Rotax 503 won't stop running

I have a 503 DCDI mounted inverted in a J3 Kitten. I am using an Ivo
in flight adjustable prop and the engine plain just runs great.

The problem is that I can't turn the engine off if it is running in
the "smooth" range, the mag 1 mag 2 switches will only cut it off if
it is at a lower rpm and running roughly.

The manual says this can be from the engine "being hot" but I actually
have a problem in that the engine is running too cold.

I took the fan and belts off and chiseled off the fan housing (weight)
and added a forced fan shroud that directs incoming prop wash through
the cyls towards the carbs.

At full throttle with the IVO set at max pitch for max rpm I get 975F
to 1,050F for EGT and 190F to 230F for Cyls (which seems too low). I
am using a Smart Single EMS 503 gauge (I tested the probes for cyl and
egt and they are accurate).

The cyls run so cold they don't even have a bar appearance on the EMS
503 guage.

These readings come from an engine that does not have the cowling on
it yet (still making it from fiberglass) so I figure once the cowl is
on the cyl temps will go up a little and I can always add a louver to
control/reduce the flow of air into the cyls if I need to "heat them
up" a little to get them into the 350 F range (so the engine will run
at the correct heat range and not too cold).

But this mag thing is throwing me, when I first start it up, I can do
a mag check, but if I increase RPM to 3,000 the mags won't check and I
can't kill the engine, I can't believe that this is because the engine
is "hot" as it has only been running for a minute or two.

Also, I would imagine that an inverted engine would have the opposite
problem and would suffer from fouling and not from "run-on" since the
cyl heads are basically flooding with fuel all the time.

Today I changed to a new type of NGK plug with a slightly longer
electrode because I read somewhere that inverted rotaxes need this
plug so the plugs don't foul. No change in how the plane runs or
whether I can cut it off or the temps.

One other thing I was concerned about with regards to these new plugs,
they got me resistor plugs but that's not what I wanted. I read that
you are not supposed to use resistor plugs if you have resistor caps,
how do I tell if I have resistor caps ? (this resistor issue is not
the cause of the won't stop problem, happens with either type of
plug).

I have run 20 gallons of fuel through this engine with the plane tied
down and this mysterious malfunctioning kill switch issue is my only
problem so far.

If I turn the mags off at 6,000 rpm, and then start backing off the
throttle, the engine will not cutoff until it gets to something under
3000 rpm.

This is the SMOOTHEST, QUIETEST, LIGHTEST and MOST POWERFUL rotax I
have ever had, it's perfect, how do I cut it off : )

Thanks

How long are you magneto grounding (kill switch) leads?

Are there any joints in the leads that could be high resistance?

Are the switch contacts dirty?

How is the earth return run?

Do you have a multimeter (available at most cheap auto shps for under
$20.00!) so you can test the resistance of the grounding lead?

Test from the disconnected lead at the magneto back to the earth on the
magneto and see where you have high resistance. This will include all the
wire to the switch, the switch itself and all the way back via the earth
return. It should be very low, less than 2 ohms or so, preferably less than
one ohm.

For test purposes with the aircraft tied to a tree, you may want to make a
short test lead to short the magneto.

Hope this helps,
Peter

Sounds like a grounding problem to me