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TBI dual tank to Carb conversion

Discussion in 'Chevy Trucks' started by 400hp408sb, Jun 25, 2006.

  1. 400hp408sb

    400hp408sb Junior Member
    from Maine
    Messages: 25

    I did a lot of searching through other posts about this subject before I posted. I apologize if I'm beating a dead horse but my problem is a little different then previous postings. I have an '87 chevy truck. Took the TBI 350 and computer out and put in a 400 carburated motor with a mechanical pump. The truck came with factory dual tanks. Ever since the swap I have not been able to use the 2nd tank. It's been stuck using the passenger side tank since that's the one I had it on during swap. I've been using it ever since making sure the electric pump wasn't turning on and having the mechanical pump suck through the electric pump. I now have the bed off after sandblasting/painting the frame and installing more lift. So now is the perfect time to finally get back the use of both tanks.
    The switch in the dash does still cause the electric pumps to turn on depending on which tank is being selected. I can put my hand on the switching valve along frame rail and feel movement and hear a noise when a helper hits the switch in the dash. But when the drivers side tank is selected (which is bone dry) the truck stays running and doesn't die. Which would indicate that the valve is not actually moving even though the drivers side electric pump is engaged. I'm not sure how long it would take to run the fuel line from the valve to the carb dry so I drove it down the street and back and it never died. I'm not sure how I would go about diagnosing the valve and if it's working properly. I know for sure it was before the swap. I think I'll be able to disconnect power to the pump, at the pump, inside the tank. Leaving the power connected to the sending unit (once I get everything working properly). I've seen the two wiring diagram pictures posted here but they seem to be for the older trucks/valves. Anyone have any ideas for me? Any help would be greatly appreciated.
     
  2. derekbroerse

    derekbroerse 2000 Club Member
    Messages: 2,377

    You did the opposite to what I did (swapped EFI and a 383 into my carbed truck)... but I did an EFI to carb on my old S15 pickup years ago.

    What I did was leave the electric pump and install a three-port Mallory fuel pressure regulator at the carb.... one inlet, one outlet, and one return line. This allowed me to leave all the wiring alone, and I didn't have to add a mechanical pump. Also gave me full control of the pressure with a turn of the allen key.

    Sounds like the solenoid-activated valve is bad... may not be travelling all the way. Seems to me the EFI version (which uses the return lines, unlike the early style carbed ones... later carbed ones used the EFI style one too) was around $100 or so. The regular non-return line ones were available as a universal replacement from semi-truck dealers for around $30.

    I've been running (crappily) this combo for a year and a half because I haven't gotten the reprogrammed prom yet (was waiting to swap in my Vortec heads first). The EFI is so much nicer than the carb ever was... especially for winter time use.
     
  3. 400hp408sb

    400hp408sb Junior Member
    from Maine
    Messages: 25

    OK I finally got it working. I kind of ended up having a backup fuel delivery system as well. Both electric pumps in the tanks work as well as my mechanical pump. The switching valve is now working properly as well. But I now have another problem. The gas gauge is pegged past full in all key positions and either tank. Which worked fine before I replaced both gas tanks and put everything back together. I used a continuity tester to make sure everything on the sending unit is getting proper ground to the frame. I am 99.9 percent sure everything there is ok. No wires were ever touched between the dash and the switching valve wiring harness. Though I did apply 12 volts directly from the battery to some of wires at the switching valve trying to find out what each one did. I can't remember if I applied power to just the contacts in the switching valve or into the wiring harness that leads back to the dash as well. Assuming I may have somehow fried my fuel gauge how would I go about testing it once I get it out of the dash? Maybe there's something else that I'm overlooking?
     
  4. derekbroerse

    derekbroerse 2000 Club Member
    Messages: 2,377

    Did you check your grounds from the sending units to the frame rails? I think that sends it to the top..... very important that they are there, and clean and tight...