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912 ignition problems

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PostPosted: Tue Jan 10, 2006 4:14 pm    Post subject: 912 ignition problems Reply with quote

Hi ! Kirk,
you posted the following message:-
I have an ignition problem with my Rotax 912 UL. I saw an old post of a
with a Rotax 912 that had a mag drop out of spec on one side. I can't
what the fix was.
The 2004 post said that the coil was changed and one of the ignition
modules. I
had heard that the ignition modules either work or they do not. No
operation. Is that true? Or are there some people that have experienced a
working ignition module (i.e. a significant drop in rpm during a mag
Well I run a 582 so don't have experience with your 912 but I am an
electronics guy and have built quite a few CDI's so I take mental note of
comments re faults. Also I have some expereience. So I'll pass on my
thoughts as it seems no one with more direct 912 experience is answering you
at the moment.
I guess your comment that modules either work or they don't while not true
might be the case a lot of times. However I recall seeing that most 912
ignition problems are due to wiring especially and very much the wires to
the modules [ basically part of the module ] but you dont want to throw away
a module for these wires as they can be repaired. The high tension lead has
also given trouble. In fact I have seen adverts for wire to repair these.
The thing is the wires are likely to be an intermittent fault. The high
tension lead not so much so. More like a fault at low RPM. The module itself
can be intermittent but yes it is more likely to fail full stop if it is the
The pickup coils also fail especially the early ones but I have no idea of
the failure mode unfortunately. However Rotax do give resistance readings
for these that although not 100% conclusive is a great help.
Bear in mind that the pickup coils and the modules are both able to be
interchanged. therefore if you have a fault on say
ign (1) then interchanging a part will swap the fault to ign(2) and
therefore that part is the problem assuming good workmanship on your part
and not getting misleading results due to intermittent faults like leads
broken inside the insulation. I think this is the most common case on the
912 remembering what I've seen from time to time..
A few hints on these leads. As I just mentioned they break inside the
insulation. The high tension lead however does not generally. Instead it
burns or corrodes at the ends. OK to find a break in a lead you might have
success gently stretching a lead when the plastic cover is warm and soft but
don't pull too much where it goes into the module because if you break it
in there you have an arkward problem. When you stretch othe plastic if the
wire lets it stretch because there is a break you may see this or be able to
feel it. The other trick is to meaure from the terminal end to the other end
of the wire with an ohm meter by using a pin through the insulation into the
wire. You can reseal the insulaion with a drop of varnish or glue or silicon
or whatever although it probably won't really matter. Don't ignore the
terminations at the ends for trouble.
Another thing is those resistor plug caps. I personally don't like them.
The resistance should measure about 5,000 ohms. If it is significantly
higher then discard them. If one is faulty I would discard them all. What I
do is use resistor type plugs and non resistor caps. This way everytime you
change the plugs you get fresh resistance that is there to suppress radio
interference. With NGK plugs this is just a matter of there being an "R" in
the type No. ie:- the 582 uses B8ES plugs and resistor caps but you can use
BR8ES and non resistor caps. Now you can similarly just insert the "R" into
your plug type No for your 912. However despite what you might have seen or
heard do not use both resistor plugs and resistor caps together as you will
have 10,000 instead of 5,000 ohms resistance to the plug. This is likely to
cause misfiring and therefore fouling due to weak spark.. Also it is bad for
the module. Yes I've seen a case made for it being OK to get rid of radio
interference but it's not the right answer believe me. Another interesting
point here is that you may actually get a better spark with 5,000 ohms
resistance than with no resistance. This is because the 5,000 ohms allows
the voltage from the module to build up higher before it arcs across the
plug gap. However it does not follow that because 5,000 ohms may be good
that 10,000 ohms is better. Don't be fooled !
Incidentally instead of using BR8ES plugs in my 582 I use Iridium plugs
and find they are a lot better for starting and running at low RPM. Also
they last and last. The type No for me is BR8EIX. The difference is at the
end ie:- IX instead of S. I is for Iridum and X is for booster gap. the S we
replaced was for standard 2.6mm centre electrode. If you want Iridium plugs
and can't work out the No for your 912 let me know what your current type No
is and I'll tell you the Iridium version.
Kirk you don't say exactly what your trouble is so I can't guide you any
closer than this general overall view. If you post that info maybe a 912
owner might well be able to better guide you but if not I hope the above
helps you sort whatever ignition problem you have. I assume you merely are
getting a high RPM drop on one ignition on ignition check at run up and/or
switch off. In which case all of the above is relavent. Anyway think about
all of the above and it should help you.
Rex Shaw

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