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C2 engine surge

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  • Richard E.
    Very Frequent User
    • December 1, 1989
    • 249

    C2 engine surge

    I’m seeking troubleshooting tips to solve an apparent engine surging issue on a ‘66 4-spd coupe. I don’t know the engine history, but I expect it is equivalent to a 350 HP-327, with relatively low miles since being rebuilt. The car has a Holley 1850-12 carburetor installed on an aftermarket aluminum intake with a heat spacer under the carb base. The carburetor is very clean and probably not very old (at least mileage wise). It has an electric throttle solenoid to raise the idle for the A/C compressor. The choke is electric. Overall, the engine runs strong, starts very good hot or cold, good acceleration with no stumbling / hesitation or backfire, smooth steady curb idle, etc. The car idles about 800 rpm w/o A/C, about 1k rpm with A/C on.
    The surging (jerking) is most prominent with foot off the gas, letting the car coast in gear. It also occurs at slow speed (in gear) with foot on the gas slightly. In these instances, the car will jerk repeatedly until gas is steadily applied. At cruise speed 30 mph and above the situation is less prominent but slightly noticeable. This happens whether the engine is hot or cold. The choke, A/C solenoid, and throttle mechanism all seem to be operating correctly as best I can tell. The situation is annoying, but the car is drivable. I would appreciate thoughts on the cause and possible remedies. Thanks in advance for taking time to respond.
  • Timothy B.
    Extremely Frequent Poster
    • April 30, 1983
    • 5195

    #2
    Richard,

    In my opinion if the tune is good the carburetor is too lean in the idle circuit. This surge has been mentioned by many people and with a carburetor without adjustable circuits it's hard to suggest a fix. Some people over on the Corvette Forum mentioned changing the main jets solved the problem but I don't see how because the main jets only effect the main circuit of the carburetor and that's not even at play here.

    Comment

    • John S.
      Very Frequent User
      • May 4, 2008
      • 427

      #3
      I agree with Timothy. I had the same issue with my 72 LT1 after I rebuilt the original carb. Lean on idle circuit. Once readjusted it went away.
      John Seeley
      67 Black/Teal
      300 hp 3 speed coupe
      65 Maroon/Black
      35k mile Fuelie coupe

      Comment

      • Richard E.
        Very Frequent User
        • December 1, 1989
        • 249

        #4
        Tim, John,
        Very much appreciate your comments. The carb does have idle adjust screws in the front meter block. Could opening them up a bit help? If the idle screw adjust is not a viable possibility, I may just try a different carb (Holley list 3367, 3370 type for example). I expect the model 1850 has a power valve, could this possibly be wrong number (typically 6.5 as I recall). I'm not an engine expert unfortunately.
        My nephew also has a 1972 LT1 Corvette, possibly I can try his carb when he is not looking!
        Thanks again!

        Comment

        • Mark E.
          Extremely Frequent Poster
          • April 1, 1993
          • 4600

          #5
          Richard,

          From your description of the problem, it sounds like the surging/jerking happens during high vacuum conditions (coasting, light load). An engine with too much ignition advance will have symptoms that can be described as jerking or a rolling exhaust.

          Is VAC connected to manifold vacuum? If yes, as a test, disconnect and plug the VAC line and see if the condition improves. If it does, there's too much vacuum advance. Since symptoms occur with the throttle closed, VAC probably isn't the cause if it's connected to ported vacuum.
          Mark Edmondson
          Dallas, Texas
          Texas Chapter

          1970 Coupe, Donnybrooke Green, Light Saddle LS5 M20 A31 C60 G81 N37 N40 UA6 U79
          1993 Coupe, 40th Anniversary, 6-speed, PEG 1, FX3, CD, Bronze Top

          Comment

          • David M.
            Very Frequent User
            • September 30, 2004
            • 526

            #6
            I seem to remember something about the distributor can cause this problem. Excessive end play if memory serves me. I searched but cant find the thread. Might have been Duke or Joe Lucia.

            Eliminate the simple stuff first.
            -vacuum leak
            -fuel system, psi, filter
            -ignition system including the amount of vac and centrifugal advance the distributor adds on top of the base timing. Where does the centrifugal begin to "come in" and at what RPM.
            -lean spot in the fuel curve. What are the plugs telling you?
            -then check the distributor

            Comment

            • Justin S.
              Very Frequent User
              • July 3, 2013
              • 305

              #7
              Decelerate button?

              Comment

              • Larry E.
                Extremely Frequent Poster
                • December 1, 1989
                • 1692

                #8
                Richard: My 66 Base engine Coupe had a similar problem. (May not have been as serious as yours)
                I thought that my original carb choke was working Ok BUT discovered that it did not
                open 100% fully when engine was warm. I changed the tension on the "divorced choke
                spring in the manifold) and has worked perfect since. That being said I am unfamiliar
                with your carb and do not know what type of choke that it has. Thanks>Larry
                Larry

                LT1 in a 1LE -- One of 134

                Comment

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