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Old 06-02-2012, 07:12 PM
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TBI to carb conversion wiring?

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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Old 06-02-2012, 09:22 PM
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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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Old 06-02-2012, 09:28 PM
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Thank you for the detailed reply, I already a dist. and regulator. I would say that you have answered any questions i had. Thanks again.
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Old 06-02-2012, 10:52 PM
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my question is why? are you planing on nagging more part than a large tbi unit can support? if you're not then it doesn't make any sense to get rid of it.
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Old 06-03-2012, 08:03 PM
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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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Old 06-03-2012, 08:10 PM
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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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Old 06-04-2012, 04:17 PM
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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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Old 06-05-2012, 12:07 AM
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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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