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  • Mixture with higher compression

    Theoretical question. All else being equal. The carb mixture is set so that its in the middle of the range, say 1 1/2 turns. If the head is then skimmed and cylinders over-bored resulting in a higher compression, would it be expected to adjust the carbs for a richer mixture to compensate for the higher compression?
    Anthony
    ’76 TR6
    Westchester County, NY

    https://triumph-tr6.wixsite.com/triumphtr6

    Carbs by Poolboy
    Good Parts: Rear hubs; Adjustable TA brackets w/ Nylatron Bushes; Throttle Shaft Bushings, Aluminum Rack Mounts
    VB: Headlight Wiring Harness
    Moss:Backup Hood Release
    World Wide Auto: Rear Lever Shocks

  • #2
    Not necessarily. Ignition timing and cam timing may have more to do with the quality of the combustion than compression...Evaluate as it as before.
    Driving a 1973 TR6
    Doing ZS carb repairs
    email kencorsaw"at"aol.com

    Comment


    • #3
      Thanks Ken. The reason I’m asking is because the color on the plugs indicate a touch on the lean side with the mixture set at 1 1/4 turns ccw turns from the full clockwise rotation. I admit that the timing was set 1-2 degrees too advanced. Using a vacuum gauge, I readjusted the timing to 1 division from max. I also richened the mixture by 1/4 turn. I didn’t travel too far with that setting, so the jury is still out. The plugs still look the same and now I experienced a stutter during acceleration every now and again. I started to wonder, given I am only one turn from full rich, if a 40 over bore and a slightly skimmed head would need a richer mixture when compared to a stock engine. That is, if a stock engine is happy at 1.5 turns would.a larger bore etc need only 1 turn or less. But like you said, there are more variables; so I will continue to tinker to find the happy zone.

      Edit: I adjusted the mixture before adjusting the timing 1 division.
      Last edited by aad; 03-15-2020, 04:28 PM.
      Anthony
      ’76 TR6
      Westchester County, NY

      https://triumph-tr6.wixsite.com/triumphtr6

      Carbs by Poolboy
      Good Parts: Rear hubs; Adjustable TA brackets w/ Nylatron Bushes; Throttle Shaft Bushings, Aluminum Rack Mounts
      VB: Headlight Wiring Harness
      Moss:Backup Hood Release
      World Wide Auto: Rear Lever Shocks

      Comment


      • #4
        You'll find the sweet spot, Anthony; just keep fooling with it and be sure to keep up with the motor oil level in your carbs' dampers...some of them hold oil longer than others.
        Driving a 1973 TR6
        Doing ZS carb repairs
        email kencorsaw"at"aol.com

        Comment


        • aad
          aad commented
          Editing a comment
          Will do. I do find the need to add a few drops of oil to the dampers every time I adjust the mixture as the oil clings to the tool etc.

      • #5
        And you may want to back off the timing a bit...try 2 in-Hg less than what you read for maximum vacuum.
        It took me a while to find my engine's sweet spot combination of timing and mixture... and I still find myself playing around with it seasonally or when I get restless....a few degrees here; an eight of a turn there, test drive, check a couple plugs, stuff like that.
        Driving a 1973 TR6
        Doing ZS carb repairs
        email kencorsaw"at"aol.com

        Comment


        • aad
          aad commented
          Editing a comment
          Will do. I’ll back it off and take it from there.

      • #6
        Anthony - I assume you're using good quality 91-93 octane gas .......
        1976 TR-6 BRG - CF57239U
        Carbs by Poolboy
        Rear Camber Kit, Rear Hubs by Goodparts
        Gear Reduction Starter by TSI
        Distributor by British Vacuum

        Comment


        • #7
          Yes, 93 octane, usually from BP
          Anthony
          ’76 TR6
          Westchester County, NY

          https://triumph-tr6.wixsite.com/triumphtr6

          Carbs by Poolboy
          Good Parts: Rear hubs; Adjustable TA brackets w/ Nylatron Bushes; Throttle Shaft Bushings, Aluminum Rack Mounts
          VB: Headlight Wiring Harness
          Moss:Backup Hood Release
          World Wide Auto: Rear Lever Shocks

          Comment


          • #8
            I'm experiencing the same issue with my newly rebuilt engine.

            GP2 cam, recurved distributor using points, head shaved to ~9.5:1, 93 octane, timing set to 15 in Hg on vacuum gauge. Plugs indicating a little lean. An occasional stumble under hard acceleration and RPM around 4K.
            Pete, Collierville TN
            1976 CF54385 U
            Mechanically restored
            Head 9.5:1(ish) + GP2 Cam

            Comment


            • #9
              Dial up the mixture, drive her, then check the plugs.
              My motor likes to run a bit rich. 9.2 CR, later PI cam. But still runs OK on mid grade. Vacuum readings are 17 HG.
              CF1634U+O Pimento/Chestnut
              2nd owner, since 1975
              Now in Fair Oaks, CA

              Comment


              • #10
                Hey Pete, I had few issues after my rebuild that required pulling the head again (machine shop error with a valve). When I put everything back together, I went with a more substantial manifold gasket (BPNW Hd?) and switched to a cooler spark plug (Champion 9 instead of the 12). Perhaps I had a small manifold gasket leak that contributed to the lean plug reading. The 9s also worked for me. It’s a process and I’m sure you’ll find the culprit.

                Anthony
                ’76 TR6
                Westchester County, NY

                https://triumph-tr6.wixsite.com/triumphtr6

                Carbs by Poolboy
                Good Parts: Rear hubs; Adjustable TA brackets w/ Nylatron Bushes; Throttle Shaft Bushings, Aluminum Rack Mounts
                VB: Headlight Wiring Harness
                Moss:Backup Hood Release
                World Wide Auto: Rear Lever Shocks

                Comment


                • prasnick
                  prasnick commented
                  Editing a comment
                  Thanks, Anthony. I used the same gasket and have also been using Champion 9s on the advice of the guy who sold me the cam. I have played with the mixture on Ken's rebuilt carbs, so I may need to go back to the baseline of 1 1/2 turns from full rich, then go from there.
                  Last edited by prasnick; 07-07-2021, 10:00 PM.

              • #11
                This thread makes me glad I sprung for the adjustable jets in my rebuild. I’m still dialing things in, but so far it seems like a bit richer is good with mine too, BP270 cam and about 9.5:1.
                Bill Connell
                1969 TR6 CC28790
                TR6 project log
                St. Paul MN

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