Rotax 912 Ignition Problems.

[JG3]

I don't think jumper leads could crank any faster than it does now. I have an almost new Odyssey battery right behind the firewall, with a heavy lead to the solinoid, and an earth lead directly to the starter motor. I have enough experience with starting engines in very cold conditions to know the importance of no-loss connections.

 

With that old module, I could start on B module then switch B off, so running only on A. The A module would cut out as soon as the engine slowed to 2500rpm, tried it many times. No way could ever crank that fast....

 

JG

 

 

[wanabigaplane]

Another approach

 

Is the soft start module a non-standard, extra device? When having this sort of problem it is advisable to get everything back to standard. Re-introduce them after the problem has been found.

 

You say you replaced one of the modules, then it worked. Then you put the old one back and it worked too. To me this indicates the problem might lie in how the devices are mounted / connected rather than the devices themselves. Everyone knows you need a good earth connection between the motor and the battery, especially across the rubber engine mounts. But you also need a good earth connection between each ignition module and the engine. I don't know whether this connection is made via the mounting lug onto the engine, or via a separate earth wire. Either way you should determine how the connection is made and confirm that you have not compromised this path by a non-standard or temporary mounting - as is often the case while you are doing tests.

 

Hope this helps. Jack.:):)

 

 

[JG3]

First very first thing I did was to disconnect the soft start module, and closely examine all connections, especially earth connections.

 

I never actually removed the B module. Just replaced the A with a new one. With the new A in place, then the B was found to be able to start the engine by itself once again, and continues to work now. This might seem to indicate a connection problem that cured itself by disturbing the wiring, but that still doesn't fit the clue that both old modules always worked at speed, but neither would provide a starting spark.... The old A module still works at speed, but cuts out below 2500rpm. I now always start only on the B module in order to monitor it, and it's never failed again.....

 

JG

 

 

[rogerCT2K]

Hi everybody, I own and fly a Flight design CT2K with a hard working Rotax 912UL (850 hours) up front. Lately the mag drop is increasing more and more, and now already outside of specs. The carbs are recently renewed and are not suspect any more (I wonder what the connection between the two is??). I joined Roan thinking they would offer me a possible solution, but all I can find is: "check ignition troubleshooting" and at ignition troubleshooting: "the mag drop should be tested at 4000 rpm and so on...". In other words nothing that we didn’t know before. I posed a question on their facebook forum but nobody has had that before or is interested it seems?

 

So my simple question: Has anybody had that before and found the reason of an ever increasing mag drop?

 

Winged greetings

 

Roger

 

 

[sseeker]

I don't wanna sound like a broken record, I'm sorry if you've already been asked, but have you checked your spark plugs for oil coating, carbon fouling etc...?

 

Regards,

 

Andrew

 

 

[rogerCT2K]

Growing mag drop

 

Hi Andrew,

 

Thanks for your quick reply!

 

Yes I have changed all the spark plugs, they are brand new now, the "old" ones all looked good.

 

The caps are also changed. I measured the old ones and they were all around 5Kohm so they were supposed to be good too.

 

Previous I did the mag check at 3000 revs and everything looked good but then I had my carbs done by Willy Lamouline of Loravia and he told me the mag check is better done at 4000 revs. That is when I discovered that the mag drop was too much.

 

Since then it increases slowly so I am a bit worried about what the future could bring.

 

Best regards

 

Roger

 

 

[bones]

I do a mag check at just above idle if the noise still continues it good :)

 

 

[rogerCT2K]

Hi Dexter and all,

 

At 3000 revs it is just perfect, no difference at all and the sound is perfect, quite something else at and above 4000, it runs a lot rougher.

 

Best regards.

 

Roger

 

 

[sseeker]

Note that it's a Rotax requirement to run the 912S at 4000rpm on the run ups unless your brakes won't hold.

 

-Andrew

 

 

[rogerCT2K]

Yep, that is correct!

 

I had to upgrade my brakes to be able to do it without blocking the wheels every time.

