Peter,
I have a few more suggestions before you reach for your cheque-book.
First, check that the plugs are not breaking down by swapping over the
plug lead pairs on each cylinder then restart the engine and see if the
fault moves to the "other" mag. If it does, then the plugs (or leads)
are the problem.
If not, reset the plug leads to their original positions.
Second, check that both trigger coils are timed correctly. You'll need
a strobe for this. Run the engine on only one mag, note the timing
position, then run on only the other mag and see if the timing is the
same on both. If one is running retarded, you will get a drop in RPM on
that mag. You don't say which engine you are using. The 914
deliberately times both mags differently (it's used to control
pre-ignition), the 912(s) should be the same.
Don't be surprised if there is a difference as engines leave the Rotax
factory with the trigger coil gap-settings varying greatly.
If you can get down the back of the engine, try and check these gaps and
the security of the clamping screws (details in the maintenance manual).
You will find four trigger coils, two for retarded timing for starting
and two for advanced timing for running.
If you have a pre-March 1999 engine check to see if the service bulletin
SB 912 026/E (for a 912) or SB 914 014/E revision2 (for a 914) were
complied with (check engine log).
Early stators had a problem with the insulation material which allowed
some solvents and fuel to dissolve it. The SB's addressed this by
replacing the whole stator with an improved replacement from Rotax -
again, your engine log should record this.
Lastly, you can isolate the problem to either the ignition modules or
the trigger coils by swapping over both sets of red trigger wires
between the stator to the modules (they are dual redundant), If the
problem remains with the same mag - the ignition modules are the problem
and if the fault swaps to the other mag - the trigger coils are the problem.
A lot to go through, but if you approach it methodically, it should be
straight forward.
Refer to the Rotax manual, you will find it covers this quite well.
.............and if you don't have one, contact me off-list and I'll
send you the latest manual in .pdf format.
Nigel
On 25/10/2010 16:39, peter.rees01@tiscali.co.uk wrote:
> --> Europa-List message posted by:
> "peter.rees01@tiscali.co.uk"<peter.rees01@tiscali.co.uk>
>
> Thanks for the response.
>
> The problem diesn't result in a 'dead cut' when selecting the rogue
> unit - just a drop of about 200 RPM (as opposed to about 75 on the good
> unit). The fault also only seems to happen when the engine is up to
> temperature (which makes ground testing significantly more difficult).
> Both of these lead me to believe that the problem is inside the box
> (though I suppose that it is possible that as the engine comes up to
> temperature, the insulation on the wires softens and the resulting
> increaded allows a damaged wire to move more than when cold).
>
>
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