changed about the fuel pump pushrod hole but on my 92 TBI block the hole was there and I just got the pushrod and the fuel pump mount plate off from Ebay works fine, maybe the roller blocks are different?
 

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Discussion Starter · #8 ·
my reason for the conversion is that i have to much motor for the stock throttle body and chip. I intend on using the stock fuel pump with a regulator. Thanks for the responses.
 

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I'm trying to PM pre-tuner, but this website won't let me. Can anyone that contacts this guy regularly draw his attention to this reply? Thanks!
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Pre-tuner, I'm new to these forums in here, I just did a tbi to carburetor swap, I was wondering if you could help me get the AC to work. I'm wondering if you could describe how you got yours to work, any help is appreciate. We may not have similar vehicles but you might be able to help me make sense of all I have in front of me.

I have a '93 Roadmaster wagon, I'm wondering how to trick the computer to let the AC run, I haven't cut any unused wiring harnesses portions yet, and have retained most of the sensors (all except for what was on the throttle body itself).

If this advice comes at a cost, name it. Thanks ahead of time!

I'm in the Chicagoland area, hell, maybe you're across the street.
 

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Not sure as to when they changed about the fuel pump pushrod hole but on my 92 TBI block the hole was there and I just got the pushrod and the fuel pump mount plate off from Ebay works fine, maybe the roller blocks are different?

That would be quite the oddball that would indicate that if this engine was original production that it would have been pulled off a the marine, industrial, or heavy duty truck line (GM is known to do this in a pinch) and pressed into vehicle production. The specified vehicle production engine for 1992 would be the Swirl Port LO5 for a 350. If would have a flat tappet hydraulic cam with no fuel pump lobe (there is an identically timed cam used for marine and industrial applications with a fuel pump lobe), the fuel pump provision would be on the block but except for face milling of the mounting pad the bolt holes and push rod hole would be unfinished. No LO5 would be equipped with a mechanical fuel pump as these cannot furnish the constant high pressure combined with the variable volume needed to run TBI or any EFI for that matter, therefore, standard production vehicle blocks were not finished for a mechanical pump so the block in your truck was originally intended for a different use that a pickup truck. You should check the bottom end as this may very well have 4 bolt mains and maybe a forged crank, but that would depend on where in the manufacturing process this was intercepted as a raw block perhaps none of the goodies went into the bottom end but if it was pulled as short block it may well have this stuff inside.


Bogie
 

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Please ignore my previous post asking for pre-tuners help, I just figured it out.* For those that were in the same shoes as I, keep reading.* I have a 93, I'm sure it's slightly different for other years, but there are two relays above the front passenger side wheel well, the outboard one is for the fuel pump, the inboard one is for the AC compressor. The AC compressor relay is closed by the AC demand circuit and the ECM provides a ground for the relays coil.* After the motor swap, the cars ECM doesn't want to turn the AC on cause it doesn't think the car is running regardless of it seeing a tachometer signal, all you need to do is provide the ground to the AC compressor relay (green with white stripe wire). The compressor will come to life and cycle properly.* However, the AC compressor will still run at wide open throttle and regardless of the power steering pressures switch position.*

You may want to wire a momentary switch to the throttle linkage to open the ground circuit you just added when you are at wide open throttle. I used a cherry part # e6800a0 switch, it has allot of stroke and the switch closes/opens at the beginning of the stroke, so the compressor turns off before wot.

Hopefully the post helps others.
 

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