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John912ULS

Video of my 912 ULS; 500 RPM Mag drop

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Dear friends,

 

My engine, a Rotax 912 ULS, has started running roughly on each mag.

 

Aircraft; UFM-13 'Lambada'. Year of Construction; 2006. Total hours; 530.

 

Previously it would always drop approx 150-180 RPM on each mag. Now, however, it's dropping over 500 RPM on each mag and running roughly with considerable shaking and roughness. It runs normally on both mags. 

PLEASE SEE VIDEO, ATTACHED

 

Spark plugs seem unlikely as they only have 29 hours on them since being gapped and fitted last year. I changed the fuel to fresh, quality fuel and that didn't' change anything. 

There's a lot of gents on here posting about the CDI modules giving trouble. It's €400 to overhaul each one at Carmo, Netherlands if I have read correctly. 

 

If anyone might be able to offer some advice which might see me resolve this less expensively I'd be very happy to hear from you:chuffed:

 

Kind regards,


John.

 

PS: Is there any risk of the spark plugs failing to spark when I'm flying?

I have been advised that the design of the ignition system offers the same level of redundancy of a classic magneto (whereby the spark will still be supplied to the plugs regardless of any ignition and/or battery problem) and therefore my CDI problem is only a startup problem -is this in fact the case?

 

 

 

 

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Could be many things. We had a lead issue once that caused a similar issue. But I would list that as a Major Defect and ground the aircraft, so yes there is a risk of something failing in flight

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Send details to rotax service agent  and consider joining the Rotax owners.com and ask the question there.   (That site has some very experienced and helpful in advice people.)

Cheers and hope to hunt the fault down soon.  Look forward to hearing the solution.

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Conas atá tú Seán?

 

I am not suggesting this is the solution but may be a contributing factor - If I read your instruments correctly, you are doing your ignition check at /about 3000 rpm - I do mine at 4000 rpm. I would go to 5000 if my brakes would hold. I think 3000 is far to low. Another thought has crossed my mind - if your engine mounts are warn, any unbalanced running will be  exaggerated.

 

Slán.

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You should always do the mag check at around 4000 rpm

I sent some after market models to a guy in NZ recently. He got back to me saying they seemed to run rougher than the older original ones but one of the originals had failed anyway and wouldnt start on it alone. Mine do seem to have a bit more energy than the originals. I told him to check the plugs first but he said he had only changed them about an hour of running before. He pulled the plugs out and noted that they were quite black and wet. So they put a boroscope down the plug hole and there was oil on the top of the pistons. Turns out the valve guide seals were leaking. He got that fixed and using my modules is now happy with everything. Seeing it is doing it on both CDI units is must be something common to the engine not the CDI units

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Posted (edited)

My first reaction to any ignition problems is..... change the plugs first.

It's cheap, it's easy, and you never REALLY know until they are changed...... 

 

And if there is no change, then you simply have a spare set of plugs for the next service. Nothing is wasted....

Edited by Downunder
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4 hours ago, Kyle Communications said:

You should always do the mag check at around 4000 rpm

I sent some after market models to a guy in NZ recently. He got back to me saying they seemed to run rougher than the older original ones but one of the originals had failed anyway and wouldnt start on it alone. Mine do seem to have a bit more energy than the originals. I told him to check the plugs first but he said he had only changed them about an hour of running before. He pulled the plugs out and noted that they were quite black and wet. So they put a boroscope down the plug hole and there was oil on the top of the pistons. Turns out the valve guide seals were leaking. He got that fixed and using my modules is now happy with everything. Seeing it is doing it on both CDI units is must be something common to the engine not the CDI units

Hi Kyle Communications,

 

Good point. Yes. It's doing it at higher RPMs also. It's very strange that they are both doing it. An engineer I know says he has encountered two cases of both CDIs failing at the same time. Odds must be very high on that.


I pulled all 8 plugs out and inspected them- they were all a normal healthy colour with minimal deposits or signs of either fouling or hot running. I'm going to changed them anyway just to be sure. I'll gap them first, of course.

