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High idle Yamaha 40

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  • High idle Yamaha 40

    I have a Yamaha 40, 2004. It has started "idling" at 2400 rpm. I replaced the thermostat today as the rubber had cracked. High idling can be a symptom for a stuck thermostat . After that the engine was normal, idling at 7-800 rpm and we took it for a test run. 1 km at full throttle, then idling, stopping and starting as normal. New 1 km run. And then the high idling returned, more persistent than ever. I notice some blue smoke. Can problems with the electrothermal valve result in high idling?

    How does the engine govern idle speed?

  • #2
    I'd remove the upper cowling and inspect very carefully the throttle cables, carb linkages, carb butterflies, etc.....get your buddy to operate the throttle controls while you watch the movement....


    • #3
      advanced Timing can increase RPM also


      • #4
        Originally posted by robert graham View Post
        I'd remove the upper cowling and inspect very carefully the throttle cables, carb linkages, carb butterflies, etc.....get your buddy to operate the throttle controls while you watch the movement....
        The problem has been intermittent for two years. We had a Yamaha service technician look at it and he adjusted the throttles, idle setting etc. Then it was ok for two months before the problem returned. If the engine was revving high (over 2000 rpm) it has usually helped to wait it out or stop the engine and wait for a while. The throttle controls look alright, but I will have an extra look at it. And look over the electrical connections to the electrothermal valve (automatic choke). My current hunch is that there is some bad electrical connection somewhere.

        Originally posted by 99yam40 View Post
        advanced Timing can increase RPM also
        Would advanced timing be intermittent?

        Thanks. I will report my findings back.


        • #5
          I have checked the throttles. Everything seems fine, no slack or lose items. The throttles move simultaneously and look ok.

          The engine starts perfectly, but instead of reducing rpm from 1500, it increases to 2100, 2400, 2600 where it fluctuates somewhat over time. Tried turning the emergency enrichment handle from "normal" to "on" and "off". In the "on" position the engine reduces rpm and often quickly stops. If I quickly pass the "on" and set it on "off", sometimes the rpm normalises to 800. After that the engine has run as normal for my test run 1 + 1 km. Just as I thought I had found out something significantly, the rpm in idle got up to 2400 again with the handle in "off".

          I give up. Yamaha service mechanics will have a look tomorrow.



          • #6
            The engine has been in for service for a week now. The mechanics have now assembled it and run it in the test pool for a while without finding any problems. Changed some sooted plugs and the anode... Curious to see how long it works this time.


            • #7
              Were the carbs ever pulled prior to the issue?

              A missing o-ring, intake gasket, etc, can cause it to lean out and idle high as well.
              1997 Angler 204 CC, 2006 F150 TXR


              • #8
                I still have to wonder if this was a F40 or a 2 stroke, along with what the timing was doing when the idle went high

                on my C40 I could take the linkage loose on the wiper/pointer on the cdi and advance the timing while idling and the RPM goes up high with advancing the timing
                Last edited by 99yam40; 07-28-2017, 08:55 AM.


                • #9
                  ^^, yep, your right.. Don't know if 2 or 4 stroke, year either...
                  1997 Angler 204 CC, 2006 F150 TXR


                  • #10
                    I have never removed the carbs, but can't say what Yamaha service have done throughout time. The high idling started two years ago and comes and goes. Lately it came and wouldn't go away easily. Here is the serial number which should identify the unit: Yamaha 40, YETO 1000142. I think I bought it new in 2004.

                    Thanks for your insights. When the problems appear again, I will ask Yamaha service to check your suggestions.


                    • #11
                      is it a F40?
                      or do you add 2 stroke oil to a tank or into the fuel?

                      or does it have motor oil and filter that you change from time to time?


                      • #12
                        Sorry. It is a 2-stroke with an internal 2-stroke oil tank next to the carb.


                        • #13
                          Hi again,
                          the high idling has returned, only a few days after the mechanics had assembled it and could not find anything wrong. I am not sure what they tried, but it included testing in a tank.

                          A typical occurrence can be like this: Start engine, rpm starts at 1500 and quite quickly increases 2000, 2300, 2400 where it fluctuates over time between 2200 and 2600. Then sometimes as if someone flicked a switch, the rpm goes down to 800. Other times I would stop the engine after a while, restart and everything is normal again, starting at 1500 and then reducing to 800 after about 15 seconds, perhaps with a short visit to 2300 before reducing rpm. Lately, before having it serviced, I could wait or restart several times with no appreciable effect.

                          Operating the emergency enrichment could result in reduced rpm. When I replaced the thermostat, the engine was normal for a while (2 km). Any ideas? Can the professional mechanics have assembled the carbs wrong, leaving out some gasket or something? They have at least previously adjusted the throttles and linkage without removing the problem for more than a while. I am still suspecting some electrical connection to be the culprit, but maybe you can figure out what is wrong.

                          In a different thread about a similar problem I got his post:
                          Originally posted by AverageBob View Post
                          Well it did it again, so I got to poking around and found that one of the 4 wires going to the ISC motor was loose. I slightly crimped it and it has gone out several times with no problem. I Don't know what year yours is Cutty, but they used aluminum wire for a time that likes to break inside the coating.


                          • #14
                            Cutty, you don't have have an Idle Speed Control.

                            Your carbs have a simple stop screw, part #7:
                            2004 40TLRC Yamaha Outboard CARBURETOR Diagram and Parts

                            You have a Prime Start for cold starting. Your cold idle is very high as well.

                            It almost sounds like the ECU is advancing the ignition, raising the idle, as the carbs would pretty much do the same thing most of the time...

                            Just for S &G's re-check the throttles at the carbs when it acts up, ascertain they are closing fully.

                            Also, remove and inspect where the ECU plugs into the wiring harness for any corrosion, loose wire etc.
                            Try slightly moving those wires (gently) and see if acts up..

                            Can you post a video when it starts acting up, preferably with the cowl off.

                            If that doesn't work, find another shop.
                            Last edited by TownsendsFJR1300; 08-03-2017, 07:25 AM.
                            1997 Angler 204 CC, 2006 F150 TXR


                            • #15
                              as i have said several times, check to see what the timing is doing with a timing light to see if that is in spec when this happens.

                              very simple to do