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Chrysler 908h4b

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  • Chrysler 908h4b

    I am picking up a Chrysler 908h4 (1984 90hp 4cyl) for a $100. Comes with controls, and extra tilt/trim pumps. It runs but has a lower unit issue which may be the dog legs on the sliding clutch. This will be replacing my 1969 Merc 800 (66cu/in), I am just tired of having to manually tilt the engine when I launch or retrieve the boat out of the water.
    With my wife and a light load in the boat (no fuel in the forward tank) @ 5300~5400 with a 13x17p we do about 33~4 mph average (according to my gps) lower unit is a 2.33:1 ratio. Anyways the merc runs good but I am underwhelmed by its top end performance, It runs more like a modern (prop rated) 65-70hp.
    The chrysler will be a winter project and I am planing on doing a complete teardown and restoration on it. They look real eazy to work on.. On the merc to get the starter out you have to remove the distributor then carbs.

    I have been reading as many of Franks posts as I can. (RIP frank, I wish u were still around to ask questions) From what I understand this motor used to be rated as a 105 and down rated in 83-84 as prop rated.
    My plan is to use the TC carbs, manifolds and reeds form a force 125. Also "square up" the bypass ports. I have a tig welder and may look at putting in a divider in the exhaust to separate cyls 1-2 from 3-4 so that the exhaust ports can be opened up slightly.. What I hope to accomplish is an honest 110hp at the prop and hopefully outperform the old merc 800..
    I am open for suggestions,
    robert.
    1965 Pacific Mariner after 2 year restoration

  • #2
    Really interesting project to follow.
    Hope you will post as the project progresses.

    What ignition system does it have?
    Prestolite without dist or Motorola with dist?

    I have a Merc 800 1972 myself and a lot of Chrysler and Force 70,75 and 120Hp..
    The 800 Merc is a wounderful engine, runs really smooth.

    Comment


    • #3
      It has the Prestolite.

      Comment


      • #4
        About the only difference in the 125 and the 90 is the crank.
        They used the same pistons and rods.

        The way I understood the HP ratings: Chrysler rated the hp at the crank.
        All others rated it at the prop.
        ​Somehow it added 5 hp to the 85's making it a 90 and took away 5hp from the 125's making it a 120??

        Good luck on the build.
        ​Just make sure the intake ports and the carbs match.

        ​They used about 100 different carbs on the 85-90-105-115 etc.


        Buy American made Cars and Trucks!!!Towing insurance for a boat,a GREAT thing.
        ​Tools to make my life easier: Battery tester, die grinder, temp.gun, compression tester, DVA meter.

        In Scenic , Beautiful San Sebastian by the sea Florida

        88 Bayliner 21CC 2001 Mercury 225 HP EFI

        Comment


        • #5
          Got it home, The plus is it has a 17 pitch stainless cupped prop.. The lower unit will not shift out of neutral the rod will not move up or down prop spins freely with no rough spots. I got the starter going so I shot a little tc oil in the cylinders and checked the compression, 155, 153, 152, 155.. (according to my gauge). Some parts are missing (carb covers, terminal block). Came with a new in the box trim pump/motor (two wire), and three wire harness of various lengths.
          I found a 1985 125hp with low compression on two cylinders, complete that looks like new, $200.. So I may be building Frankenstien...

          Comment


          • #6
            Low comp on 2 adjacent cyl??
            That usually means a bad head gasket???
            You might have a runner with only a bit of work??

            All the Force props even the Stainless are cupped.
            It doesn't have to have a beveled edge for it to be cupped.
            A 17SS is about equilivent to a 19 alum.


            Buy American made Cars and Trucks!!!Towing insurance for a boat,a GREAT thing.
            ​Tools to make my life easier: Battery tester, die grinder, temp.gun, compression tester, DVA meter.

