High idle is often a sign of lean mixture. So, perhaps it's a leak in the carb mount block, but it could also be that the cowl causes some air pressure buildup at the carb inlet, which would lean the engine and cause high idle. Do these engines exhibit the same symptoms when run without cowls?

If so, placing a baffle in the airstream just ahead of the carb and perhaps increasing the air outlet area in the carb might reduce the cowl pressure and thus the idle.

Also, I suppose air pressure in the cowl could cause the metering diaphragm on the carb to see a different pressure, which could also cause leaning. If you haven't already, consider soldering a short piece of brass tubing to the metering diaphragm cover's little breather hole. Run a piece of fuel tube from the new brass tube into the fuse, where there should be no "ram air" pressure to mess things up.

That's about what I can think of that might cause high idle in flight, if it's lean related. Pretty easy things to try (especially running without the cowl) anyway to try to eliminate possibilities.

J