I am starting to assemble the M20 which arrived in a dismantled state. What is the consensus regarding using a phenolic spacer when fitting the carb to insulate it from enegine heat? Should I fit one?
I think you should definitely fit a spacer. Apparently, in days of yore hot starting was a problem for DRs because of fuel vapourisation. Others would know more than me about this.
Fuel vapourisation due to heat from the engine is not a problem when the engine is running, neither is it a problem when starting from cold.
It becomes a problem when you try to start a hot engine. After 20 to 30 minutes of the engine being stopped, it has cooled sufficiently to no longer be a problem.
The spacer is to limit the heat soak into the carb which occurs as soon as you stop the engine. I use a 1/8" (3mm) spacer which works fine for me.