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Airplane Light Sport Aircraft ROTAX Engine 912: Rough Idling & Carburetor Synchronization

Curious about Airplane Light Sport Aircraft ROTAX Engine 912 rough idling & carburetor synchronization? We’ve got answers to your questions here.

Cold Starting

When the engine is equipped with a choke (enriching circuit), always keep throttle at idle or choke will not function. As soon as it starts, can increase throttle slightly for smooth running 2000 RPM, gradually disengage choke. Run at 2000 for two minutes then 2500 RPM for warm up.

When cold starting using primer system, you can crack the throttle and prime while cranking engine too avoid flooding (similar to other aircraft engines cold starting procedure).

Smooth Idling

Smooth idling is important to gear box health and achieved mainly through proper idling speed and carburetor synchronization.

If after proper idling speed/carburetor synchronization have been achieved and the engine is still not idling smoothly, consider:

  • Air and fuel systems
  • Carburetors (jets plugged)
  • Gear box may need servicing

Correct minimum idle speed is:

  • Typical optimum idling speed is 1800 RPM
  • Avoid setting warm idle speed above 1800 RPM or choke (start) will not function properly
  • Extremely light weight propellers can allow for lower idle settings for unique applications, but never allow idle speeds below 1400 RPM as this can cause rough idling and engine stoppage.

Rough Idle Gearbox Clatter Sound

  • Fine line between idling at correct RPM and idling too low.
  • Clatter is hard to hear but experience will help to identify the torsional vibration damper working, which causes the gear box clatter.
  • Rough idle will wear out the propeller splines on the propeller shaft.

Carburetor Synchronization

  • Carburetor synchronizations are recommended initially, annually, every 100 hours and when the idle becomes rough or the gearbox clatters
  • Carburetor synchronization is an involved process and should only be done by qualified repairman trained specifically in throttle settings and carburetor adjustments.
  • “Carb sync” kit has two vacuum (not pressure) gauges that are matched to read the same, actual values are less important. A pressure dampening orifice is installed between the gage and the carburetor to dampen out the pressure surges in each line for steady gauge readings.
  • Connecting the “carb sync kit” is accomplished by hooking each gauge to each carburetor via the carburetor balance tube.
  • Mechanical throttle settings are first set then carburetors are adjusted so “carb sync kit” needles read identically.
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