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Hi,
Its a long shot but the plug leads are also screened. Bearing in mind that 4
plugs are powered by the LHS mag and the remaining 4 by the RHS mag if one
lead from each magneto set were shorting out the engine would be effectively
firing on 3 cylinders.
We had a situation where one of our leads was shorting out which caused a mag
drop on one side. Stripping back the screen at the plug end fixed the
problem.
What you describe certainly sounds as if one cylinder is not firing on each
set of mags. When both mags are live the problem would dissappear.
Due to the high voltage in the plug leads a short is much more likely here.
Crank the engine with the plugs unscrewed (but grounded) and it should be
possible to identify the faulty lead. Just be sure there is no fuel vapour
around (ie: drain the float chambers and disconnect the fuel).
PS: Rubber gloves are reccommended unless you want a shock !
Hope this helps.
Carl Pattinson
G-LABS
----- Original Message -----
From: Keith Hickling
To: europa-list@matronics.com
Sent: Friday, February 27, 2009 4:56 AM
Subject: Europa-List: large mag drop on both mags, 912uls
Hello all,
On first running my new 912 uls a couple of weeks ago, I got a mag drop pf
just over 500 rpm on one mag and just under 600 rpm on the other mag,
starting at 4000 rpm. All else was OK; oil temp and press, EGTs, CHTs etc,
and the engine ran quite smoothely; even on 1 mag with the rpm drop it was
not obviously rough or misfiring. Prop is Airmaster, operating correctly -
testing done on fine pitch.
Our local Rotax man thought it was probably not a real mag drop but a tacho
problem, though it sounded real. So yesterday we checked it with an optical
tacho and it was real. The optical tacho showed a rpm drop on the prop from
1700 down to 1490, which is equivalent to a 500 rpm drop on the engine.
Another rotax owner here suggested it might be the mag ground leads
shorting to the screen, as I had left the screen quite close to the end of
the wire where it enters the ignition modules. So today I stripped the
screen a bit further back, disconnected the screen ground and connected the
mag shorting wires to 2 temporary switches, bypassing the ignition switch
completely. This made no difference, still the same mag drop, eliminating
partial shorting of the mag wires as a cause.
The plug gaps are correct, and the plug colour looks OK. The throttle
cables are set correctly. The engine always starts first go, easily, and
runs smoothly. Our local engineers have no ideas, although they are not very
familiar with rotaxes. So I would really appreciate any advice on where to
go from here - what should I look at next?
Thanks,
Keith Hickling,
New Zealand,
Mono XS, Kit 613.
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