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HELP.. Mercury 850 (85 HP) Issues

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  • HELP.. Mercury 850 (85 HP) Issues

    Ok, so I bought my first boat in April. It is a 1972 Larson Trihull with a Mercury 850 attached. I have taken the boat out approximately 5 times since I purchased it and it has ran like a champ until yesterday.

    It started up and idled great. It went into gear and ran moved nicely across the water. As soon as I crossed the no wake zone, I ran into issues. I put the throttle approximately 3/4 of the way down. The motor started to go, but then fell flat on its face. It would not move at all. It wouldn't open up and I could only go approximately 10 MPH. It kept bogging down.

    I puttered around the lake for 5 hours, mainly fishing, and then brought the boat home. The whole time on the lake it bogged down going more than 1/2 to 3/4 throttle, but ran like a champ below 3/4 throttle. At home, I ran it on muffs and it ran like a true champ (even at full throttle). It did not bog down at all while on the muffs. I thought maybe it fixed itself, ya right. So, just in case it didn't fix itself I went to the local marina and got some suggestions.

    After speaking with them, I came back home and I ran a compression test. I got 115 PSI in each of the cylinders. I then went out and bought some BG fuel additive (suggested by my Marina) and x4 new spark plugs. I added the BG additive to 6 gallons of fuel (nearly double the suggested amount) and changed all of the spark plugs. I was feeling confident that this may solve the problem, so back to the lake I go.

    At the lake, it started and idled just fine. I got past the wake zone and got similar results. This time it seemed to go a little faster, but still the bogging issues. I figured I needed to run the entire tank through the engine to clean it out (with the BG additive). While riding around, the engine would fluctuate and it seemed like it was going to take off, but it never did (it did this several times).

    After I drained the first 6 gallons, I switched to my second tank. This time I disconnected the fuel filter (figured it maybe clogged) and ran fuel straight to the motor. Still no luck and lots of bogging and sputtering. We put the filter back on. I then had my friend pump the primer bulb while I got on it (the engine) and "boom" it seemed to take off. We did this several times and each time it seemed that pumping the bulb got the motor running correctly.

    So, my question is this. Does that mean its a fuel pump issue? To my knowledge, it had never been re-built or replaced. If its the fuel pump, then why does it run perfectly while out of the water? Is there a way to test the fuel pump? What PSI should the fuel pump be putting out?

    If it is the fuel pump, where can I find a replacement? What goes bad in them? I looked everywhere online and cannot find a place to by a new or used fuel pump for my motor. I believe my engine is a 1972 or 1973 Mercury 850 (85 HP), SN 7083192. The fuel pump has the #73433 etched on the top of it (probably a part number). Any help would be great. Thanks in advance.


  • #2
    Re: HELP.. Mercury 850 (85 HP) Issues

    Jimmy;

    The s/n would indicate that your engine is Canadian built 4 cyl:

    85 1976 EL 4366802 - 4590511 7071373

    From here:

    http://www.maxrules.com/models/Merc_...es/60-100.html

    Parts listing:

    http://www.crowleymarine.com/mercury-outboard/475.cfm

    Service Manual p/n: 90-86134--4

    http://kingston.kijiji.ca/c-cars-veh...AdIdZ285464523

    If you look under Fuel Pump in the spares listing you will see the Check valve and Diaphram kits which will allow you to repair your fuel pump.

    The reason it runs perfectly out on the water when your friend is pumping the bulb is that the bulb is now providing the necessary fuel pressure in lieu of the defective fuel pump.

    Gerry

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    • #3
      Re: HELP.. Mercury 850 (85 HP) Issues

      Thank you very much. You are 100% correct about my motor being at 1976. I still don't understand how you got the year from reading my serial number, I looked at the above website and still cant figure it out. I appreciate you taking the time and responding to my post in such detail. Thanks.... I am going to order the diaphram kit.

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      • #4
        Re: HELP.. Mercury 850 (85 HP) Issues

        I noticed that you ran it at full throttle on the muffs; don't run it above 1200 rpm when not under load!
        If you have these carbs with the flattop there's a filter inside that might be plugged
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        remove them, unscrew the fuel fitting and look inside.
        I had one that was full of lint.

