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rotax 4-stroke - starting
912UL, 912A, 912F, 912ULS, 912S, 914UL and 914F Engines
All information provided here is correct at the time of publishing to the best of our knowledge, but Rotax recommendations may change at any time without our knowledge. It is the engine owners responsibility to ensure that the engine is fit for flight at all times. The information is provided here free of charge and with no obligation from us to the consumer. To ensure that you have the correct and most up to date information, you should refer to the current Installation, Operators and Maintenance manuals for your engine. The information provided here may not be valid for aircraft outside the UK. Click here for ConAir SportsTerms& Conditions, including website usage policy.
Heavy duty starter motor
Gold in colour
900W
The minimum current required for cranking is 75A.
Sprag
The sprag clutch is a one way device. It provides a sort of permanently engaged starter. A simple but effective device that engages in one direction of rotation but free wheels in the other direction. If the sprag clutch becomes damaged it can slip resulting in a low cranking speed which in turn can cause kickback.
Standard starter motor
Black in colour
700W
Occasionally the brushes wear out, but a starter motor is generally good for 1500+ hrs
Solenoid Clicking
Most likely caused by a very flat battery, poor connection or a faulty starter switch.
Battery
Small / degraded batteries can cause damage to engine components. High cranking speed is essential to ensure nice clean starting. Battery should be 16Ah (a battery with at least 200A cranking current should be selected.
Start Engaged Light
A starter engaged light is required on all LAA aircraft and should ideally be used on all installations.
Low Cranking Speed
Could be caused by a number of faults but often results in kickback. Symptoms are similar to the engine hydraulic locking. Causes of low cranking speed include:
- Poorly charged or defective battery
- Battery of insufficient output (starting current or capacity)
- Defective starter motor
- Faulty starter solenoid
-Battery charging system operating incorrectly
- Damaged or worn sprag clutch or sprag clutch housing
- Wires of excessive length to the starter motor
- Bad or corroded electrical connections
- Wires of insufficient current carrying capacity I.e. Too small a diameter or too high a resistance
Kickback
The engine appears to hydraulic lock, Ii.e. the engine often fires once or twice and then stops violently. The main reason for this is a low cranking speed.
Possible Remedies
Not necessarily in order....
- Check battery condition (charge)
- Check charging system
- If difficulty is experienced with starting, try reducing the plug gap to 0.5mm
- Check gearbox friction toque - increase if low- click here for friction torque details
- Replace sprag clutch - click here for replacement kit
- Warm engine & oil (pre-heat engine)
- Check operation of starter motor (strip & clean)
- Install heavy duty starter - click here for starter motor details
- Fit soft start module - click here for soft start module details
- Use new flywheel with retarded timing - click here for details of flywheel
- Fit
Rotax smd module with advance start - click here for smd module details
- Ensure throttle movement is full & free - throttle fully closed is essential for the choke to work.
- Carry out normal pre-start checks, then crank the engine over with the magnetos turned off until the oil pressure rises. Max cranking 10 seconds then rest starter for 2 minutes.
- Start the engine - check oil pressure.
- Cold start - full choke, closed throttle, crank until it fires and catch on throttle. Gradually reduce choke and re-apply as necessary.
- Hot start - no choke, closed throttle crank until it fires and catch on throttle.
- Warm start - by experience! Quite often ¼ throttle is needed. The fuel may have evaporated into the inlet manifold and gives an over rich mixture. Some throttle will clear it.
Other possible causes of starting problems
- Incorrect oil grade
- Incorrect use of oil additives
- Lead contamination internal to the engine
- Incorrect trigger gap
- Loose or damaged trigger
- Incorrect installation of the magneto flywheel
- Loose or damaged flywheel
- Poor connections of any electrical component
- RF interference
- Incorrect maintenance of the gearbox (friction torque incorrectly adjusted or pre-load on disk springs in gearbox incorrect)
- Poor carb set-up
- Blocked carb jets
- Blocked vent off carb bowl
- Improper use of the mixture enrichment (choke)
- Insufficient movement of choke
- Resonance between engine & propeller
- Mal-aligned crankshaft
- Damaged bearings
- Bent propeller shaft
- Damaged gears
- Out of balance carburettors
- Fouled, old or incorrectly gapped spark plugs
- Incorrect spark plugs
- Engine mounting system not stiff enough
- Shaking during the start cycle which causes the fuel in the carbs to froth and spill into the vent lines
- Carb shaking - which might be fixed by installing the Rotax airbox which rigidly fixes the carbs
- Low pre-load on gearbox shock absorbers
If difficulty is experienced with starting in cold weather, try the following:
- Check to see if the choke is full on and both are equal. Proper set up of the cables is critical
- Increase starting jet size (located in float bowl) to allow more fuel
- Modify choke system - see Rotax Service Information SI-03-1998
- Reduce spark plug gap to minimum - see Line Maintenance Manual
- Make sure you have a good battery & charging system
- Check battery wire size, connections & grounds
- Check battery size
- Measure cranking current - minimum 75A
ConAir designed the Soft Start Module to overcome starting kickback and to increase the life of the sprag clutch. Rotax have now incorporated the feature into their new smd modules, calling it with Easy Start or Advance Start.
Recent engines are fitted with the new smd module as standard - the smd module with this features have a yellow label on them.
Engines fitted with these smd modules, or with soft start modules will start and run “softly “ for a few seconds, after which the rpm will increase by a small amount (2-300 rpm).