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1999 Honda 25 horse low vacuum

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  • 1999 Honda 25 horse low vacuum

    Cleaned carbs and installed all orings and float bowl gaskets. New intake manifold gasket. New insulator gaskets. New muffler gasket. New spark plugs. Compression is 180 in all three cylinders. All three cylinders have spark. Put vacuum gauge on and all three are at about 6 inches of Hg. Spraying carb clean into the top and middle cylinder causes a bog. Spraying carb clean in bottom cylinder does little except a rev up slightly once in a while. Put my hand over the top two carbs and it bogs . Put my hand over bottom carb and no change in rpm. I believe I found that the throttle linkage bushing is leaking vacuum on the top two carbs but can't quite tell on the bottom carb. Based on an exploded diagram for the carbs on ********** it doesn't look like that bushing is replaceable(or just nla). top two cylinder plugs are dry(but fire is evident based on carbon)bottom cylinder is wet.
    On good running v8 engines I've seen 16-20 in Hg.
    Shouldn't outboards be the same?
    thought about swapping coils to see if there is weak spark on the bottom cylinder.
    thanks for any insight.

  • #2
    Are you familiar with the procedure used to synchronize your carbs? It's not so much about how much vacuum they're pulling. The important thing is that they are all pulling the same! If I were looking at the scenario you are descibing, that's where I would start.

    There's a good chance the top 2 carbs are doing all the work on the idle, with the bottom just kind of coasting along for the ride. Get that botton cylinder pulling it's share of the load, and you may be amazed at how much better that engine runs.

    Sync can be done with a single gauge, but it's SO much easier using 3.

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    • #3
      I am not familiar with the sync procedure on this engine. I know that each manufacturer is specific in their process. I do have a four gauge set up and all three show around 6 inches while the engine is running. Would this explain why the bottom cylinder does nothing when I spray carb clean into the carb?
      Still concerns me that when carb clean is sprayed onto the outside of the carb where the throttle plate shaft goes through the engine bogs down.

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      • #4
        If all 3 are showing the same or very similar, it's not likely worth messing with trying to get it any closer than that!

        Sync procedure, if you want to mess with it, starts with the bottom carb as the base value. You then match the other 2 to that one. Pretty easy simple really, especially if you're set up with a 4 gauge gang already.

        Is there an issue that's drawing your attention? One that you would notice without the use of a spray can?

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        • #5
          It's boggy until it gets warmed up and it starts hard.
          It's acting like it's missing a cylinder.
          the top two plugs are dry and the bottom one is wet.
          Just doesn't seem right.
          if the top two carbs are bogging when sprayed the bottom one should too.
          it doesn't.
          it doesn't do anything.
          the engine bogs when I put my hand over the top two carbs.
          it doesnt do anything when I put my hand over the bottom carb.
          bottom carb is getting fuel and when I put my hand over it fuel does spray up into the venturi just like the top two.
          this engine isn't running one hundred percent .
          It also bogs when carb spray is sprayed onto the outside where the throttle plate linkage comes out of the carb in the top two carbs.
          can't really tell on the bottom because it's tucked up against the engine block.
          I'll have to run it again and get a pic of the vacuum gauges. My memory may not be serving me right on all cylinders being identical in vacuum.
          It is close though and that should be enough to run better than it is.

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          • #6
            If it's not dropping out when you put your hand over it, it's already dead.

            How recent was this compression check?

            Have you put a set of plugs in it lately, or swapped them around to see if the dead cylinder moves?

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            • #7
              Compression test was just a few days ago.
              All right around 180 psi.
              Plugs are brand new as well.
              Swapping spark plugs makes no difference .

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              • #8
                I didn't see, but is the dead cylinder making spark? No spark will cause a wet plug.
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                • #9
                  Yes all three have spark.
                  When I ran it this evening the bottom cylinder has 7 inches of Hg and the other two have around 5.

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                  • #10
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                    For the lack of better ideas, I'd get them synced back up and see what happens.

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