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1979 Mercury 9.8hp

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  • 1979 Mercury 9.8hp

    just bought used 9.8hp mercury, 1979 according to serial number. Have it running good and in neutral goes through the RPM's really nice, idles well, starts easy, etc. But I got it on the water on my 10 foot avon inflatable and when in gear it does crank up very high on the rpms and in turn is slow. that size motor on that little inflatable should fly. am i losing power somewhere? does it need tuned up better? it rev's up nicely in neutral but when there is a load it is slow even at full throttle. only had it out in water twice, and during the first run at one point in time with two people on board it actually started to speed up too fast and started to plane a little bit but not since. what should i do?


  • #2
    Re: 1979 Mercury 9.8hp

    so you seem to be getting the rpm's just not getting the speed? whats the condition of the prop? is it banged up? it could be slipping too, the rubber hub inside the prop that is, if this is the case you will have to take your propeller to the prop shop and have it re-hubbed

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    • #3
      Re: 1979 Mercury 9.8hp

      Originally posted by 1988regal View Post
      ... But I got it on the water on my 10 foot avon inflatable and when in gear it does crank up very high on the rpms and in turn is slow. ..
      I'm not sure what you're trying to say here, not sure where you're talking about rpms or where speed.

      Are you saying the engine revs up to high rpms?

      Or are you saying is not putting out high rpms ("runs slow").

      Or are you saying the rpms are high, but then the rpms slow? ("crank up very high on the rpms and in turn is slow")

      Or are you saying is that it does give high rpms, but the speed is very slow?

      Yea, I would think a 9.8 would push the zodiak nicely, let us know what it's doing.

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      • #4
        Re: 1979 Mercury 9.8hp

        Sorry, i'll explain better. When engine in neutral, when giving it gas it will increase RPM's nicely and really crank up. but once in gear, i am not getting it anywhere near that high of rpm's, even at full throttle and in turn, it is going slowly. i tried adjusting the angle of trim and lowering the weight in the boat but no matter what it still just cruises along slowly, not anywhere near where a 9.8 should be able to go. at one point the first weekend i had it in the water with two people in it, i actually was going so fast that my girlfriend got a little scared because we started to get airborne off other boat wakes. i had to repair rewind starter and cleaned up spark plugs and now it seems to run well when its just sitting in tub of water or in the water in neutral. but once i shift to forward gear i can't get it to get up into higher rpms.

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        • #5
          Re: 1979 Mercury 9.8hp

          OK, so you're saying:

          Earlier, it ran really fast - good high rpms and good, fast speed.

          Now, in gear, on the water, it does not run at high rpms and will not push the boat fast.

          So something changed, right? That is the critical part, imho. Please correct if any of the above aren't correct.

          Did you make any (ANY) changes between the time it ran great and now? Something changed -either something you did, or something happened to the engine, probably in the spark or fuel supply/delivery system.

          Since it once ran great, we can assume (but maybe shouldn't) that the plugs are correct type and gap. You might double check that.

          It could be bad fuel or contaminated tank/pickup, or bad fuel lines. This can be easily checked/ corrected by using a known good/clean fuel tank with fresh, properly mixed gasoline with known good hose and primer bulb, connections, etc. Or you can look closely at all aspects of this from the pickup on to the carb. and make sure everything is clean and tight, and the fuel is fresh.

          Unfortunately, sometimes just cleaning /changing the tank and putting in fresh fuel doesn't help because the bad fuel has already gummed up the carb (see below). But sometimes it works miracles and is easy to do.

          Make sure the connections to the spark plugs and back to the coils are nice and tight and the wires are in good condition.

          Other than that, getting more complicated, you need to do some basic assessment of compression and spark (not only that it's present, but the strength using an adjustable gap spark tester).

          If it's not the spark, or compression, another step would then be to clean and /or rebuild the fuel pump and carb. Not super hard or expensive, but do require a service manual and a "kit". Might be something you'd have a shop do if you're not into DIY.

          Others will chime in, but these are things that I've done to trouble shoot bad running Merc 9.8 a little older than yours.

