Internal cylinder temperatures vary in an air cooled engine much more than in a water cooled engine because airflow is required.  These internal cyl. temps will increase to the point of pre-ignition under long idling or short stop and start conditions on hot days.  It has happened to a number of members, and they have reported this, in the past.  A cold plug will help that, but once you resume normal driving speeds and the engine cools to normal operating temps., the cold plug will be too cold for those conditions.  Hi octane gas covers all these issues. I have used it exclusively in my Savage and the plug burns clean.

Check the content of your gas on the pump. Here in AZ the state mandates additives in the summer blend (starting about April 15) to combat smog. My four cyl bikes ran like crap for 4-6 months a year. (until the blend changed back). Compression was above 10 for all of them.
As for corn...it better come from Kentucky or Tennessee!

 octane in Illinois and Wisconsin are 87    89    93,10 cents more as you go up per gallon.