 

Best regards

 

Roger

 

 

[carlsnilsson]

Roger:

 

I'm unsure if you have a 912 80 hp or 912S, but I assume ignition layout is similar. On my older 912UL, ignition circuit A goes to plugs (1T, 2T) and (3B, 4B) (one pair per coil) and circuit B goes to (1B, 2B) and (3T, 4T). So, theoretically, all being equal, each circuit supplies a symmetrical set of plugs and should produce same rpm. I assume all your plugs are gapped the same? Does the engine start with the same facility on each circuit separately? On my 912 the power input to the ignition modules flattens off (at about 35V equivalent AC on a multimeter) at around 3000 rpm. If previous answers were "yes", then I would put a multimeter (AC volts) on each of the ignition circuit power input lines (red wires) with the engine running and see if the readings were similar from starting rpm to about 4000 rpm. The mag drop indicates that the plugs on one circuit are not getting the same power as on the other - in your case above 3000 rpm. That's either due to input to the modules or within the modules (expensive) - or maybe one coil has a problem - unlikely but possible - or there is leakage on one or more HT leads - more likely. All this assumes your spark plugs are really identical. However, if this problem has been progressive over time and apparent through changes of plugs and caps, then it would appear to rule out the plugs. Leakage in or around the HT leads is the most likely cause, in my opinion.

 

Carl

 

 

[icebob]

Hi Roger,

 

Have you tested the HT leads? mine played up exactly the same but at that time i was on a fly away and minimum tools with me it was found by running the motor at night, you could see the spark, so replaced the whole set when i got back home(temp fix with insulation tape - to get me home), problem fixed.

 

Bob.

 

 

[Yenn]

I don't fly with a Rotax, but I am wondering if you are down on revs at full throttle on takeoff. Have you noticed a lessening of revs at take off or a lack of power. From what I am reading here it seems as if it is only a drop when you are testing. Can you run up to take off power to check revs without doing a take off?

 

 

[rogerCT2K]

Hi Carl and all,

 

I must say I already suspected the HT leads, so my next job after the vacation (we are in south Tenerife now until 3 april) is changing them all by new silicone ones from a motorcycle shop nearby, I already did this by a few of my cars with excellent result. It seems a bit of a mess to do so, but i guess it is the next logical step. I must say that there is no visible change in take off revs, but then I don't know when the problem started since I always did the check at 3000 revs until lately. They have always been too low since the Germans fitted a neuform 2 blade propeller on my new plane with a lot of pitch. The take off revs are only 4600 in my case (80Hp engine). On the other hand it makes my CT go like hell (240 km/hour) at top speed with 5600 revs.

 

Many thanks to all for the suggestions, we should all fly together and have a good Belgian beer on my account afterwards.

 

Just let me know when you come over!

 

Best regards

 

Roger

 

 

[rogerCT2K]

Hi all,

 

While the wife lies here baking her but of in the Tenerife sun I found the readings (of my flydat) I wrote down the first time I did my ignition check at 4000 rpm:

 

Both: 4000 rpm right front 673° back 734° and left front 711° back 781°

 

A 3930 rpm 710 782 744 811

 

B 3800 rpm 720 844 692 843

 

The high temperatures (reading in degrees Celsius) on "B" jumps out, could it be that a faulty coil or damaged HT leads cause higher temperatures?

 

Rotax says: the max speed drop on one ignition should be max 300rpm, so these readings are very good and the difference between the two should be max 115rpm, so too high in my case. Since then it got worse.

 

They also say that a wrong needle position in the carbs can cause a high mag drop. Has anybody have an idea what the logic behind that statement is?

 

Best regards

 

Roger

 

 

[carlsnilsson]

Roger:

 

Tenerife eh! I'm real sorry for you! I'm right in the middle of changing my HT leads now, which I had put off doing for far too long. I had other problems for the best part of two years (no wings on the a/c!) that took priority. Then I did some tests that suggested (falsely, I believe) that the plugs were firing OK under compression. The real test is how the engine runs - or doesn't run! Don't know what the C2K is fitted with, but my Rotax 912 UL had HT leads (probably even older than my engine - viz. about 19 years) that B. Rotax supplied with the 912. These are the blue shielded leads. Note the shielded bit. Under the braid, the leads are only 5 mm in diam. I don't think that's enough insulation for a long life. Talked to Wal at Bert Flood's a few days ago. He doesn't like those leads much either. He suggested I make up my own using 7mm (black) leads. Slight problem is that, if you can buy the older 7mm HT cable (you can, with some difficulty) it is unshielded. How important the shielding is in keeping radio noise tolerable, I'm not sure, because I haven't tried. Rotax seem to have dropped the shieding on their later installations, so maybe it's not really necessary if the ignition unit is mounted on the engine and the leads are short. Hey, maybe that's why Rotax went to engine-mounted ignition units? I'm shielding my HT cables with a braid sheath that you can buy (GA use it) by the metre. I've looked on the net and cannot find any HT cable that is both shielded (with a grounded sheath) and at least 7mm inside that sheath. "Magnecor" cable seems the most sophisticated noise reduction lead, but the leads have to be made up for you and fitting the ends to the tapered screw ends of both the coils and caps might not be practicable. RF noise suppression is not just about hiding the radiating leads under a shield - the braid sheaths also serve as distributed capacitance which, coupled with the 5K resistors in the caps, form some sort of R-C circuit to dissipate the RF. Because of the greater diameter, shielded 7mm leads will have less capacitance than 5mm leads, but I don't expect the effect to be too significant. Magnecor and other leads work with a different sort of distributed capacitance. If your present installation has unshielded leads, then you don't have a problem with HT cable - if they are, I suggest you contact me next month and I'll tell you (a) whether I have solved my problems or I'm full of hot air and (b) how to make up your own shielded 7mm HT cables.

