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HS6: Front Carb low vacuum 444-544

posted by  tommyt544 on  Sun Jun 11 19:12 CST 2023
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last visit: Thu Oct 16 11:11 CST 2014
[RELATED]

Trying to figure out why I have such a low vacuum from my front HS6 as I do from my rear. My synchrometer registered an 8 on the rear and a 2 on the front. I know that it's all relative to the tool but that's a lot. I've been able to even it out by increasing the idle and increasing the enrichment but that's not a long term solution.

The linkages were all detached including the carbs and the accelerator. Jet flats all at the same number of turns. Did a dry compression and all my cylinders were around 155psi from my B20B low compression head/B20 block. Couldn't see any vacuum leaks but I couldn't get under the car.

Any thoughts?








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    posted by  someone claiming to be on  Wed Dec 31 18:00 CST 1969 [RELATED]

    Update:

    Tried attaching a set of HIF6 carbs and the same issue. Linkages all disengaged.

    Rear carb appears to be working just fine.

    Sprayed to see if there was any increase in RPMs along the manifold. Nothing.

    Valves are fine.

    Thinking there could be a flow issue in the intake portion of the manifold? It's a siamese 4-2-1. Tempted to try another manifold or header/intake setup.

    Doesn't seem like a carb problem if both sets show the same lower vacuum. This is a freshly done engine too. Good dry compression readings.

    Help!



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      posted by  someone claiming to be on  Wed Dec 31 18:00 CST 1969 [RELATED]

      Check for worn camshaft lobes.

      You can have good compression even with a worn camshaft.
      --
      Eric
      Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
      Torrance, CA 90502
      hiperformanceautoservice.com or oldvolvosonly.com



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        posted by  someone claiming to be on  Wed Dec 31 18:00 CST 1969 [RELATED]

        This would really suck. It's a freshly rebuilt engine.

        Any hints on how to check for wear without a tear down?



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          posted by  someone claiming to be on  Wed Dec 31 18:00 CST 1969 [RELATED]

          To check for worn cam lobes, disable the ignition by disconnecting the coil power or trigger wire.

          Remove the valve cover and engage the starter motor with the car out of gear and parking brake on and watch the rocker arms to see if they going up and down equally.
          --
          Eric
          Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
          Torrance, CA 90502
          hiperformanceautoservice.com or oldvolvosonly.com



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    posted by  someone claiming to be on  Wed Dec 31 18:00 CST 1969 [RELATED]

    How are your valve adjustments ?? You note good and even compression, but since vacuum begins with the valves, I'm not wondering if that wouldn't be a good place to start.



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    posted by  someone claiming to be on  Wed Dec 31 18:00 CST 1969 [RELATED]

    How are your valve adjustments ?? You note good and even compression, but since vacuum begins with the valves, I'm not wondering if that wouldn't be a good place to start.



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    posted by  someone claiming to be on  Wed Dec 31 18:00 CST 1969 [RELATED]

    Try spritzing a water spray around all the areas of possible vacuum leaks. Use carb cleaner if you like to live dangerously. Listen for a drop in idle speed with water--a rise if carb cleaner. - Dave



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