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 Posted: Tue Mar 21, 2006 10:27 pm Post subject: Late Model Rotax 914UL3 Mag Drop |
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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.
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Configuration:� Rotax 914UL3 S/N 4418895 (one year old); pusher; Airmaster/Warp Drive 63" CS prop.
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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.
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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
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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).
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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.
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Hope this long winded missive help future 914 owners experience a smoother engine commissioning period.
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Regards,
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Tam Pho
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