The Rotax ignition has two distinct trigger pulses, a delayed one for low
engine RPM (starting) and
another advanced one for higher RPM.
The two
trigger pulses happen each time the notch on the flywheel passes the spark
timing pickup coils, one when it closes the magnetic field and
another one when it opens again.
As the voltage
generated depends on the speed at which the
magnetic field varies, it is dependent on engine RPM. Rotax used to just use
a zener diode to suppress the first (leading) impulse at low RPMs whilst still
passing the delayed spark during startup.
With powerful
batteries and particularly when it's quite cold (since the zener
diodes are a bit temperature dependent), the ignition could
trigger on the first, leading impulse even during starting, which
results in the observed kicking and sudden stops.
These take an enormous toll on your sprag clutch (and other things).
Starting only on one
ignition circuit only can help but using the soft start mod is better. The
units are a few hundred dollars but it is very basic electronics so easy to
make your own if you are into electronics.
Newer Rotax motors have the soft start incorporated.
I can send a circuit diagram if anyone wants one.
I must admit to liking aircraft motors where the prop is bolted to the end of
the crankshaft.
Rob Thompson
0429 493828
From: [email protected]
To: [email protected]
Date: Tue, 26 Aug 2014 19:37:17 +0930
Subject: [DOG mailing list] Super Dimona Starter Motor
Hi I am Colin Turner President and CFI of the Scout Gliding Club in South
Australia.The Scouts own a HK36TC Super Dimona which is having starting
problems.Ever since we purchased the Dimona from the Scouts in NSW it has
always been rough to start.The starter motor never turns the engine over
smoothly it is always jerky but up until recently it has always started
relatively quickly.Of late it has become much more jerky and it takes a lot of
attempts to start it. The battery is a year old but it is still reading
reasonable voltages no load and loaded. I have attempted to start it jumpering
another good battery with no discernible change.The starter relay cuts in OK
and the starter motor seems to run smoothly but what appears to be happening is
that the starter clutch is slipping. It is now at the stage that it is serious
enough not to use it. Has anyone had similar problems who could perhaps give an
opinion of the possible cause and what would be involved to fix it? Regards C.
R. TurnerBranch Commissioner Air Activities Scouts SAPresident & CFI Scout
Gliding Club
