I haven't been following this closely, but a comment anyway. On some chain saws I have had apart that don't have a large phenolic carb insulating spacer, they have a small spacer such as you are using. But, this is important, they have insulating sleeves ON THE BOLTS that mount the carb, and fiber washers under the nuts that hold the carb on. This prevents the heat from transferring through the bolts into the carb...

AV8TOR