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1976 Mercury 85 hp Thunderbolt.

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  • 1976 Mercury 85 hp Thunderbolt.

    I am told this engine is shot. It is worn so that it won't draw fuel at low speeds. It stalls when put in gear and is hard to start unless the idle is set really high.

    The carbs have been cleaned, timing checked, new plugs by a professional.

    Is there any way to rebuild this engine so that it idles properly and run when put into gear at low speeds.
    Attached Files

  • #2
    Re: 1976 Mercury 85 hp Thunderbolt.

    Was a cmpressio test done? If not it can't be said the engin is shot and even then it can be overhauld.Depends on time and money.A overhaul ain't cheap!


    • #3
      Re: 1976 Mercury 85 hp Thunderbolt.

      I believe the compression was good. Would the overhall be cheaper than a new engine or a newer engine?


      • #4
        Re: 1976 Mercury 85 hp Thunderbolt.

        Originally posted by lathehand View Post
        A overhaul ain't cheap!
        Neither is a new motor

        Seriously, you need to weight the costs vs risks. Are you mechanically inclined? Willing to get your hands dirty and search for used repair parts? An overhaul could run around $500-$800 in parts.

        An older motor will be very expensive to maintain if you need to constantly run to the local repair shop for every small thing. Parts are getting harder to find and few mechanics are willing to work on them anymore. A motor of this vintage needs a do-it-yourself owner to look after it.

        Oh, and who told you the motor is shot ?


        • #5
          Re: 1976 Mercury 85 hp Thunderbolt.

          A certified repair guy, has all kinds of certificates and diplomas and stuff


          • #6
            Re: 1976 Mercury 85 hp Thunderbolt.

            This is very interresting post,I own a 72 Merc 80 hp and have my fair share of problems with this motor.But in respect to do it yourself aspect these motors are very straight forward and easy to work on.And yes parts are getting harder to come by.When these motors are running descent they put most newer motors to shame,not to bad mouth newer motors but you cant beat a good old 2 stroke.In my town here in Arizona the Mechanic's wont even touch mine (it's old junk).I am guessing because most newer generation marine Mechanic's dont have the expertise of this era.So it's up to the individual what they want.Newer Motor = $$$$ - older motor parts time $$$$.
            Your call good luck.


            • #7
              Re: 1976 Mercury 85 hp Thunderbolt.

              Do as much testing as possible on your motor.Compression,spark to cylinders at least.If compression is good(all w/in 10% of each other)and spark is good(bright blue spark jumping at least a 7/16"gap tester)then you shouldnt have a bunch of money invested to get it running(provided lower unit is sound).Really,do those tests(or have done)and see exactly where you are.then report back.good luck.


              • #8
                Re: 1976 Mercury 85 hp Thunderbolt.

                These are good engines and worth fixing in my opinion,especially if you have the solid state ( 4 coils for 4 cyl's) spark system, just took mine (77' 850) out again today and runs, starts and shifts like a champ.

                Some of the dealers wont work on these because they wont come apart,the mechanics are new and they dont understand it, some parts 'could' be nla (no longer available),and why bother to learn and work on these when they can sell somebody a new one with a 5 year warranty and have full factory support, then have them walk them through it and pay em' (it seems there have been a lot of issues with the new ones btw)

                these older engines are a pain in there side because they are so good,very well built, fairly simple and they last a long time!

                dont be fooled by some mech/salesman, find someone that actually knows how good these are.


                • #9
                  Re: 1976 Mercury 85 hp Thunderbolt.

                  I agree, most new mechanics don't know how to fix these old 2 strokers so they are garbage to them. Not so. Old Mercs are hard runners but they do need TLC just as the new ones do.

                  Outboards do get old but think of how many hours you'd have to run that thing for it to be "shot". That's almost impossible unless it is a log bronc.

                  Do your own compression test, wet and dry. Watch the "pump up" on each cylinder. Determine the worth of the lower end and if it's solid, (no water in the oil) then put a few bucks into the motor. Buy a carb kit or two, a FP kit and make sure the ignition is hot at the plugs. Linc it and sync it and that engine will be red hot.

                  Sometimes the rings get carboned up and even varnished up from improper/cheap oil or a poor mix. If the cylinders look good try to work the rings free with a good engine cleaner and de-carbon them the best you can. There are spray cans with excellent products to do just that.

                  If you can make it run, use a GOOD quality synthetic at 50-1 and break it in on that. That engine will spring to life with each passing use. Mercs don't die from overuse. They do and will die from poor maintenance.

                  Get yourself a good manual and work it over. Then hang on to your hat, those 850's were a good motor.


                  • #10
                    Re: 1976 Mercury 85 hp Thunderbolt.

                    The reason it wont idle and pull fuel is low crankcase pressure due to the reed block labryth seals are worn on reed blocks. Inline engines that have been run hot or overpropped(lugged) usually have these worn and cylinders have excessive wear around transfer ports. It will still have good compression but run like crap on low end as cylinders "cross talk" thru reed block. This is same problem on the OMC 3 cylinders as seals are made on block. You can overhaul this engine for around $850 and that with everything new...
                    When in doubt,Trim it out!!!
                    Business Owner & Certifed Technician
                    Boats I own & use
                    20ft Javlin w/250 Junkrude
                    17ft Bullet w/200 Merc (SOLD!!)
                    13ft Allison w/150 I-6
                    17ft Hydra-Sport 175GT
                    15ft Allison w/75 Stinger(project)
                    14 x 48 Drifter with 1100SS Merc


                    • #11
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                      Re: 1976 Mercury 85 hp Thunderbolt.

                      Sounds hoakie to me, how could it be shot if you have good/even compression?

                      I have a '77 500 Thunderbolt and I am told that many Merc mechanics won't work on these because they have no cylinder head. From what I understand, the crankshaft with all of the rods and pistons have to come out together from the bottom of the block.

                      They must be good motors though, there are many still on the water. I was told that if you're going to fix it, be sure that yours has no distributor and uses triggers, otherwise it may not be worth fixing.

                      In your case, I would sure look further into the definition of "its shot" before I would give up on it. The reed idea sound right to me, on the 2 stroke dirtbikes it would act the same way.
                      '78 Sylvan Sea Master
                      '77 Merc 500, 4 cylinder Thunderbolt
                      55# Minn Kota V2 Powerdrive

                      Three-fourths of the Earth's surface is water, and one-fourth is land. It is quite clear that the good Lord intended us to spend triple the amount of time on the water than taking care of the lawn.