            In Scenic , Beautiful San Sebastian by the sea Florida

            88 Bayliner 21CC 2001 Mercury 225 HP EFI

            Comment


            • #7
              Just an update.. I picked up the 125. (IIRC it is a 1986) Cyl #2 and #4 approx 20psi compression.. I pulled the head off (had a brand new head gasket, zero carbon on the head) and #2,4 has galled piston on the exhaust side of the bores.
              The 125 has a decent lower unit, it shifts and spins smooth, very little corrosion. It leaks a little oil for the shift pin, The thrust washer must of spun or it has the wrong one. It has a #3 mark on it... 2.0:1 ratio, 19P aluminum prop, plastic flair washer..Has a stupid "hydo foil" bolted on it..
              Has blue coils and cdi boxes (one box is burnt on the back) that were grafted onto it, thank god they did not cut the plugs off the stator..
              Tilt trim works and is corrosion free. I assume it has mostly been run in fresh water.

              The 90 lower unit will not shift, the shift rod is stuck (light tapping with the handle of a hammer will not budge it) #4 mark 1.73ratio. Stainless 17P, has the plastic flair and tail cone. power head has great compression... 150+ varies about 3lbs between cylinders. Broke 3s bolt holding on the exhaust extension, 2 on the lower cowl, 2 on the rear mid leg cover.. it was used in salt water.. Has a newer trim cylinder, upper part of the tilt cylinder is "rusty" the tilt tube is only coming out if I cut and hammer it out..
              I think I may have enough to put one together..
              My plan is to pull the 125 block apart and see if the 2 bad cylinder will clean up. If they do I will use the rods/pistons out of the 90. If not then the 125 crank can go into the 90 and I will modify the ports to match the 125 timing.. I am hoping I can use the 125 block as it will be a less work..
              I will post pics when I take them..
              Robert. .

              Comment


              • #8
                Good luck.
                ​I've found along with PB Blaster and Krol(?) a mix of acetone and trans fluid helps free stuck bolts and stuff.


                Buy American made Cars and Trucks!!!Towing insurance for a boat,a GREAT thing.
                ​Tools to make my life easier: Battery tester, die grinder, temp.gun, compression tester, DVA meter.

                In Scenic , Beautiful San Sebastian by the sea Florida

                88 Bayliner 21CC 2001 Mercury 225 HP EFI

                Comment


                • #9
                  Tore into the 125hp tonight. (1258x5A). Cylinders #2 and #4 have piston galling,the rings did not break/catch on the ports. No broken bolts or use of penetrating oil, even the bypass screws came out. The bores measure 3.3125~8 standard. This is the biggest 2 cycle engine I have ever worked on.. I have had a few 2 cycle dirt bikes growing up as a kid (1982 cr125 and a 1984 cr500) and am very familiar with galling a piston.. I used to clean the aluminum from steel liners with muriatic acid as it would aggressively attack the aluminum and usually just clean the steel liner. I think I have a very good chance of saving the liners without re-boring.
                  The plan now is to take the pistons/rods out of the 908h4b since it has more corrosion in the water jackets and use them in the 1258x5A block..
                  Any idea as why #2 and #4 galled.. no signs of piston over heating or detonation. I think it may of been lack of lubrication. The re-circulation feeds into cylinders1,3 and the small reeds are in perfect shape.

                  Comment


                  • #10
                    Took a few hours to get the pistons out of the 908H4B. Only 3 bolts did not break on the exhaust cover, and I had to drill 4 head bolts to get the head off. I did not split the case on the 90hp to get the pistons out (I was done breaking bolts)
                    The 125 (1258X5A) came apart with no troubles, even the bypass cover came off..
                    What is the deal with the three rings pistons. Will there be any problems using them in the 125? The rings are the same as the two ring pistons. I can see having a little bit more "drag" from the extra ring,

                    Comment


                    • #11
                      Sign up today
                      3 rings.Shouldn't be a problem


                      Buy American made Cars and Trucks!!!Towing insurance for a boat,a GREAT thing.
                      ​Tools to make my life easier: Battery tester, die grinder, temp.gun, compression tester, DVA meter.

                      In Scenic , Beautiful San Sebastian by the sea Florida

                      88 Bayliner 21CC 2001 Mercury 225 HP EFI

                      Comment

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