peter,
without proper loading on the engine a lean condition will arise because the engine will turn faster than it is supposed to with the throttle setting under normal load. This is exhibited on a machine when descending with throttle because the prop is being forced to turn faster and the engine rpms increase yet no change in throttle. If you return to level flight after descending the engine regains the load and EGTs will drop because the engine is now turning the proper rpm to throttle relation.
If the gearbox is at the improper ratio and has too much ease to spin the prop at WOT a lean condition will arise. The engine must be properly loaded at WOT and if you reduce the gear ratio it is possible to bring the engine RPM, prop pitch, and throttle setting in to harmony. I have found some great information on the web regarding this exact issue.
Exaserbating the problem is that my 72" prop is VERY light and since it is not heavy would not normally require such a high gear ratio. If I was not running a Kiev and was running a Warp Drive I found out that the gearing would be fit for that prop. The Kiev is very light and efficient. After speaking to Kiev I was informed that my gear ratio is probably too high, but would work much better if I had a faster aircraft. Since we are slow compared to airplanes the lower ratio is recommended. I get superb thrust at high speeds and personally I don't like going 100MPH to achieve full thrust potential from the propeller.