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Yamaha F100A has got the sneezes

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  • Yamaha F100A has got the sneezes

    Hi I have serviced my Yamaha F100A but it has not cured the sneeze.

    I have done:

    New plugs
    New fuel filters
    New Anodes
    New timing belt
    4 carb kits carbs carefully serviced spray and wire and daylight through every hole. Float level checked and set on all 4.
    Brand new super fuel.
    Service if the prime starters - similar wire / spray through all holes etc...

    But in idle it sneezes - see the video.

    I have thought (maybe wrongly) that it was lean sneeze. The stock needle setting is 2 out. I have run it at 1.5 out. 2 out. 2.5 out. 3 out.... etc.. no difference.

    I have also run it while pumping the bulb to keeo that hard to make 100% sure that each float bowl is fuly full. No difference.

    It starts perfect. Runs smoothly. Just sneezes.

    I don't know if it will do this under load - I can't test that at the moment.

    Any ideas would be much appreciated.

  • #2


    • #3
      might need to do a heated sonic bath clean of those carbs.
      the idle/pilot jets and passages may be hard to get cleaned properly.

      also did you follow the link and sync procedure in your service manual after reintalling the carbs to be sure that are all synced together properly?
      they all need to be able to have the throttle plates open the same amount especially at idle.
      only way to be sure is to sync them with the plates completly closed, not just sitting one the idle cracked open position.


      • #4

        Ok I am a dumb ass

        i wrote in the original post that it sneezed when idling.

        it is the opposite - it idles well but a little high- it only sneezes over 2k rpm.


        Also no I have not balanced the carbs - it’s that really likely to fix a misfire ?


        • #5
          Sneezes above 2000 RPM in gear when pushing the boat?

          Like voltage, testing without a load means virtually nothing.


          • #6
            boscoe99 i have only run it on the muffs so I do not know if it sneezes when in gear and pushing the boat - you are right it may not


            • #7
              since this is a 4 stroke, is this thing popping out the exhaust or backfiring thru the carbs?

              my mind was thinking 2 stroke lean sneeze at idle.
              I cannot see the video unless I create an account and sign in


              • #8
                I have trying to upload the video - this time on youtube.
                video, sharing, camera phone, video phone, free, upload


                • #9
                  Video is viewable now, but I have no idea what would cause that.
                  it does not sound like it is happening in the exhaust side.
                  intake Valve not closing off completely/sticking open somehow maybe.

                  firing a plug at wrong time, but very intermittent.
                  timing light might rule that out if you monitor each plug while it is happening
                  Last edited by 99yam40; 3 weeks ago.


                  • #10
                    OK I have got a timing light somewhere I might try that. It's so small and quick that it will be hard to spot.

                    It was the same as this before the service I did - in other words the service didn't cause it or make it better. I didn't sonic the carbs but they were very very clean and when it's at 2.5k revs like in the video the throttle is open and the main jet is doing all the work - the main jets are definitely not blocked.

                    I was very careful with the timing marks when I did the timing belt. And after I wound it by hand a few times to make sure that they stayed in the right place. Anyway as I said it was doing it before.


                    • #11
                      I am not even sure a 4 stroke can have a lean sneeze, like the 2 stroke motors are.
                      with the timing light at least you can try to see something different on one of the plugs.
                      hitting at the wrong time just sometimes, or not hitting at all sometimes causing raw fuel to hit the exhaust where it would light off from the hot burning exhaust gases in there


                      • #12
                        Valve clearances ?


                        • #13
                          We had a similar tick not long ago on this forum, something in the air box I think. You need to look on past postings.
                          I notice your oil dipstick wobbles, could it be letting air in? Unlikely, but investigate: whilst engine running, push it in, pull it completely out, to see if this influences the tick (miss fire?).


                          • #14
                            Dipstick tested no difference.

                            Air box clean as a whistle.

                            Carbs unltrasonic cleaned.

                            Primary and secondary side of coils tested - in spec.

                            HT leads tested - in spec.

                            Spark jumps 11mm - to spec.

                            Compression test gives 175PSI+ on all four pots.

                            Still misses........


                            • #15
                              A clue???

                              I was checking for air leaks due to the prime starters - were the orings in place etc..

                              I saw what I thought was a crack in the prime start body. Pretty easily a piece came off - it had been glued / bodged.

                              The piece missing is right from the bottom - I think it takes fuel in from the bowl and uses it to enrich the mixture at the start.

                              Anyway maybe air has been getting in there.... ?

                              I don't see how this will solve the prob because it didn't show on the vaccuum balance and also when the prime starter is warmed up it expands the plunger and blocks all this up....

                              In other words when the engine is warm and missing like the video then this hole is plugged anyway...

                              But it is so far the only component that I can find that has any defect so it's the best lead I have got at the moment.

                              266 plus shipping ouch.

                              Broken prime start body.jpg