tbi to carb , vacum gage reading / perfomance help

Discussion in 'Engine Performance + Maintenance' started by efipaneltruck, Jun 30, 2020 at 1:15 AM.

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  1. efipaneltruck

    efipaneltruck Newbie

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    hi there
    i did a carb swap on a tbi . i stripped down everything...computer wiring etc..it was a real mess before...now i can see clear

    carb is a 650 cfm edelbroke
    it start and idle perfectly...start even better than before
    before that , i did a compression test ( wet and dry )and all cylinders are good

    with a vacuum gauge test
    at idle it get 21 but the needle is not steady , it kind move vibrate slightly between 20 and 21

    with the help of someome pressing gas pedal and keeping it at 3000rpm range
    the needle goes to 23 and vribate a little more between 22 and 23

    the truck feel good around the block but when i get on freeway and or up hill . it doesnt have much power

    any idee?

    also i have a fuel pressure regulator an holley hly-12-881 with a gauge
    when engine cold, i start it , pressure is 6psi
    engine get warm and it end up just under 5psi
    is that normal ?
    should i use the vacuum port at the fuel pressure regulator?
     
  2. RawbDidIt

    RawbDidIt I'm Awesome

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    Carbs need between 6 and 8 psi, but I'm wondering if that might be chalked up to the gage being off, get a rental from the parts store and double check your initial findings. Low pressure would account for lack of power during full throttle applications. Not sure where the reference pressure for the regulator pulls from, recommend reading the instructions.

    Sent from my SM-N976V using Tapatalk
     
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  3. Busted Knuckle

    Busted Knuckle OBS Enthusiast

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    Yea... The reason it had TBI is because in overdrive you will not have enough vacuum to pull fuel out of a carb. And before the flow stops completely, you will be operating in enrichment mode. It should run fine by NOT using overdrive. What is your trans controller ?

    You also have the issue that without the ECM the torque converter clutch will not lock in OD and you will burn up the converter, pump, or something else.

    And another one, the in-tank fuel pump cannot be dead headed or it will eventually fail. They are made to recirculate excess fuel back to the tank. You can piece together a recirculating system and just tap into that with your holley reg. The internal spring and diaphragm in your TBI is what set the circulating fuel pressure (around 13 -15 psi if I recall).
     
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  4. RawbDidIt

    RawbDidIt I'm Awesome

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    Interesting points. I was under the assumption that the fuel pump was replaced with an inline pump designed for carbs.

    Also didn't know any of the things you mentioned about overdrive or the problems it could cause with a carb setup. Thanks for the knowledge.

    Sent from my SM-N976V using Tapatalk
     
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  5. Erik the Awful

    Erik the Awful I'm Awesome

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    You don't need torque converter lockup, it's just for fuel mileage. Transmissions before the 700 didn't have lockup and ran just fine. If you want torque converter lockup, it's fairly simple.
    https://www.hotrod.com/articles/hrd...-install-a-700-r4-4l60e-trans-carbureted-350/

    Where does that info come from?

    That's a bypass FPR. You're fine with it so long as you have it plumbed with the fuel return.
     
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  6. efipaneltruck

    efipaneltruck Newbie

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    yes i plumbed it with the return line



    any idee about the vacuum gage needle ?
     
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  7. evilunclegrimace

    evilunclegrimace Does not always play well with others

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    That is not true, there are plenty of carburetor equipped engines that had over drive and ran just fine. TBI was introduced because it is able to control air/fuel mix MUCH better than a carb.
     
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  8. Busted Knuckle

    Busted Knuckle OBS Enthusiast

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    "That info" comes from the fact I was a designer for the electronics at GM, and that's one of the main reasons to switch to injection. (Others: No choke, improved fuel accuracy, throttle response, ability to cut fuel off entirely, etc)

    The trans can't be operated in OD without TCC lock or it will overheat.
    The trans can't tow anything without TCC lock, even in 3rd, or it will overheat.
    Even with TCC lock, the trans can't tow in OD until 1997 when the lubrication at the tailshaft was improved.
     
  9. Schurkey

    Schurkey I'm Awesome

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  10. Busted Knuckle

    Busted Knuckle OBS Enthusiast

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    What exactly are you concerned about ? Those readings seem OK. The needle is always shaky. And the bigger your cam, the less vacuum and the more shaky it gets.
     
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