Sputtering at heavier throttle input, Low Idle

Discussion in 'Troubleshooting & Diagnosis' started by MP81, Aug 20, 2019.

  1. MP81

    MP81 Veteran Member

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    Check engine light goes on for a second or two after starting then goes out - same as it always had, back when it didn't have any running issues.

    Do these things have a fuel filter? I'm thinking not, as I do not remember seeing one anywhere, but I figured it'd be a good place to start if it does.

    If not, I think I should start with easy things - warm the car up, then pull the filter off and watch for streams to see if the accelerator pump is working, correct?
     
  2. RickM

    RickM Veteran Member

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    Your fuel filter is in the carb, right where the fuel line connects to the carb.

    If you rebuild your carb just replace the filter and be done with it.

    Rebuilding the CCC carb requires a few special tools to do it correctly.

    After the rebuild you need a dwell meter and a tachometer to dial it in.

    You will be looking for a varying dwell at idle and a steady dwell around 30 degrees at 2500-3000 RPM's

    Adjustments are made at 5 places:
    • The Idle Air bleed valve
    • The Rich Stop adjustment
    • The Lean stop adjustment
    • The Idle Mixture adjustments
    • There is also a throttle position sensor adjustment that can be checked without the engine running.
    If you are careful the and dont mess with the Rich Stop and Lean Stop adjustment screws you should be able to dial things in with just the Idle Air bleed and Idle Mixture screws (2) at the base of the carb.

    All of the adjustments need special tools. The carb tools are getting harder to find, look on ebay for Thexton brand CCC carb adjustment tools. You can get a tach/dwell meter anywhere.

    Any 81 service manual will have all of this info.

    Good Luck
    RickM
     
  3. MP81

    MP81 Veteran Member

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    So I'm going to have an extra pair of hands/eyes today and I'd like to take a look, visually, at this.

    Whether or not the carb needs a rebuild (it does - it's 38 years old, and I don't think it ever has had one), I feel like there should be some specific thing causing this, since it happened all on its own this year, and had never happened before. I'm an engineer, so I need some real proof it is the carb (or if not, something else), before I pull the thing off to race to rebuild it.

    I have to imagine if the O2 was bad, I'd be getting a check engine light, but I don't. I'm definitely expecting to focus a lot on the accel pump on the carb side - from what I've read, the best way to test that is to warm the car up for about 5 minutes (mainly to make sure all the bowls are full), then turn the car off, and open the throttle. I think it said something about watching the streams of gas. While playing with the throttle, what else should I be paying attention to? It just seems so odd that it "feels" rough (and doesn't really go anywhere) for a few seconds, then all of a sudden, smooths out and pulls much harder. To me, that means something is at fault, not just a carb that is old.

    I've also read that perhaps checking the dwell on the carb, as is, could tell a few things?

    I'm just trying to exhaust every avenue before taking the thing off, especially if it doesn't fix it.
     
  4. MP81

    MP81 Veteran Member

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    All right, so I'm going to try and get my observations from today down with some associated pictures before I head to bed, and if I need to expand more tomorrow I can.

    First what I did was play with the throttle before even starting the car - last time it was started was a week or so ago, so the main bowl wasn't full. What I noticed were slight "fuel bubbles" coming from this kind of "crevice" here in the corner between the gasket and the raised area around the choke flap. You could hear it and see it "lightly" bubbling there when you'd open the throttle.
    P1130310.JPG
    P1130313.JPG

    With the car running and the choke flap fully warmed up, it looked like I was getting a good solid spray in the primaries:
    P1130317.JPG

    After revving it, the car seemed to only want to idle at like 500-600 rpm, lower than it should be, but it stayed there and didn't stall, but it was a little odd.

    I noticed that this rod was a bit wet where it goes into the carb body:
    P1130322.JPG
     
  5. TheFastForester

    TheFastForester Veteran Member Lifetime Gold Member

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    The old accelerator pumps have a black rubber ring on them that gets degraded with alcohol-based fuels. Time for a carb rebuild.
     
  6. RickM

    RickM Veteran Member

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    Rebuild your carb and change your O2 sensor and see what happens. Your carb has already been serviced in the past since the idle air bleed valve plug has been removed. Therefore I would not trust how the rich stop and lean stop adjustments are set. Your idle adjustment screws have likely had their plugs removed also. I would want to go through all of the adjustments to be sure. That means checking with a dwell meter to be sure you have a varying duty cycle. That will verify your computer is seeing the O2 sensor and adjusting the carb to the 14.7:1 air/fuel ratio.

    It really is not that complicated if you have the special tools and a dwell meter. The special tools just allow you to make the adjustments to the rich stop and lean stop settings when you rebuild the carb and to make adjustments with the car running. If you are careful when rebuilding the carb you may not need them. But it is likely, especially if you change the O2 sensor that you will need them since the new sensor will not supply the computer with the exact same values as the old sensor.

    Once you get the CCC carb dialed in it will run beautifully for a long time. GM did a really good job with their first attempt at a computer controlled carb.
     
    Last edited: Aug 29, 2019
  7. MP81

    MP81 Veteran Member

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    So that's why I'm seeing the wetness when it comes back up. Makes sense.

    What plug are you speaking of? To my knowledge, this thing has never been off the vehicle or rebuilt. I didn't think that large screw there (the Idle Air Bleed Valve) had a plug - every image I've seen shows it wide open. The smaller hole (for the MCS screw, I believe) that's further down the carb body there still is blocked off. The idle adjustment screws are the two that are on the bottomish area of the carb, yes? Those still have their plugs as well.

    Looks like I need the Thexton 370 and 380 tools?

    I saw someone mention an "OTC Monitor 85" on a CCC thread on a Corvette forum regarding adjustment and what not. Is that something useful to look for?
     
  8. RickM

    RickM Veteran Member

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    Here is what the carb, if untouched, should have. notice the Idle Air Bleed Valve has a protective cover over it. The circled plug is for either the lean or rich stop adjustment cant remember which. The other plug (rich or lean stop adjustment) is next to it, in between the idle air bleed and the circled plug.

    Yes you need the Thexton tools to adjust the lean and rich stops.

    RickM

    [​IMG]
     
  9. MP81

    MP81 Veteran Member

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    Okay - no image I found had that cover over the idle bleed valve, so that makes sense then. It may still have never been disassembled though - would need to ask my uncle if he had anyone do that ever.

    Either way, my understanding is likely: the accel pump seal is worn out due to ethanol in the fuel, causing extra fuel to get by when it is "active", causing the car to bog, hard, until it stops dumping in fuel, which is when it resumes normal acceleration. Seem about correct, regarding overall root cause?

    Cool, I'll check ebay for the 370 and 380 tools (even if I don't use the 380 tools, I'll need the 370 for the disassembly/reassembly, yes?) and then find myself a nice dwell meter and then order a rebuild kit. I'll have basically 5 days to complete this and get it dialed in before dyno day, so...yep. Oh well!
     

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