912 ULS starting but stalls

Wow! Two start techniques (Sam & Kevin), both of which differ from mine - before I state mine - could we have diffrent ignition systems?

My 912ULS is a 2022, with "Soft Start", first run December 2024. Now has Hobbs 132 hrs.

Cold Start Procedure

Select Fuel Tank On

Boost Pump On - observe pressure rise to usual.

Boost Pump Off

Ignition - Both

Throttle Closed

Choke Full On - Choke will not remain on, kept on, with two orange plastic clothes pegs (laugh if you wish, it works for me).

Start is instantaneous - Immediate removal of one peg = 1/2 choke

Second later RPM automatic increase to 2400 - remove second peg (no choke) - rpm drops

Throttle Advanced to 2400 rpm. 

(The above all happens very quickly - almost continuous flow of movement)

Engine has very slight roughness, which disappears as it warms.

As engine warms, rpm rises to above 2500 - slow engine to below 2500, until oil temperature reaches 50C

Hot Start Procedure

As above, without Choke & small throttle advance to 2000-2500 rpm😈

There appear to be several different starting procedure methods mentioned in this forum, so we will provide the one specified in the Rotax Operator's Manual (referenced in the attachment)

Now that you have the official starting procedure, there are a few additional things to consider.

You mentioned that your idle RPM is 1,745 RPM after warm-up, which is too low and can lead to premature gearbox and carburetor wear, potentially causing starting issues. (Minimum recommended idle speed is 2,000 RPM)

Since you have been idling below 2,000 RPM, it might be a good idea to check the friction torque on your gearbox. This is a simple procedure that should be done every 100 hours to ensure the gearbox remains within limits. An out-of-spec gearbox could cause the engine to run rough, especially at idle, which may exacerbate starting issues.

(Friction Torque inspection procedure Video): https://www.rotax-owner.com/en/videos-topmenu/elearning-videos/60-friction-torque29

If you provide your engine S/N, we can check whether your engine is equipped with the Advanced Start Module. It is also important to have a strong/properly charged battery. A low battery may not allow the starter to spin the engine fast enough, which can cause starting issues.

Hi RO,

Thank you for "the official starting procedure" - it is deficient on two points;

Boost Pump -

Where fitted, should be activated prior to engine start, to determine it is fully operational and ensure "priming" of fuel delivery system.

In a cold start there should be no need to run the Boost Pump during engine cranking.

For a hot start, the Boost Pump should always be run, prior to cranking, to expel any vapour through the return line and replaced by cool liquid fuel.

In a severe vapour lock situaton, using the Boost Pump continually from pre start to smooth running, may be necessary

Choke-

I can only comment on my engine in my moderate to hot climate -There is no need to continue with full or even half choke (Step 9) to warm engine. I would suggest that running the engine "rich" when it doesn't need it, may lead to spark plug foaling. I accept that in cold climates the choke may be required for a longer period after start.😈

I think starting throttle setting really depends on where your idle rpm is set. If it is set at 1500 rpm (too low) then throttle is needed at start. If your idle rpm is 2K then no throttle is needed. I think there are variables that others may have on their individual engine's and sometimes it's not the way Rotax has it setup in the manul so you may need to adjust for your specific startup

For many it isn't a one size fits all and variables need to be considered.

Kevin Stewart wrote:

Do you have a soft start function on your engine that is somehow staying on too long after cranking that is holding the ignition timing in the retarded state?

How do you tell?