503 won't start when hot

Hello all,

Sorry, I don't know too much about 2-stroke engines so if you need more info, just ask.  I have a Rotax 503 engine.  2 bing carbs.  Ducati ignition.  Installed on a powered parachute.  Engine starts and runs great!  Until I shut down after a flight.  At which point I repack the parachute and would like to taxi the PPC back to the trailer.  Except that after shut down, I cannot for the life of me get it to start again.  I could wait 20 or 30 minutes and then it will start again.  I can continue doing this but just wondering if anyone has any ideas why?

Fuel lines are all PVC so I don't *think* its vapor lock.  They stay pretty cool out in the breeze.