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compression 50 hp yamaha 4 stroke

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  • compression 50 hp yamaha 4 stroke

    I have a 50 HP yamaha 4 stroke OB, measured the following compression Cyl 1/100 psi, 2/140 psi, 3/100 psi, 4/100 psi.
    understand that 90-100 psi is normal. Why would cyl 2 read 140 PSI. and what can be done?

    Did the leak-down test with 100 PSI the air loss was zero

  • #2
    Re: compression 50 hp yamaha 4 stroke

    This is a 4 cylinder 50HP motor?....Regardless, if any of your compression readings are on the high side, where's the problem?

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    • #3
      Re: compression 50 hp yamaha 4 stroke

      It could be that way because #2 is not at fault and the other three are. Why are you doing a compression test? It may also be because that cylinder is using oil. A leakdown test would prove which cylinders are actually at fault. An engine is considered to have normal compression if the numbers are over 100 and within 10% of each other. Yours are not.

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      • #4
        Re: compression 50 hp yamaha 4 stroke

        I always thought 4 strokes had a lot higher compression than 2 strokes. Yours looks like a bad reading on a 2 stroke.
        I would try to find someone to get a reading on a different 4 st 50, just to make sure.

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        • #5
          Re: compression 50 hp yamaha 4 stroke

          As long as an engine has compression over 100 psi it will likely run very well. Maybe not making 100% of available power but nevertheless a good running engine.

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          • #6
            Re: compression 50 hp yamaha 4 stroke

            he said he did a leak down test and got zero leakage.
            So I wonder if his tester or method may be the problem

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            • #7
              Re: compression 50 hp yamaha 4 stroke

              Gentlemen, thank you for your response
              Engine problem was rough running, unable to get to max speed and difficult to start.
              Took the carburetors apart and cleaned the float chamber and nozzles, engine runs better.
              Did the following tests:
              Leak-down test 0 %
              Compr. test carbs removed open trottle closed trottle
              cyl 1 210 200 100
              cyl 2 210 200 110
              cyl 3 210 200 130
              cyl 4 210 200 135

              It seems obvious that the problem is related to the carburetors is there anyone who can direct me which part of the carburetor can cause this problem?

              Again many thanks gerry

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              • #8
                Re: compression 50 hp yamaha 4 stroke

                Your carbs have not been synced properly.
                Need a 4 tube manometer and follow directions in manual.
                Seem you did not take compression 1st time with carb throttle plates cracked open some and some of them are closed too far.
                All need to be open the same for motor to run properly

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                • #9
                  When you do a compression test, it should be done at WOT. Whomever told you 90-100psi is normal was completely wrong... 90-100psi on a 2 stroke you're thinking about rebuild time. 4 strokes have higher compression...... Your compression is very good at 200 WOT.

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                  • #10
                    why would a mod bring up and post on an old thread??

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                    • #11
                      Sign up today
                      for what i know 50hp 4 stroke is 9.5x14.7 is--------- 139 psi
                      Compression Ratio 9.5:1

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