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1992 Weber carburator jeting

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    1992 Weber carburator jeting

    After 14 years of being grounded I am getting the boat on the water this year.
    All was going well until, while at idle the engine would start running ruff and die too much gas.
    After troubleshooting everything else I overhauled the carb. I removed the jets for cleaning but to my surprise all 4 jets were not the same size orifices (from the factory). I was expecting the primary's to be the same and the secondary's to be the same sizes. But no all 4 have different jetting. Port side jets are leaner than starboard. Ordered 1ea.primary and secondary stock jet and to my surprise they had different
    orifices
    than the factory ones installed on this carb. I reinstalled the original jets that were removed for cleaning and engine is running smooth again.
    My question is it normal to have all 4 jets different sizes on marine engines?

    #2
    I would not know, but I am about to find out as I going to be rebuilding my 95 5.7 with a weber.
    It does not sound right to me.
    Both primary jet should be the same, as both secondary jets should also be the same imo.
    1995 Maxum 2400 SCR LUNA DE MIEL
    1988 Bayliner 2455 (sold)
    1976 Tahiti 16.5 I/O (sold)
    10 ft livingston (lost in fire )
    1987 18ft. Seaswirl cuddy (lost in fire)
    "Is it better to be on a boat thinking about God, or be in church thinking about boating?"

    Comment


      #3
      PlynHooky Actually, praise to Weber for recognizing the needs of the engine..here's what happens at higher RPM's...

      Consider the firing order, 1-8-4-3-6-5-7-2-1-8-4-3...etc... The carburator supplies air/fuel to the manifold plenum chamber...each cylinder's vacuum then draws on the available air/fuel from the manifold chamber just below the carburator. If you look at the firing order 7 draws right after 5. That means that the two cylinders are competing for the available air/fuel in the chamber. It stands to reason that a bit more fuel will be needed to satisfy 5 and 7. Then 1 and 2 are competing much the same way...2 is "fighting for fuel...right after 1 gets what it wants. When 1 draws from the chamber the flow has to make a u-turn all the way from 8. And so on. A close (very close) examination of the spark plugs will show the difference between a uniformly jetted carb and one that has different jets. I used to use a magnifying glass when racing high-performance GM blocks. The net result was different jets for each hole.

      This is even more evident when using multiple carbs...each venturi is more dedicated to the closest chamber, hence, different jets means better performance on the track. Dedicated port fuel injection does not have this issue as each nozzle feeds the individual cylinder.

      Temperature readings would also bear this out. If you were to have a temp sensor above each cylinder you would see the temp difference on an equally jetted carb where the area above #7 would be the hottest. Hence the spark plug color being an important factor in determining jetting.

      Racing or higher performance manifolds help this situation over the stock manifolds by the length of the tubes to the cylinders and their placement entering the plenum chamber. The straighter the tube, the better the performance. Some manifolds have straight tubes leading to the cylinders...these manifolds would require more attention to individualized jetting...

      Makes sense...?
      1988 3888 "Liberty"
      Twin Cummins 6BT's 210hp
      Onan 8.0
      Boating Raritan Bay

      Comment


      • PlynHooky
        PlynHooky commented
        Editing a comment
        Thanks Nickp,
        I needed some one to explain in detail.
        I did find out a lot about Weber carbs and jetting from the different websites and phone calls.
        Just as you explained marine carbs may have 4 different size jets

        The numbers on my jets are port: primary120/415 starboard 120/401 secondary: port 120/308 starboard 120/377 at least that's how I put them back in. The port side will have more gas. The number's were real hard to read even under magnification. These numbers needed to be converted/measured to find the orifice sizes.
        (I did size them and hope I got them back in the correct positions). I did not know they were all 4 different when I removed then for cleaning. Metering rods 6542. When I rebuild another carb I will take more pictures, tag and bag every part as I remove them.
        26 Oct18 went to lake for a test run. Carb works well at idle
        (I did adjust), mid-range and wot. I has a 200rpm flux as soon as the secondary's start to open (3200rpm) all the way to wot (4000rpm).
        Timed engine today to 8 deg. btdc (it was set at 12). I will see if that fixes the flux next time out.

      #4
      That slight blip is likely due to not getting enough gas when it makes the transition to the secondary...does the Weber have an "accelerator pump" that gives it a shot when the secondaries open.,..?

      Which Weber do you have...?
      1988 3888 "Liberty"
      Twin Cummins 6BT's 210hp
      Onan 8.0
      Boating Raritan Bay

      Comment


      • PlynHooky
        PlynHooky commented
        Editing a comment
        Carb is 9772S (9000 series)
        The flux of 200rpm is constant from 3200 rpm and up wot.
        After re-reading your 1st explanation I may have the port and starboard jets installed opposite. I have the port side running the larger orifice jets.
        I am going to the lake today to see if the timing helped.

      #5
      Good luck with it...keep us updated...
      1988 3888 "Liberty"
      Twin Cummins 6BT's 210hp
      Onan 8.0
      Boating Raritan Bay

      Comment


      • PlynHooky
        PlynHooky commented
        Editing a comment
        Got on the lake this afternoon. Perfect fall day.
        Timing change helped somewhat.
        Now the flux is a solid 50 to 100 rpm at 3600/3700 the flux is more noticeable when throttling from wot to 3600 than bringing it up from idle to 3600. The rest of the rpm ranges are stable.

      #6
      Is it possible the float might be a bit low...? This would have the effect of "almost" running out of fuel at higher RPM's and the RPM's would fluctuate as the carb tries to empty and then refill and empty and refill, etc... It would not be a drastic change in the float bowl but maybe enough so that the carb does not feed enough fuel to the venturi in a constant manner. Having said that, the bowl will never go to empty...it's just that the level would not be sufficient to sustain the desired RPM...hope that was clear...?

      (sorry it took so long to get back to you)
      1988 3888 "Liberty"
      Twin Cummins 6BT's 210hp
      Onan 8.0
      Boating Raritan Bay

      Comment


        #7
        Changed the timing to 4 deg. btdc to see if that could be the problem. There was no change in flux at 3,600 rpm. I am changing the timing back to 8 deg. btdc.
        I now have removed the carb. and found one of the small round screens that filter the float needle and seat was installed backwards/upside down.
        This would restrict the amount of fuel that could go to the float needle/seat on that side. The float levels were off slightly so I installed new floats that I had on hand and set the levels as per specks.
        There is still at least 10 days of good weather to go out and try these fixes.

        Comment


          #8
          How do I put my boat picture on my profile?

          Comment


            #9
            Originally posted by PlynHooky View Post
            How do I put my boat picture on my profile?
            Click on your user name at top of page. Click my profile. Tap on the avatar icon box to edit/add your pic.
            Dave
            Edmonds, WA
            "THE FIX"
            '93 2556 5.7 Bravo II 2.0:1 18 1/4x19 P
            (.030 over-Vortec top end-part closed cooled)
            The rebuild of my 2556 https://www.baylinerownersclub.org/f...76?view=thread
            Misc. projects thread
            https://www.baylinerownersclub.org/f...56-gctid789773

            Comment


              #10
              Originally posted by PlynHooky View Post
              There is still at least 10 days of good weather to go out and try these fixes.
              Any day on the boat is better than a good day on shore...
              1988 3888 "Liberty"
              Twin Cummins 6BT's 210hp
              Onan 8.0
              Boating Raritan Bay

              Comment

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