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912 engine cough in humid conditions

in 912 / 914 Technical Questions 11 years ago

  • Re: 912 engine cough in humid conditions

    by » 11 years ago

    Will do. Thanks again for everyone's input. Steve
      
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  • Re: 912 engine cough in humid conditions

    by » 11 years ago

    Roger: I have finally been able to adjust the prop. angle and do the carb synch procedure. Between getting parts and vacations etc., it took some time. I appreciate your patience. The carb synch procedure I think definitely shows something wrong with my left carb. It showed 25% less vacuum at low rpms than the right and, in fact, I really couldn't get it to idle well at all. I was shooting for about 1700 for an idle, did the mechanical synch (on my gauges, 30s versus 40s), brought each mixture screw out 1 and a half turns, hooked up the gauges, and right away noticed the substantial difference between the left and right carbs. The other thing that happened was it would settle in at 1700-1800 rpms after I reved it and then I noticed the left carb started dumping fuel out of the relief tube and the engine rpms would drop, the vacuum gauge would vary wildly (left side) then it would either stop and the rpms would come back up again or it would die. I tried making the cable adjustment and idle screw adjustment on the left side but it really had no impact on the vacuum reading. It seemed to run fine if I kept it above 2000 rpms (albeit still with the vacuum reading differential.) If I brought the rpms above 4000 the vacuum difference would close substantially but still not the same. I suppose its possible this is not related to my "cough," but obviously something is not working properly in the left carb. Again, any thoughts or suggestions you may have would be appreciated. Steve M.
      
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  • Re: 912 engine cough in humid conditions

    by » 11 years ago

    Hi Steven,

    I have that vary problem in my shop right now. The left carb is pulling way too much vacuum while the right vacuum is about normal. For me that means an obstruction in the idle circuit and the carb has to come completely off and all orifices cleaned and blown out. This is why I like standard dial gauges over the electronic ones. You can diagnose with the dial gauges.

    Sounds like you have the same issue. You can battle the unequal issues for 1-2 days and then have to pull the carb any way and clean it or just do it now and be done with it in an hour.

    Here is my example and your gauge numbers may be very different.

    First, the carbs sync fine at 3500+ rpm because I'm out of the idle circuit., but at idle my right carb is around 14-15 inches of vacuum while the left carb is pulling 21 inches. I never see that unless the high vacuum side has an obstruction. So off it comes tomorrow to get cleaned and then a fresh carb sync.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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  • Re: 912 engine cough in humid conditions

    by » 11 years ago

    The other difference is that my left carb, the one spewing fuel and with the vacuum fluctuation, is pulling the lower number. Does that matter?

    Also, any possibility this is related to my original problem, the occasional cough at 4100-5000?

    Your belief is that I likely have some physical obstruction in the idle circuit?

    What causes the fuel to be pushed out the overflow?

    Again, thank you for your time and expertise. Maybe I'm finally getting to the root of this.
      
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  • Re: 912 engine cough in humid conditions

    by » 11 years ago

    Hi Steven,


    Yes it could be part of your problem.
    Vacuum gauges usually have number indications from 0-30. The higher the number the more vacuum it is pulling. The lower numbered gauge will be getting more fuel/air than the one with the higher number. If one is a much lower number and puking fuel out the vent then you may want to remove the carb or carbs and give them a good cleaning and check the float level. You may also have a bad or obstructed float needle valve which still requires
    dis-assembly and at a minimum a cleaning. If you have many hours it may be time to rebuild them. Check and make sure you do not have a bad float.

    p.s.
    If the engine is shaking really bad on the ground then the floats have a hard time controlling the fuel level and sometimes fuel can come out a vent until you smooth it out.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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