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Late Model Rotax 914UL3 Mag Drop

 
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PostPosted: Tue Mar 21, 2006 10:27 pm    Post subject: Late Model Rotax 914UL3 Mag Drop Reply with quote

We spent the last month diagnosing a mag drop problem while commissioning our new 914.  If you have a "new" 914, you may be interested in our findings.
 
Configuration:  Rotax 914UL3 S/N 4418895 (one year old); pusher; Airmaster/Warp Drive 63" CS prop.
 
Symptoms:  Deep RPM drops (400 - 450 RPM) on both A and B ignition tests; mild detonation in the mid-RPM range (2000 to 4500).  Sluggish during drops.  All temperatures and pressures are within specs.
 
We consulted with several well known Rotax Authorized Service Centers in the US and one in Canada and were advised to try the following:
    - Check plug caps resistance and trim ignition wire ends before reassembly (use dielectric grease to prevent future corrosion).
    - Check primary and secondary coil resistance
    - Swap A and B ignition modules
    - Synchronize Carbs
    - Synchronize Carbs
    - Synchronize Carbs
    - Check propeller for excessive pitch setting
    - Verify return fuel line is not obstructed
    - Replace spark plugs
    - Clean carburetors
    - Verify float levels
    - Perform float needle valve seal check
    - Check carb vent system
    - Move carb 1/3 throttle needle position from 2 to 1
    - Change main jets from 156/158 to 160/164
 
We performed all except the last step without arriving at a solution.  As best as we can tell, the proper settings for our carburetors are specified in Rotax SI-914-015 (pg 3, pp 3.3.2 and 3.3.3).  The document specified 156/158 main jets and position 2 for both carb needle positions for 914 with newer air box (with optimized enrichment system).
 
We finally consulted with Mike at Rotec Research in B.C. Canada and arrived at the solution in a short 2 minute phone call.  Mike told us that the proper main jet specifications should be etched on our air box below the model and serial numbers.  We verified that we have the correct jets (156/158) not 160/164 normally prescribed for earlier 914s.  He then suggested that we move the needle settings to position 3 on both carbs to achieve a slightly richer mixture in the mid-range.  We performed the mods, resynchronized the carbs and now have a very smooth running 914 with mag drops both below 200 RPM.  We suspect variations in manufacturing and environmental factors contributed to our problem (near sea level; around 60 deg F ambient conditions).  We credit the Rotax documentation system for the confusion factor.
 
Hope this long winded missive help future 914 owners experience a smoother engine commissioning period.
 
Regards,
 
Tam Pho
 


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