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  • 2 stroke to 4 stroke

    Hello everyone
    I just bought a boat and found out I got rocked. Will only run up to 3/4 throttle. I did a compression check and top cylinder is 120. Bottom is 90.
    it is a 1992 yamaha 40hp 2 stroke. I found a used 2002 50 hp yamaha 4 stroke and wondered it this would be a good buy. I heard 4 strokes are much slower. I do not need to go any slower than I am at 15mph. This is on a 18 ft pontoon.
    thanks

  • #2
    Could be your 40HP 2 stroke just needs some de-carbon and/or carbs cleaning or a tune up.....I'd check it out good before committing it to the junk pile...

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    • #3
      I was hoeing so. But the 120 psi in one and 90 psi in the other scared me. I tried a can of seafoam in a gallon of gas and nothing. I am going to buy some ring free tomorrow and use 2 oz in a gallon to see what that will do. I would love to fix that compression

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      • #4
        The rings might be stuck. All carboned up as they say. I had a 1977 OMC 85 with even mo' worser compression and I fixed it.

        Pulled off the exhaust plate - this exposed the exhaust ports in the cylinders. Rotate the flywheel to bring the rings directly into the center of the exhaust valve. Squit them liberally with liquid wrench, PB blaster etc or MUCH better yet....AeroKroil.

        Fashion a chisel-shaped tool out of hardwood. Take that tool and tap, Ok BANG on the rings. Two stroke rings are prevented from spinning in their grooves by pins. That makes them stick easier than those in a 4 stroke.

        Spin the engine and repeat for each cylinder, but especially the low PSI ones. Repeat this over and over for a few days.

        Put it all back together and run some de carb through the intake. I forget what its called, but get some.

        Shoot in a whole can at idle and then shut it down and let it soak for 30 mins. Fire it up and run the hell out of it for a bit, WOT.

        Re-check your compression.

        Smile.

        Go fish. Take beer. DWLV15AVMAAJ0ao.jpg:large.jpg

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        • #5
          Will try that

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          • #6
            I looked at the exhaust cover and there is no way my hands will fit down at the bottom to get the bolts out. I did see that if I take the plugs out and put the top one in and spin by hand I can only turn it a bit then pressure is to hard to turn. I took that one out and just put it in the bottom cylinder and I could turn it easy I felt a little pressure but not enough to stop it. I can see both pistons are pretty dark. I squirted some ed 40 in the holes will spinning by hand will lit that soak over night. Will se tomorrow if I can lower the bottom cover some how to get to the bolts

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            • #7
              Pull all the plugs. Shoot the cyl's with solvent. Run the starter for a sec or two to toss the solvent everywhere.

              What do you have to lose at this point?

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              • #8
                I looked at the exhaust cover and there is no way my hands will fit down at the bottom to get the bolts out. I did see that if I take the plugs out and put the top one in and spin by hand I can only turn it a bit then pressure is to hard to turn. I took that one out and just put it in the bottom cylinder and I could turn it easy I felt a little pressure but not enough to stop it. I can see both pistons are pretty dark. I squirted some ed 40 in the holes will spinning by hand will lit that soak over night. Will se tomorrow if I can lower the bottom cover some how to get to the bolts

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                • #9
                  To get ride of any confusing I always call it ed 40. But used blaster. Put a straw in the cylinder raised the piston in question and squirted it in there with the outboard as vehicle as I can. If this dont work tomorrow I will try some ring free in it or seafoam.

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                  • #10
                    To get ride of any confusing I always call it ed 40. But used blaster. Put a straw in the cylinder raised the piston in question and squirted it in there with the outboard as vehicle as I can. If this dont work tomorrow I will try some ring free in it or seafoam.

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                    • #11
                      Not sure why my phone is double posting.
                      thanks for the help

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                      • #12
                        Do you have a tach? What is 3/4 throttle?

                        Are you sure it's propped correctly? Just because that's the prop that was on it, doesn't mean it's correct.

                        A 40HP or a 50HP on a pontoon isn't going to make much of a difference, if any. And 15-ish mph... yeah, that's about all you can expect from a rig like that.
                        2000 Yamaha OX66 250HP SX250TXRY 61AX103847T
                        1982 Grady Weekender/Offshore (removed stern drive & modded to be an OB)

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                        • #13
                          Yum

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                          • #14
                            This rig does not have a tac. One is on order. 3/4 throttle is about an inch away from full. If I push it that far she stalls

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                            • #15
                              I dont mo d the 15 mph. Just do not want to move slower if I go with the 4 stroke. Keep hearing 2 strokes are faster. But hoping to resurrect this one

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