The only thing I noticed is that some airheads had problems with valve seats comming out of the heads when they got to hot.
A lot of those times the heads have been overhauled to use unleaded fuel.They just didn't do a good job.
It did happen even more when they were used for a sidecar.
On events in the netherlands where sidecars were used to take handicapped people for a ride out, and the bikes were driving slow for a long time it was a common airhead failure.
My solo bike's have had hours of idling on muddy forest tracks,trying to get through in serious heat.
Had no heat problems with more than 400.000km....
Just my experience.