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By andyhill296
#1873143
Hi.
I look after a motor glider with a Rotax 912s.
Recently we have had issues when starting from cold.
Once it has started , it runs faultlessly.
I have checked the spark plugs, I've operated them to see if I am getting a good spark.
I am.
I have checked the float height visually. One is slightly higher than the other.
Finding info on setting up carb height is proving difficult.
I suspect the right carb float height is set too high.
After checking the carb bowl and refitting. The engine starts 1st time from cold.
Another point that I have read is that the starter must turn the engine over at least in the range of 250 plus rpm.
Ours is only around 200 rpm.
It is a new battery.
Think I have the problem of low starting rpm. Possible age issue with the starter motor.
One set of floats too high.
Any pointers please.
Many thanks.
By Aerials
FLYER Club Member  FLYER Club Member
#1873173
I'm unsure at what rpm the magnetos begin to operate but your figure of 250 is, to my mind a little low. I think it's about 300.
Spinning the engine draws a lot of current from the battery so it is essential that to minimise voltage drops across electrical joints each part of each joint in that particular circuit is absolutely clean. Poor contact cleanliness inside the solenoid is one possible cause too, but try investigating that after all the other things. Don't forget that a good return back to the battery is an essential too.
I have insufficient knowledge to comment on the float levels except that they must be the same and as close as you can get them.
One more thing to examine is the (mostly) metal tube that goes between manifolds across the top of the engine. Around about the centre there is a smaller tube welded on. It should be airtight. Ours was found to still have a plastic transit cap taped over it after a considerable number of hours. It fell off!
By Brooklands
#1873212
andyhill296 wrote:Hi.
...
I have checked the float height visually. One is slightly higher than the other.
One set of floats too high.
...
Many thanks.

Andy,

Have you weighed the floats? Sinking/heavy floats is a known issue on 912s - some of them have a design fault and absorb fuel.

As Aerials said 200 - 250 rpm sounds too low to start the engine.

Brooklands
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By T67M
FLYER Club Member  FLYER Club Member
#1873214
Remember that the Rotax has a gearbox, and depending on the setup, the tacho may read engine RPM or prop RPM. 300rpm is about the minimum on the engine side for starting, but a prop speed of a little over 200rpm should be fine.

Heavy floats due to fuel absorption is definitely a common problem and causes starting problems and rough running. There's a specific weight defined in the maintenance manual and SB to check them. Given that it starts easily after dropping the bowls, this sounds like the first thing to check.
By Nick
FLYER Club Member  FLYER Club Member
#1873218
Contact CFS Rotax agents, reference the sinking floats. I have had 3 sets so far all FOC. They are still having problems with them.

Nick
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By Charliesixtysix
#1873228
Agree with check weighing the floats and if not already done, do inform your inspector of the problem too.

If it is a BGA motor glider it is most likely on a C of A and will need Inspector sign off for work done.
User avatar
By Sooty25
#1873232
T67M wrote:Remember that the Rotax has a gearbox, and depending on the setup, the tacho may read engine RPM or prop RPM. 300rpm is about the minimum on the engine side for starting, but a prop speed of a little over 200rpm should be fine.


Every Rotax I'be been close to has indicated engine RPM. Never seen prop rpm on it's own. All rpm's in the books are engine rpm's.

I had low cranking speed, and poor starting caused by a high resistance in the negative battery cable. You can easily test this by clipping a jump lead between battery negative post and the engine block, and see if the cranking speed improves. You can also prove the positive side between battery positive post and the starter solenoid.

Even if the cause of poor starting does prove to be floats, low cranking speed needs to be investigated and cured as well.
User avatar
By ArthurG
FLYER Club Member  FLYER Club Member
#1873234
I had intermittent starting problems with a 912S, which ran perfectly once she had decided to start. As far as I can gather, most of the ignition units fail in the end. If, after failing to start, you take a hairdryer to the ignition units for a few minutes, making them hot to the touch, and then she starts, there's your problem. At about £1400 at the time, it was an expensive fix, but she never gave starting trouble again. My engine had done about 700 hours. I tried all the other cheaper possibilities first.
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By Shoestring Flyer
#1873245
Almost certainly an ignition module problem. Some say that heating the modules with a hair dryer before starting will prove?
A pair of genuine Rotax modules will now set you back £1900.
Alterntives are to have them repaired by Carmo with mixed results :-
https://www.carmoelectronics.co.uk/inde ... _2711_4730

or another alternative is to fit Ignitech Modules( requires LAA/BMAA Mod):-
See here:
https://afors.com/aircraftView/48888

Personally if you can afford it I would bite the bullet and fit a pair of genuine Rotax modules.
By andyhill296
#1873325
Hi all

Thanks for your replies.
The floats are brand new.
I will weigh them this weekend.
I will try the hair dryer tip.
The cranking speed does seem low. I read that if you do achieve the cranking speed, this causes major problems.
I'm surprised that rotax do not give much info on float levels. Only to weigh them.
I've been busy reading up on float issues. Some checks involve using a special tool to check the height of the floats whilst the carb is upside down.
This is a lot of work on our aircraft.
User avatar
By Charliesixtysix
#1873334
andyhill296 wrote:Hi.
I look after a motor glider with a Rotax 912s.
Recently we have had issues when starting from cold.
Once it has started , it runs faultlessly.
I have checked the spark plugs, I've operated them to see if I am getting a good spark.
I am.
I have checked the float height visually. One is slightly higher than the other.
Finding info on setting up carb height is proving difficult.
I suspect the right carb float height is set too high.
After checking the carb bowl and refitting. The engine starts 1st time from cold.
Another point that I have read is that the starter must turn the engine over at least in the range of 250 plus rpm.
Ours is only around 200 rpm.
It is a new battery.
Think I have the problem of low starting rpm. Possible age issue with the starter motor.
One set of floats too high.
Any pointers please.
Many thanks.


The comment that it starts from cold after checking the carb seems to indicate the problem lies elsewhere than the ignition modules or cranking speed (albeit cranking speed may be a tad on the low side).
User avatar
By T67M
FLYER Club Member  FLYER Club Member
#1873441
Sooty25 wrote:
T67M wrote:Remember that the Rotax has a gearbox, and depending on the setup, the tacho may read engine RPM or prop RPM. 300rpm is about the minimum on the engine side for starting, but a prop speed of a little over 200rpm should be fine.


Every Rotax I'be been close to has indicated engine RPM. Never seen prop rpm on it's own. All rpm's in the books are engine rpm's.


At least all Diamond DA20-A1 and A100s, and some Aquilas indicate prop RPM.v I'm sure there are others too.
User avatar
By RichJordan
#1873571
Hi,

Had same issue on my 912ULS powered CTSW this year. Having previously been very quick to fire up, it got more and more reluctant, but ran beautifully once started.

I did the float swap, having found mine were now slightly out of spec (free, as outlined above) but to no avail.

Then warmed the ignition modules with a hairdryer before first start - started on first crank.

I’ve now swapped to the Ignitech modules. As well as being usefully cheaper they have long wiring harnesses so they can be moved from being directly mounted to the top of the engine, to somewhere away from the heat and vibration (mine are cockpit side of firewall, behind dash). They are able to be wired to the aircraft 12v feed, so don’t rely on RPM for initial spark and have “soft-start” ignition timing pre-programmed.

I’m very happy with them.