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          • #6
            Re: 1979 Mercury 9.8hp

            yes, you have it about right.

            the weekend that it was running fast, by the end of that weekend it was also running slow with no changes. since that i have replaced the fuel hose and primer bulb from tank, changed it to 89 octane gas instead of 87, taken out plugs and cleaned off ends, made sure wires were attached correctly etc. the guy i bought it from had cleaned the carb and put it all back together and i've run nothing but new gas through it. where is the fuel filter located on that? do you have any good sites or references to consult for what i need to tune it up? I am pretty good with tools but have not had much experience in outboards. i got the repair manual from the library but it covers all merc's prior to 1980 and is kind of generic. thanks.

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            • #7
              Re: 1979 Mercury 9.8hp

              I doubt if the 89 octane gas made a difference, but 87 is fine. With lower compression engines, the higher octane can actually fire less efficiently. It's made to work in higher compression engines. But, I doubt if that would make a night and day difference like what you're describing.

              You can get an OEM service manual for your engine. I'm not sure if they have one for yours, but I got mine at oldmercs.com, and I also have an OEM "owner's manual" which has a different mix of info in it and I find handy.

              I've found the Seloc service manuals for the little mercs to be OK, so maybe you can find one locally, quickly.

              OK, there are others on this board more expert than I am, but this is what I'd do if it were mine, more or less in order (in other words, I'd work through this until it helped, and might not do everything else until I got the inclination to check out the entire engine).

              The fuel filter is probably right near the fuel pump and that would the first thing. I don't know where all this is for your engine, but for mine, the fuel line comes into the engine area (under the cowling), and attaches directly to the filter on the starboard side of the engine (right side as you're facing forward in the boat). It could be a tiny cannister type thing, or as simple as a little screen inside a disk shaped plastic bracket/cover. Then, the next thing in line is the fuel pump which could be a flat outer metal part which sandwhiches rubber diaphragms and gaskets. From there the fuel gets pumped directly into the carb.

              I'd see if the primer bulb sucks flat and/or if pumping it gives you the RPMs and speed when trying to run at high speed. If it sucks flat, there's something restricting fuel before that point - air vent closed or clogged, clogged pickup inside the tank (could be intermittent from debris in tank), etc. If pumping it helps, it's probably a fuel pump issue and that's generally very easy to rebuild, like a 10 minute job and your local Merc dealer should have the rebuild kit and it should be cheap.

              Double check spark plug specification # and gap. Is it possible you accidently changed the gap when you cleaned them? Use the actual recommended brand whether that be Champion or NGK, etc. I doubt brand could make a night and day difference, but in the long run is probably best. Local dealer can give you this info if you're not sure.

              Check compression in both cylinders. Should be >=100psi and fairly even (within ~10%) If one or both is really low, it could make a night and day difference. Report results and if bad, a decarbon procedure might help.

              Check actual strength of spark using an adjustable spark tester. This is the one I have: http://thexton.com/index.cfm?pageid=42&search=404, available many places on line or at auto parts stores. Thers's other similar ones and somewhere you can find home made versions.

              This will tell you the condition of the ignition system. Weak spark in one or both cylinders can cause a night and day difference. If you report results, and they're not good, folks can advise you on how to figure out which parts might be bad, how to troubleshoot, etc. A lot of this stuff is very easy to fix.

              Since the guy said he cleaned the carb, that might not be a problem. However, it could have gotten messed up and cause your problem. The service manual will tell you how to take apart, clean, adjust etc.

              Comment



              • #8
                Re: 1979 Mercury 9.8hp

                lol number one rule of thumb... like d-boat says MAKE SURE THE VENT IS OPEN ON THE GAS TANK! it runs off suction from the fuel pump so if the vent is closed, your engine wont draw fuel properly

                i dont know if you have the walbro or tilliotson carb but they are fairly identical, your fuel filter is identified as part # 27 on the diagram ( http://www.crowleymarine.com/mercury_parts/297/50.cfm ), i dont believe the 79's used the cylindrical fuel filters before they went into the fuel pump in the carb, just the circle screens.

                also make sure your mixture is 50:1 for your motor, you can go a little richer i usually mix mine at 45:1 but too rich you may have reduced performance... if it was running fine one weekend now all of a sudden its not, its probably a simple fuel delivery problem, also check to see that your choke linkage (when off choke) is completely open and not at all turned or blocking the barrell in the least

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