 

Carl

 

 

[carlsnilsson]

Roger:

 

as to the question about carbs - surely it depends on how the plugs are connected to the ignition units. Don't carbies tend to feed each side of an engine - that would be why, if they are not properly synchronised, the engine runs rough? Now, on my 912, as detailed in my earlier reply, each ignition unit feeds two plugs on each side of the engine (i.e. 4 plugs). So a maladjusted carbie should, to first approximation, effect rpm on both ignition circuits equally. Don't know for sure, haven't experienced it, but that's what I would expect. (Often wrong, though!)

 

Carl

 

 

[rogerCT2K]

Hi,

 

In my opinion the HT leads do not have to be shielded at all!

 

 

[rogerCT2K]

Hi Carl,

 

My carbs are comletely redone by a professional under my supervision, absolutely nothing wrong with them!

 

How the position of the needles (with perfectly synchronised carbs and needles in the same position) could affect the mag drop is just a mystery to me!

 

Rotax does not provide any explanation either!

 

Roger

 

 

[carlsnilsson]

Roger:

 

Hi,In my opinion the HT leads do not have to be shielded at all!

I suspect this is the case, but I have not tried it. Could you tell me if this opinion is based on a test - or whether your present leads are unshielded and, if so, about how long are they. Mine are all around 1.0 m. You would have to turn the squelch down or off on the radio to get a valid comparison. One thing I am pretty sure about, the 5mm diam inside the shield of the old blue Rotax leads is insufficient for safe insulation of HT as the leads age. The older 912 system had the modules, coils and connections packed inside an Al box and the HT lead shields were clamped to the box at the entry point - which was as far as the shield braid went. I note that on all my leads there is dark discoloration for about 1 cm plus just inside that clamping point and before the leads go inside another protective rubber tube. I have a suspicion that this darking is due to discharge through the translucent insulation, which is less than 2mm thick. Not sure, but no other explanation springs to mind. I'll cut one soon and see if the discoloration extends through the insulation.

IceBob made a good point about looking at the unit running when it's reasonably dark. If it is sparking where it shouldn't, it should show up. I should have taken the cover off my "Interference Suppression Box" and taken a look.

 

One test I did do, after a lot of thought, is crank the engine over with one circuit off and, for the "on circuit", one pair of plugs connected in the open air and one plug connected (in the open) and its mate disconnected. I know some will say that's a no-no, but I do not think it likely to damage the ignition module at cranking rpm of 300. If anything is at risk, I would say it is the coil with a disconnected plug. The HT voltage wants to discharge somewhere and hopefully, that's not back through the insulation in the coil to the primary or ground. If that happened, I think Rotax would be regularly replacing a large number of dead coils around the world and I don't think that has happened. Anyway, in this case the plug with the other end of the coil disconnected from its plug went on sparking just as though there were two plugs on the coil. This was definitive in my view - there had to be leakage for the the one plug to spark. One plug on its own should not spark. I only wish I had taken the lid off my anti-interference box to observe where that might have been.

 

Re the carbie issue - I agree - Rotax's statement is a bit of a mystery. One way to test it is to have one deliberately maladjusted carbie and see if it effects the relative rpm drop. Not on my list of priorities.

 

Go back out in the sun and join your wife! I don't think you have a serious problem either. My engine, when it did occasionally start and let me fly, ran beautifully with full normal power - no missing at full rpm. Once it's going, there seems ignition power to spare - even apparently with severe HT leakage.

 

Carl

 

 

[rogerCT2K]

Hi Carl and all,

 

Mine were never shielded and my radio works perfect, so don't worry about that.

 

They are probably leaking now and my radio is still ok.

 

By the way, if you would open the ignition box and see what is inside and compare it with the price (900Euro's) you would realise that Rotax is very willing to sell you another one every year, the profit must be enormous! No wonder they put it on the engine and let it vibrate happely along with all the rest.