 

Thanks for your help,

 

John

 

 

 

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3 hours ago, Downunder said:

My first reaction to any ignition problems is..... change the plugs first.

It's cheap, it's easy, and you never REALLY know until they are changed...... 

 

And if there is no change, then you simply have a spare set of plugs for the next service. Nothing is wasted....

Hi Downunder,

 

Thanks for your reply. Yes, I've bought 8 plugs and I'll change them all and see what happens.

 

John

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9 hours ago, skippydiesel said:

Conas atá tú Seán?

 

I am not suggesting this is the solution but may be a contributing factor - If I read your instruments correctly, you are doing your ignition check at /about 3000 rpm - I do mine at 4000 rpm. I would go to 5000 if my brakes would hold. I think 3000 is far to low. Another thought has crossed my mind - if your engine mounts are warn, any unbalanced running will be  exaggerated.

 

Slán.

Go raibh maith agat, Skippydiesel, an Éireannach thú?

 

Yes, I tried the mag check at higher RPMs, same large drop and rough running unfortunately.

 

Regards,

 

John

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Hi John

 

Yes its highly unusual that both fail at once but as its a 2006 model engine they are probably the old 4 wire units. Mostly one will fail and you replace that one then usually the other isnt far off failing then either so its a possibility they both are not well. At that age the capacitors are probably just about had it anyway they will have lost most of their capacitance and they are not replaceable practically

 

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What you do need to check especially if they are 4 wire ones is you need to remove and clean the earth lug connection for both the modules. This could be the source as well. A bad or corroded earth will produce similar and it will affect both modules

 

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16 minutes ago, Kyle Communications said:

What you do need to check especially if they are 4 wire ones is you need to remove and clean the earth lug connection for both the modules. This could be the source as well. A bad or corroded earth will produce similar and it will affect both modules

 

A good point in damp old Erin - I am for ever extolling the virtue,  of making sure the earth return any 12 V  system is in excellent health - running an additional earth ring wire is a cheap and easy way to achieve this.

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50 minutes ago, John912ULS said:

Go raibh maith agat, Skippydiesel, an Éireannach thú?

 

Yes, I tried the mag check at higher RPMs, same large drop and rough running unfortunately.

 

Regards,

 

John

 

Other than an Irish education and Father (from Claire) I confess to only being associated with the old sod.

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One possibility would be the trigger coils. On the back of the engine they are mounted and adjusted in 2 groups (one group at 1-2 o'clock, the other group at 7-8 o'clock).

Each of those groups variously fires both ignition modules, so if a group is out of adjustment, it could affect both ignition modules.

 

These are near impossible to check with a conventional steel feeler gauge, as the magnets on the rotor make the trigger gap feel tight even when it is not.

However, some gauge sets come with non-ferrous feelers (brass?) and these work very well, especially if you dismantle the set and are reaching in only with the single gauge.

 

It's well worth checking this gap on all engines: correct gapping also gives better starting, and poor starting on a Rotax can lead to expensive problems.

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"Go raibh maith agat"

" It Literally it means "[I wish] that there were goodness at you" - in other words, "I wish that you might have goodness".

spacesailor

 

 

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1 hour ago, IBob said:

One possibility would be the trigger coils. On the back of the engine they are mounted and adjusted in 2 groups (one group at 1-2 o'clock, the other group at 7-8 o'clock).

Each of those groups variously fires both ignition modules, so if a group is out of adjustment, it could affect both ignition modules.

 

These are near impossible to check with a conventional steel feeler gauge, as the magnets on the rotor make the trigger gap feel tight even when it is not.

However, some gauge sets come with non-ferrous feelers (brass?) and these work very well, especially if you dismantle the set and are reaching in only with the single gauge.

 

It's well worth checking this gap on all engines: correct gapping also gives better starting, and poor starting on a Rotax can lead to expensive problems.

No offence but I consider an out of adjustment trigger to be an unlikely cause - unless they have recently been installed. Reason; there are no wearing /moving parts once installed correctly, they remain in adjustment. If recently installed they may not have been secured and go out of adjustment.

