Carby Tuning

Discussion in 'Gas Engines' started by Les Hartley, Jun 27, 2014.

  1. Les Hartley
    Joined: Jul 2012
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    Location: Australia

    Les Hartley Junior Member

    Hiya Guys from Australia,
    I have a Volvo Penta 170A which has triple carbies petrol drive. Can anyone point me in the right direction as to tuning these for the best running results. The motor is running ok but the carbies have not been re-tuned for a couple of years and my regular marine mechanic has hung up his spanners. Any advice will be gratefully recieved..

    Many thanks Les Hartley
  2. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    To precisely tune carbs you need a flow meter; a tapered glass tube with a small ball that rises in the airstream. The glass is calibrated in metric or imperial quantities.
    The main target is to get the carbs perfectly in sync when the engine idles.

    If your Volvo runs fine, I recommend to just clean the flame arresters and replace the filter cartridge.
  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    You can get very close with a length of moderately large vacumn line. Assuming the carbs are in good order and tuned properly, you just need to sync them. Place the vacuum line at the mouth of the carb and listen to the air flow past it. Do this will all the carbs and you'll note slight difference in pitch. Adjust the mixture screws to even them out. If they have low and high speed adjustments, start with the low and then move to the high side once the idle and off idle are dialed in.

    You can buy a "Uni Sync" gauge for about 20 bucks, which will make things a bit easier, but you're just syncing, not tuning.

    Tuning is a bit different and usually requires a exhaust gas sniffer, so you can get the air/fuel ratio dead on. If not interested in the last 1% of efficiency, just make sure the carbs are clean, all set the same, especially float, needle and jet heights and sizes.

    The usual routine is to set the idle speed first (you'll need a tach), then make mixture adjustments, which will toss off the idle again, so reset them to match and "chase" each carb down, until they're both sucking the same and idle and slightly off idle. Do it again with a high speed mixture screw is so equipped, except you don't need to fool with the idle setting any more (they should remain the same).

    Basically you lean or richen the mixture screw to get the highest RPM. Turn the idle mixture screw in (clockwise) by 1/2 turn increments until the engine RPM drops. When the RPM drops from turning the screw in, the mixture is too lean. Turn the screw back out (counter clockwise) 1/4 turn. Blip the throttle a bit and let things settle down. If you get a RPM drop immediately when you screw in, then back the screw out in 1/2 turn increments. Too far out drops the RPM from being too rich. Look for the idle mixture screw setting that gives you the highest idle. When you are satisfied that you have the highest idle and the idle adjustment screw is off the linkage, you can go to the next carb.
  4. Petros
    Joined: Oct 2007
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    Petros Senior Member

    does it use the SU carbs with the vacuum operated piston? I am very familar with those since many of the cars I owned and worked on many years ago had those. They are easy to adjust the balance and mixture if you know how, not such a mystery as the mechanics like to make you think (many of which are clueless about the wonder SU carb anyway).

    If all the throttles are opening at exactly the same time they are considered "in sync", which can be done by listening to the a tube put in the throat as PAR suggested, or you get the flow meter (I have used both methods, the simple flow meter is nice but not necessary). Than spary down the throat with carb cleaner with the engine running (rev it up to keep it from stalling out when you do this), this should clean off the build up on the throttle plates and in the throat. look down inside the throat with a flash light (engine off!) to inspect to see if there is fuel residue built-up, or if you cleaned it all off after the threatment.

    ONce the throttles are balanced, the throttles and inside the barrels clean, you just adjust the idle speed (screw at the throttle belcrank, where the cable or linkage attaches to the carb) and you should be good. If it has SU carbs I will tell you how to adjust the mixture correctly, if not the mixture is set and can only be changes by taking it apart and changing the jet size, which should not be necessary.
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Agreed it's not as hard as it might seem and the hear method works fine, unless you have major issues or adjustments to make. Level the playing field with fresh cleaning, rebuilds and settings, then sync them up.

    I'm also familiar with the SU's having had a few on various rides. I had twin side drafts on an MG that I tried replacing the oil enrichment circuit, with a mixture of alcohol and 60 weight. It would have worked well, if I played with it long enough, but the first run's popping and backfires shied me off the idea, before I bent something.

  6. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    they are conventional down draft carbs not like SU's
    The engine will run very smooth at low idle speed if you get them right, the float level needs to be the same on all 3 and everything else mentioned above
    Last edited: Jun 29, 2014
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