 

I am an electromechanical engineer, but if I would be an electronic weirdo i would desing and produce my own box that performs much better then that one.

 

It gives about 6° up to 1000rpm and 24 or so when "more". That is about as sophisticated as the mechanic system in the old VW's made in the second world war.

 

Best regards

 

Roger

 

 

[carlsnilsson]

Roger:

 

Not as sophisticated as the old VW beetle that I drive! I have a mechanical (centrifugal) advance distributer for mine that works a treat. Also, many is the time that I've wished my Rotax would start with the same reliability and ease that the VW does! Being an old "wierdo" myself, I have designed (but not yet built) a substitute power source for one ignition module just for starting - runs off the 12V battery. Not too difficult - my ambition is to be able to hand start the ba*****d.

 

Cheers

 

Carl

 

 

[rogerCT2K]

Hi Carl,

 

We will probably lose our high "Cadet" grade now, because this has nothing to do with the subject but I had 3 Beetles. With the oldest one (1958) I drove back from Germany with a split cylinder head, the one that is hardly reached by the cooling fan, and it still brought me and my dad home (with a good puff of smoke behind). Brings back old memories. The system was beautiful in its simplicity and worked perfectly, probably the only good thing we can thank the greatest dictator ever for. Ferdinand Porsche must have had a very bad night when he thought of that wonderful idea. It is also possible he just stole it from somebody else. I have made an aircraft engine from an old Subaru EA71 and it goes like a bat out of hell every time one touches the starter button, on only one ignition system that the Japanese stole from the humble Beetle. It is still in my garage and every now and then I let it run a bit, just to enjoy the noise coming out of a good old flat four with a direct coupled two blade propeller. The sound of music, even the neighbors enjoy it! (When they are awake of course). To be honest my Rotax 912UL still runs good, although a bit rougher then before but it has cost me more just to have the carbs done (700 Euro’s) than I ever paid for one of my Beetles.

 

Best regards

 

Roger

 

 

[rogerCT2K]

Hi Dexter,

 

Those measurements were taken the first time the "professional" did my carbs (for only 700 Euro's) and told me the mag drop was too big. Since then it has gotten worse. The plane I had before my CT2K was a Rans Coyote ESII, one could land it on a dime while asleep, only good so because the fantastic German Hirth engine did quit on me 12 times. My CT2K flies like a dream but lands like a drunken Dinosaur, I had already twice a "vache" (that is how the French say that the engine is nice and quit all at once) with my 912, once in the middle of the French alps. That is more than enough!

 

Cheers

 

Roger

 

 

[carlsnilsson]

Roger: no worries. I too have had a number of Beetles thru my life, first one being a 1958 1100. I think it was the first model with the one-piece big rear window. In 1959 my mother also bought an 1100 and I recall her being upset that the dealer didn't tell her about the 1200 that came out a month or so later! I still have that car, sitting in the back yard and simply needing some TLC and a paint job. In about '94 I had the engine thoroughly rebuilt by a specialist VW mechanic who had bought up all locally available 1100 cc spares and had them tucked under his caravan! Taught my daughter to drive in that car and since then, it's been resting - except for an occasional engine start to blow out the cobwebs. So the engine only has about five hours on it! I was driving a 1972 model until a few years back, until the rust beat me and a great 1973 restored model came my way courtesy my ex-wife. That's what I now drive - if only the Skyfox/Rotax would let me be, I should get the 1959 VW on the road. What I love about them, is that if they do stop, I can generally get them going again. Look under the hood of a modern car, and I don't know what many of the parts are or do.

 

Now, to make this relevant to flying, I will note that the steel exhaust tube that my old mechanic-friend had so much trouble locating in Tasmania for the 1100 engine - and could only buy a length such that I had a generous surplus to my needs - turns out to be the very tube I need for the exhaust of the Rotax 912 UL!

 

I now note that the darkening on my HT cables for about 1 - 1.5 cm just inside the box lid before they hide inside a rubber tube is evident on all 8 cables and suggests HT shorting to the lid, which is closer than the edge of the box/clamp. This would explain similar (mis)behaviour on both ignition circuits. I'm pretty sure that's the smoking gun, but the proof of the pudding will be in the eating. When I have confirmed that the new HT cable has fixed my ignition problems and tested whether or not the shielding makes any difference to radio noise on my installation, I'll get back to you. I'll also ask our kind webmaster to pass on my email address to you.

 

Thanks for the convers.

 

Carl