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        • #5
          Re: HELP.. Mercury 850 (85 HP) Issues

          Originally posted by merc850 View Post
          I noticed that you ran it at full throttle on the muffs; don't run it above 1200 rpm when not under load!
          If you have these carbs with the flattop there's a filter inside that might be plugged
          Click image for larger version

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          remove them, unscrew the fuel fitting and look inside.
          I had one that was full of lint.
          I didn't run it very long with muffs on, but I will keep that in mind next time. I will check out my carbs and see if they are similar to the ones u posted.

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          • #6
            Re: HELP.. Mercury 850 (85 HP) Issues

            ive had the same exact issues for all of last fishin season and tried alot of different things to no avail. i finally decided to take it to a mechanic. by the way i have a 1977 850 merc. anyway he ran some tests and found that my stator was bad. i havent replaced the stator because i now have a hole in the water jacket and am debating getting a newer year. hope this helps.

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            • #7
              Re: HELP.. Mercury 850 (85 HP) Issues

              Well, today I received my fuel pump rebuild kit. It only cost me $5.00 and was very simple to install. While I was installing it, I noticed a thin fuel line leaking that runs from the bottom of the engine to the top of the engine. I don't know what it is for, but I replaced it with a new yellow fuel line. I will post some pics maybe someone can tell me what it does and what it's technical name is.Click image for larger version

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              • #8
                Re: HELP.. Mercury 850 (85 HP) Issues

                Originally posted by vadigger View Post
                ive had the same exact issues for all of last fishin season and tried alot of different things to no avail. i finally decided to take it to a mechanic. by the way i have a 1977 850 merc. anyway he ran some tests and found that my stator was bad. i havent replaced the stator because i now have a hole in the water jacket and am debating getting a newer year. hope this helps.
                Thanks for the post. If my latest updates (fixed leaking fuel line and rebuilt the fuel pump) then I will focus on the Stator. I want to rule out the cheap things first. Did the mechanic give you a quote on how much it would be for the part and labor?

                Comment



                • #9
                  Re: HELP.. Mercury 850 (85 HP) Issues

                  bump bump bump

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                  • #10
                    Re: HELP.. Mercury 850 (85 HP) Issues

                    That small rubber tube is the bleed system on your inline 4 motor. Any oil that pools on the lower main bearing is pumped thru that hose to the upper main bearing. Keep an eye on that yellow stuff you added, to make sure it holds up to fuel and temperature conditions in the motor.

                    Comment



                    • #11
                      Re: HELP.. Mercury 850 (85 HP) Issues

                      I have noticed some wires that need attention/replacing.
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                      • #12
                        Re: HELP.. Mercury 850 (85 HP) Issues

                        You might look over the whole motor harness. If it is real bad, consider replacing it. You can try surplusunlimited.com for replacements.

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                        • #13
                          Re: HELP.. Mercury 850 (85 HP) Issues

                          Originally posted by Chris1956 View Post
                          That small rubber tube is the bleed system on your inline 4 motor. Any oil that pools on the lower main bearing is pumped thru that hose to the upper main bearing. Keep an eye on that yellow stuff you added, to make sure it holds up to fuel and temperature conditions in the motor.
                          I watched fuel move from the bottom connection up to the top connection, does it always bleed this way? I will keep an eye on the yellow hose, it was the only hose that i could find with a small size hole. The hose is a Holley and it made for fuel.

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                          • #14
                            Re: HELP.. Mercury 850 (85 HP) Issues

                            Originally posted by merc850 View Post
                            I have noticed some wires that need attention/replacing.
                            Click image for larger version

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                            Yeah, I have noticed a lot of wires that need attention. I guess the times are catching up to the old wires.

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                            • #15
                              Re: HELP.. Mercury 850 (85 HP) Issues

                              Originally posted by Chris1956 View Post
                              You might look over the whole motor harness. If it is real bad, consider replacing it. You can try surplusunlimited.com for replacements.
                              I will check out the website, thanks.

                              Comment

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