Rotax 503 won't stop running

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