I can't speak to Ed's airplane, but my C85-powered airplane produces no RPM drop at run-up. I suspect this is the case for two reasons - one is that it's really not yet hot enough under the cowl to produce a large enough temperature rise in the incoming airflow, and the other is I don't think the engine is sucking enough air at run-up RPM to accurately reflect the effects of carb heat.
I've seen the same symptomology in our new O-360-equipped aircraft. So far only limited ground runs have been conducted, with CHT's being kept at very conservative levels in accordance with Mahlon Russell's engine break-in guidance. The lack of RPM drop in this new aircraft/engine seems to back up the two theoretical causes mentioned above.
As for engine clearing, I have two small anecdotes.
1) student pilot cleared an O-200 in a C-150, only to have it backfire and blow the carb off the engine. Besides the obvious need for a change of shorts after the very successful off-field landing, there were a lot of "how the heck did THAT happen?" conversations!
2) student pilot in O-320-powered C-172 failed to clear the engine on a simulated engine failure on downwind leg. It was one of those really muggy days when most of us would never think of carb ice. The engine quit when he found himself too low to make the field and tried to add a shot of power. "Airport Road" was where we found him, white as a sheet and shaking like a leaf, but otherwise none the worse for wear!
|