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18 hp fastwin carb main jet

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  • 18 hp fastwin carb main jet

    Hello, I have a 1964 18402 18hp fastwin that I just bought. The carb looks like someone tried to change the welsh plug with a chainsaw. My question is the main jet. It's a 63d. I have a carb from a 1969 18902B 18hp fastwin here and it uses a 67d main jet. This got me thinking so I checked a few others and a 1968 18802A 18hp fastwin uses a 57d. That's a big drop in fuel at wot. All mentioned use the smaller 18hp carb. The main jet or orifice plug as omc likes to call it is all over for size. Anyone have any ideas where I should be for 18hp here at 500ft elevation 50-95 degrees average? I also have a larger carb here from a 1971 25hp using the same 22ci block. I was thinking of getting a 25hp 22ci intake manifold and bolting it together. Any thoughts on this as well? Been working on old outboards for many years and don't believe that I have read anywhere or heard of anyone trying this. Any way, Thanks for any input.....

  • #2
    Re: 18 hp fastwin carb main jet

    Sounds like you must have the chart so I won't bother posting it, but for the record, models 18402 thru 18603 (1964-66) use a #64 orifice. Models 18702 thru 18803 (1967-68) use a #57 orifice. Models 18902 thru 18305 (1969-73) have a totally different carburetor so it really doesn't matter that they use a #68 orifice. There has to be a good reason why they dropped from #64 to #57, but I confess I don't know what that reason was without doing a lot of research. In any case, bottom line is simply use what the chart says. It is for conditions fitting what you describe, and no fiddling is required.

    As for the 25hp stuff, surely you know that the block may be the same, but the pistons and cylinder head are different, along with a balanced crankshaft. Putting a 25hp carburetor on a 18 will NOT make 25hp. In fact, you may actually lose some hp along with poor mid range performance. That is because the air flow velocity through the venturi is slower because the engine is not developing the hp because the compression ratio is lower because of the pistons and head, etc, etc. BTW, yes, I tried it. Oh, and before you ask, no it won't help to put the 25hp head on a 18. The different pistons go with the different head.
    Last edited by F_R; November 19th, 2013, 06:30 AM.

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    • #3
      Re: 18 hp fastwin carb main jet

      When did they go to non-adjustable high speed mixture?
      Could that be the change from #64 to #57?
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      • #4
        Re: 18 hp fastwin carb main jet

        Originally posted by F_R View Post
        Sounds like you must have the chart so I won't bother posting it, but for the record, models 18402 thru 18603 (1964-66) use a #64 orifice. Models 18702 thru 18803 (1967-68) use a #57 orifice. Models 18902 thru 18305 (1969-73) have a totally different carburetor so it really doesn't matter that they use a #68 orifice. There has to be a good reason why they dropped from #64 to #57, but I confess I don't know what that reason was without doing a lot of research. In any case, bottom line is simply use what the chart says. It is for conditions fitting what you describe, and no fiddling is required.

        As for the 25hp stuff, surely you know that the block may be the same, but the pistons and cylinder head are different, along with a balanced crankshaft. Putting a 25hp carburetor on a 18 will NOT make 25hp. In fact, you may actually lose some hp along with poor mid range performance. That is because the air flow velocity through the venturi is slower because the engine is not developing the hp because the compression ratio is lower because of the pistons and head, etc, etc. BTW, yes, I tried it. Oh, and before you ask, no it won't help to put the 25hp head on a 18. The different pistons go with the different head.
        Thanks for the response. I know the 25hp uses different pistons and head. Been through that years ago. Makes sense putting the larger intake manifold and carb on wouldn't help with less compression. I'm still trying to figure out how the same engine with the same carb and no baffle/air box could drop 7 jet/orifice sizes. I would really like to get to the bottom of that. It's possible when the 20hp came out the 18hp was detuned further with a smaller jet size? In my experience building many full race 2 stroke engines with all being equal you cant drop 7 jet sizes on the main without a serious burn down. There has to be more to it......

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        • #5
          Re: 18 hp fastwin carb main jet

          Originally posted by Tim Frank View Post
          When did they go to non-adjustable high speed mixture?
          Could that be the change from #64 to #57?
          1964 was the first year for evinrude with the fixed main jet on the 18hp. There were a lot of changes in 1964.

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          • #6
            Re: 18 hp fastwin carb main jet

            Originally posted by slx12001 View Post
            Thanks for the response. I know the 25hp uses different pistons and head. Been through that years ago. Makes sense putting the larger intake manifold and carb on wouldn't help with less compression. I'm still trying to figure out how the same engine with the same carb and no baffle/air box could drop 7 jet/orifice sizes. I would really like to get to the bottom of that. It's possible when the 20hp came out the 18hp was detuned further with a smaller jet size? In my experience building many full race 2 stroke engines with all being equal you cant drop 7 jet sizes on the main without a serious burn down. There has to be more to it......
            You have your head on straight. You can't just drop 7 sizes without something else also changing. That would make it so lean it probably wouldn't even run. There has to be something else to it. That "same" carburetor is different somewhere. Perhaps smaller venturi?

            EDIT: I just did some comparison shopping. 18 and 20 horsepower used the same jets in any given year. In other words, both changed in 1967 & 1968 over the 1966 models. So it isn't a horsepower issue.
            Last edited by F_R; November 20th, 2013, 01:48 AM.

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            • #7
              Re: 18 hp fastwin carb main jet

              Originally posted by F_R View Post
              You have your head on straight. You can't just drop 7 sizes without something else also changing. That would make it so lean it probably wouldn't even run. There has to be something else to it. That "same" carburetor is different somewhere. Perhaps smaller venturi?

              EDIT: I just did some comparison shopping. 18 and 20 horsepower used the same jets in any given year. In other words, both changed in 1967 & 1968 over the 1966 models. So it isn't a horsepower issue.
              I have 3 18hp carbs here. 2 newer 1 older. All venruri measure the same in/out center and look identical. I'm very interested in figuring this out now. I know the older ones with 2 needles used an air box. These carbs do not use an air box so air flow should be the same......
              Last edited by slx12001; November 20th, 2013, 03:14 AM.

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              • #8
                Re: 18 hp fastwin carb main jet

                The only thing that I can think of is for elevation changes. Say 500ft to 5000ft is roughly 20% loss in power you would need a smaller main jet due to less available air. But the parts breakdown shows the engines shipping with these jets. Not for high elevation, hmm You mentioned that you have a jet chart for this era. I would love to see it. Thanks for any help.

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                • #9
                  I want to know this too. I'm running a '61 powerhead and I have both size jet between the two carbs I have on hand. I've only tested the 57 and it actually seems too rich at WOT. Which makes me think using the 64 I have will be going in the wrong direction.

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                  • #10
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                    Originally posted by stephenspann27 View Post
                    I want to know this too. I'm running a '61 powerhead and I have both size jet between the two carbs I have on hand. I've only tested the 57 and it actually seems too rich at WOT. Which makes me think using the 64 I have will be going in the wrong direction.
                    A little late to the party. The last post was almost 4 years ago.

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