RE: Glow plug eater!
247F hot? My engines will run 220F on day and as much as 280F the next depending on the weather. Its been said time & time again, never tune to a temp.
on the topic though, I reckon the mc9 is too cold for 20%. try the Mc8 or the Mc59. Its likely that you are either too lean or too rich or any of the above mentioned..