I think, when you shut down at a faster than normal idle you pumped the exhaust system full of gas/air mixture--then on restart you have an ignition source and BOOM.It is surprising that it could happen after an extended shutdown.
I think correct the idle/low speed carb problem and I would check the point gap/dwell.The points may be at the stage of closing up due to rubbing block wear as to cause intermittant spark.
A lot of "seem to be" carb problems are corrected by attention to ignition system.
