Pete,
I too recently experienced an increasing amount of "sub idle resonance" on
start up from cold on a 912 - exactly the same as your description.
I contacted Nigel Beale at Skydrive who informed me that this could be
either the sprag clutch slipping and not turning the engine fast enough
before the first cylinder fired or an inadequate torque friction setting in
the gearbox. In any case, Nigel recommended immediate action to fix it
because he warned that repeated bouts of sub - idle resonance would
seriously damage the gearbox.
I decided to remove the gearbox (straightforward job, with due care to avoid
damaging the mating surfaces, from the Rotax Maintenance Manual) and take it
to Skydrive for re-shimming and was delighted to find that this has
completely fixed the problem.
Apparently, due to operational experience, Rotax have found it necessary to
increase the specified friction torque (which reduces in service due to wear
and a number of factors such as too slow an idle speed and carb imbalance).
I'm not saying this is the answer to your problem but it may be worth you
checking your friction torque and if necessary getting it re-set to the top
end of the latest specified band, as recommended by Skydrive.
Regards
Roger Mills
G-BVUV
Time:
From:
"Duncan McFadyean" <ami@mcfadyean.freeserve.co.uk>
Subject:
Re: bad cold starts
<ami@mcfadyean.freeserve.co.uk>
<<what the Rotax dealer in the UK
describes as 'sub-idle' >>
Which in another language is torsional resonance. Every engine has
this,
especially with a larger flywheel (the prop) coupled by a spring (the
face
cam device at he front of the gearbox) to the crank.
This natural resonance usually occurs at sub-cranking speeds and is
not
excited by the smooth constant-level torque supplied by the starter
motor.
The starting system is usually designed to crank the engine above the
resonant speed. Once the engine starts to fire then, if this is at the
resonant speed, the resonance is massively excited. Resonance absorbs
a
vast amount of energy so, even if the engine is weakly firing, it may
not be
able to "power through" the resonant band.
So the problem could yet be one of the starter not spinning the engine
fast
enough to get through this low speed phase; Or improper adjustment of
the
preload of the "dog clutch" which effects the resonant frequency. Or
different prop inertia.
Duncan McF.
----- Original Message -----
From: "Pete Lawless" <pete@lawless.info>
Subject: RE: Europa-List: bad cold starts
<pete@lawless.info>
>
> David
>
> Yes that is exactly what it does. My engine started cold fine but
would
> not come up to idle speed. Ran as if only 2 cylinders were firing,
very
> rough with lots of vibration. It would take several attempts to get
it
> to run smooth. Once it had fired up once then it was ok for the
rest of
> the day. I believe it is the kick back that damages the sprag
clutch
> but the bad starting, going into what the Rotax dealer in the UK
> describes as 'sub-idle' it the result.
>
> Regards
>
> Pete
>
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