 

The perplexing part is that the effect appears to be the same on both ignition circuits - could be the earth as suggested giving a weak spark that is OK when both systems operational.  Worth the effort (no cost) to make sure all related earth returns are in excellent condition.

 

Kind of bad luck. if both modules are failing at the same time, but possible. As this is a most expensive fix leave to last.

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11 hours ago, Blueadventures said:

Send details to rotax service agent  and consider joining the Rotax owners.com and ask the question there.   (That site has some very experienced and helpful in advice people.)

Cheers and hope to hunt the fault down soon.  Look forward to hearing the solution.

Thanks Blueadventures, I am trying to find out who that is :-) 

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8 hours ago, Kyle Communications said:

What you do need to check especially if they are 4 wire ones is you need to remove and clean the earth lug connection for both the modules. This could be the source as well. A bad or corroded earth will produce similar and it will affect both modules

 

Thanks- actually this very thing was told to me by a respected Rotax maintenance engineer here in Ireland over the phone also today

 

Pardon my ignorance but would you be so kind as to explain how exactly I can find this. I'm given to understand it's a 10mm head nut somwhere close to the CDIs? :chuffed:

 

Kind regards,

 

John

 

 

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9 hours ago, skippydiesel said:

No offence but I consider an out of adjustment trigger to be an unlikely cause - unless they have recently been installed. Reason; there are no wearing /moving parts once installed correctly, they remain in adjustment. If recently installed they may not have been secured and go out of adjustment.

 

The perplexing part is that the effect appears to be the same on both ignition circuits - could be the earth as suggested giving a weak spark that is OK when both systems operational.  Worth the effort (no cost) to make sure all related earth returns are in excellent condition.

 

Kind of bad luck. if both modules are failing at the same time, but possible. As this is a most expensive fix leave to last.

Agreed that once correctly installed and tightened, the trigger coils are unlikely to give problems. And no offence taken.

I mentioned them because I was considering what might effect both 'mags'. And also because I have seen trigger coils that seemed to be correctly adjusted, but were not once we got in there with the non-ferrous feeler gauges.

 

However, if there is a common grounding point, as suggested by others here, that would seem a much more likely suspect.

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Follow the longest black wire from both modules and they go under a bolt that is into the inlet manifold..it is easy to find. Both modules are anchored under the same bolt. You need to remove them and clean up the lugs and the area on the manifold then rebolt them and try it

 

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Just a general note on mag checks. An idle one has validity for a dead cut (off position check) and an "out"  situation. The 1500 rpm for older motors is probably to get any  retarding effect of the impulse action out of the equation. Also generally, especially with plugs located well apart, cutting one bank will have the effect of retarding the ignition and this will have a bigger effect on rev drop, the higher the revs you use. The rough running though often present  really shouldn't be there unless you are quite lean, something else has to cause it. In general aviation engines it's usually lead on the plugs if you are on 100LL. Nev

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16 minutes ago, smokeybear said:

Carb balance is worth a look

I absolutely agree. So much so it is one of my 100 hr checks  - doesnt take long to do a pneumatic/vacuum check  and it can make the world of difference to a smooth running efficient engine. Its a pity that Rotax have made the adjustment screws so fiddly.

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Hi Guys

not withstanding what you have said, which is all good stuff To me the fact it does the same thing on each ignition says it is probably NOT the ingnitin system but something more basic / mechanical.

The start should be back to basics 

cylinder leakage test ( check valves rings etc)

take the valve covers off and check the valve lift is the same on all valves. No need for dial gauges and fancy stuff just visual with a 6 inch ruler 

A very thougher visual inspection particuarly for vacuume leaks around the carby stubs and balance tube 

drop the float bowl and check for contamination like wise the fuel filter for water /dirt

 

you may well find a problem, some years ago I had a similar problem with a 912 it turned out to be a split carby stub causing a vacuume leak, and lean mixture when the manifold vacuume was high closed throttle. At wide open throttle it made little difference. 

Just something to consider

 

cheers

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 Normally an air leak will have more effect when the vacuum is higher and the flow lower ie at idle as said. If it idles OK it's a small leak.  Still worth investigating and correcting.  Nev

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