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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