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Carburettor problem

in 912 / 914 Technical Questions 9 years ago
Gents,

Your expertise is called for. I have an 80hp 912 with 800hrs on it. It has been serviced regularly as per the Rotax documentation throughout its life.

I have had a period of rough running that I have struggled to diagnose. It is particularly rough between 2000-3000 and 4000-4500 rpm. I have checked the carbs, plugs, compression, etc. I have changed all the jets, needles, O rings, diaphragms, etc. I have even checked the throttle shaft O ring.

In the end someone kindly loaned me a set of ignition modules and a pair or carbs. I swapped all of these parts over and the engine ran smoothly.

I refitted my original ignition modules and there was very little difference.

I refitted my original right-hand carb and the engine ran very slightly rougher.

I finally refitted the original left-hand carb and the engine went back to running roughly.

So in summary it looks like I have one carb that is slightly below par and one that has a definite problem.

I have stripped the worst one down and I cannot see anything wrong. Does anyone have any suggestions or am I better off cutting my losses and just finding another pair of carbs?

Thanks,

Kevin

  • Re: Carburettor problem

    by » 9 years ago

    Not sure about the design for the Rotax carbs but most carbs of this type have adjustable needle position which thus governs the mixture at mid and high ranges. Is there a c-clip that inserts in one of several grooves on the main needle? If so, it needs to be in correct groove to provide correct air/fuel ratio.
      
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  • Re: Carburettor problem

    by » 9 years ago

    Thanks James, I have already experimented with the needle position and it only made the roughness worse. These carbs are so simple I can't believe that I cannot find the fault.
      
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  • Re: Carburettor problem

    by » 9 years ago

    Hi Kevin,
    Did the symptoms start out of the blue or after some maint?

    You need to start at the beginning and be very through or you may be looking for this gremlin for a long time. My money says you have an air leak or debris or a part issue inside.

    This could be the rubber carb sockets with a fine crack and may not show up until there is enough vibration to open it some Flex them fairly hard and check for any cracks. It may be the small ball bearings in the carb body from the MFG that have lost there epoxy coating. It wouldn't hurt to re-seal them. Check diaphragm position and the piston position within the carb body. You could have sucked some tiny piece of debris into one of the carb body holes in the interior.

    I would pull both carbs and strip them. Flush all holes, ports and crevices with carb cleaner from a spray can with the small red nozzle and then follow that with some high pressure air. Make sure to clean out the idle jets down through its center. Then examine all parts being re-installed. My bet is if you do this and check for those air leaks you may be good.

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

      
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  • Re: Carburettor problem

    by » 9 years ago

    Hi Roger,

    The symptoms started at the same time I had an inlet valve problem and so the symptoms were masked. It was only when I replaced the valve that I was left with the rough running that I now have.

    I have stripped the carbs multiple times doing exactly what you have detailed above. I have inspected the carb rubbers numerous times and found nothing. I can rule these out as the culprits since I used them with the replacement carbs without any trouble.

    I have already flushed all of the passageways with carb cleaner and blown them through - gently.

    This is why I have resorted to the expertise of the forum in the hope that someone else has had the same problem and found some more obscure problem. I must admit that I haven't looked at the ball bearings so I will check that out, it has got to be something like that.

    Thanks,

    Kevin
      
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  • Re: Carburettor problem

    by » 9 years ago

    I have now had an opportunity to check the ball bearings and they all seem securely epoxied in position so I guess it is not that :-(

    Are there any suggestions for a bench type test that would allow me to find any air leaks?

    Thanks,

    Kevin
      
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