With regard to mixture rich for runup, then leaving it there and then wondering how long it would take to foul the plugs...it depends on how rich/lean the system is calibrated at idle and the density altitude of the airport. Certainly, if nothing were noticeable, the effect of fouling the plugs would be cumulative. I flew a Super Cub on floats that would lead the plugs fairly quickly. One had to lean dramatically to keep the plugs clean. Idling along to the takeoff point didn't help, either.
I lean my IO-520 powered 185 after start and leave it there (even during the runup) until ready for takeoff, then set it for the density altitude (I primarily fly in the mountains), not necessarily full rich.
With regard to everything forward, and leaving it there after the runup, has anyone ever experienced carb ice waiting for takeoff? When flying a carb equipped airplane, especially this time of year in the northern hemisphere when the temp/dew point gets more cozy, I alway begin the takeoff roll with full carb heat. Normal RPM is confirmed and then the carb heat is reset to full cold with a corresponding increase in RPM noted (if flying a fixed-pitch prop).
Blasphemy or practical application and understanding of an internal combustion engine? I would like to think the latter, and the years and hours have confirmed it. My IO-520, with over 1200 hours on a Continental factory reman (with original cylinders...no top overhaul), with good compressions and oil analysis also confirms that I might be doing something right.
I am from the internet, though, so investigate and think for yourself on this stuff.
