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Carbs from a '77 50 HP on a '78 55 HP?

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  • Carbs from a '77 50 HP on a '78 55 HP?

    I have an annoying idling problem with my '78 55 HP Johnson outboard (see Other topic).It seems one or two of my carbs have some kind of problem, since I think it's running too lean at idle, but not always. I've cleaned and rebuilt the carbs dozens of times, so I'm out of ideas here. It might be an air leak somewhere or a float that's stuck sometimes, or something else.I have two brand new carbs of a '77 50 HP rope start model (part no. 322990). These carbs have adjustable idle mixture orifices. The carbs on my 55 HP (part no. 321700) have fixed idle orifices.My question is: are the carbs exchangeable and are the idle mixture orifices the only difference? (apart from the size of the high speed orifices, which I should exchange also).If so, can I try these carbs and see if there is improvement?


  • #2
    Re: Carbs from a '77 50 HP on a '78 55 HP?

    Ok I made a mistake, the carbs from the '77 50 HP have a different part number and they also have fixed idle orifices.So to 'renew' my question: are the carburettors from a '77 50 HP exchangeable with the carburettors of a '78 55 HP?

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    • #3
      Re: Carbs from a '77 50 HP on a '78 55 HP?

      Yep, and DO NOT mix up the jets. Use the new (different) carbs as produced. Carbs are calibrated (jetting is a part of calibrating) for what they are not what they're on.

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      • #4
        Re: Carbs from a '77 50 HP on a '78 55 HP?

        Ok, let me get this straight (English is not my native language, so I want to be sure) I can just mount these new carbs and don't need to change anything on them?Is it wise to try 'easy' first and don't run at WOT and then check color of the plugs to be sure it doesn't run lean? I'm assuming that the 5 HP difference of both motors doesn't have anything to do with carbs and jetting then.

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        • #5
          Re: Carbs from a '77 50 HP on a '78 55 HP?

          Thats correct. If the carbs are clean then just bolt them on and go. Yes, going easy is always a good idea on the first try.

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          • #6
            Re: Carbs from a '77 50 HP on a '78 55 HP?

            I had a 76 Evenrude 55 that I got to know inside out over the years. I had the exact same probem. I doubt the carbs are the problem. I would check the reed valves for cracks or missing pieces. The first and easiest check is while the engine is running see if either carb is blowing back at you at all, just cup the carb like you are trying to choke it out and see if any air blows back towards you.

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            • #7
              Re: Carbs from a '77 50 HP on a '78 55 HP?

              Dhadley, Dean E,Thanks for your response. I'm gonna try these carbs anyway, it's an easy job to do and I have them lying around anyway. If it doesn't work out the reeds will be the next thing to check. I've checked for visual blowback already, but did not see anything. Another tip I got is to check blowback with a strip of tissue.Dhadley, do you know if the reeds of the '77 50HP are also exchangeable with the 55 HP? I have this 50 HP (rope starter) in a brand new condition, so I can easily borrow some parts to isolate the problem.I will let you know how it works out.

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              • #8
                Re: Carbs from a '77 50 HP on a '78 55 HP?

                I mounted these new carbs from a '77 50 HP (rope starter) on my '78 55 HP. Boy does this make a difference! It does idle all day, also in gear. The throttle response got a lot better, it reacts terrific instantly.But I also have a new problem, because the idle RPM's are too high, it won't shift easy.I retarded the ignition timing @ idle to approx. 2 degrees to slow it down a bit, but it doesn't run that smooth then, it shakes. So my guess is it's running a bit too rich @ idle now.With the new carbs I also mounted the cast (iron?) air silencer instead of the plastic one that was on the motor originally. Could that be the cause, or should I mount the idle orifices from the old carbs (assuming they are different)?

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                • #9
                  Re: Carbs from a '77 50 HP on a '78 55 HP?

                  If youre sucking air somewhere youll get a higher idle. Look for that first. Make sure the linkages are all correct. You might have one carb slightly open. We assume youre talking about idling too high when its in the water, not on a flushette.

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                  • #10
                    Re: Carbs from a '77 50 HP on a '78 55 HP?

                    Yes in the water. I'll check linkages and air leaks first.

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                    • #11
                      Re: Carbs from a '77 50 HP on a '78 55 HP?

                      Dhadley,You were right (as always) the top carb was slightly open, because the linkage wasn't correct. I can adjust the idle now like I want it, so it finally responds like expected and behaves consistently.But I still find it doesn't run smooth below 2000 RPM. I can feel the boat shaking and see the motor shaking. The response on the throttle is terrific and it runs very smooth above approx. 2000 RPM

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                      • #12
                        Re: Carbs from a '77 50 HP on a '78 55 HP?

                        If you havent done it in a while it might be a good idea to do a decarb. Some models of that type didnt idle the greatest but it should be passable. We have fooled around with idle jetting to help the situation but you need a pretty good stock of idle jets. Since we know the carbs are working well we might try working with the idle jets only.What jets are in there now? What jets do you have? Do you think the motor is too rich at idle?

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                        • #13
                          Re: Carbs from a '77 50 HP on a '78 55 HP?

                          I decarb it every season. I'll check the idle jet sizes today. The only idle jets I have are the ones from the previous carbs (30, upper carb & 31, lower carb)I don't know if it's running too rich, the way it runs reminds me a bit of the time one spark plug wire was loose. That makes it shake also. But I'll check the spark plug color too after running idle for a while. That would give a clue I guess.

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                          • #14
                            Re: Carbs from a '77 50 HP on a '78 55 HP?

                            Hi CorBI have a 1980 55 and that didn't idle too good when I first got it. I did the decarb, rebuilt the carbs, fitted new plugs, fitted a new thermostat and checked all the other relevent things like fuel hoses, fuel tank and primer bulb. Those things imnproved it no end, but it still wouldn't idle smooth. I don't think it ever would have, no matter what I did to it. So I've just increased the idle speed a tiny bit, so it'll still go into gear nice without clunking, and it'll also idle nice and smooth. Maybe your idle speed is just set too low?

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                            • #15
                              Re: Carbs from a '77 50 HP on a '78 55 HP?

                              Steviecops,Thanks for your response (a bit late, I know)Most of the things you mentioned have been done already.The new carbs have both .35 idle jets (upper & lower) and the old carbs had .30 in the upper and .31 in the lower carb. I'm not sure now if I solved most of the problems because I've eliminated a 'defective' carb or that it just runs better because of the richer idle jets.Anyway, it idles a lot better now, but it still shakes, could that be caused because the idle jets are now the same while there was a slight difference in the sizes with the original carbs?The oil puddling problem is still the same. In my opinion there is too much oil accumulating in the air silencer. I'm thinking of trying the (new) reeds from the '77 50 HP also, just to eliminate possible defective parts. The part numbers of the reeds differ. The originals have part number 387431 according to the parts manual and the new reeds from the 50 HP have part number 317116 and there is also D3 stamped behind the part number. Additionally it says 4 & 2 cyl 5.Could there be much difference between the reeds and do I take a risk exchanging them?

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