By cycling the prop at those mid-range RPM's you still gain a fair wad of extra Manifold Pressure, effectively I would think, increasing the volumetric flow through that or those cylinders - if more air or in this case charge is entering the cylinder then I gather that the operating pressure would be higher at the power stroke. Maybe this technique "mimics" higher operating pressures and temperatures by cycling the prop at lower RPM's. I'd like to find out more though if anyone has any ideas?
Good guess, and it sounds reasonable, but wrong.
When you cycle the propeller, you're slowing down the engine. fixed drive or geared, you're slowing the engine speed. When you slow the engine speed, you slow down the airflow through the engine. You're not increasing pressures or temperatures. Nothing is being mimicked.
Your engine is a wind machine. Pistons move up and down and draw air into the engine through the induction, and expel it through the exuast. For reasons of self-perpetuation (to make it go) we add fuel and set it off with a spark. The faster that engine runs, the more air it draws in. The slower it runs, the less. The engine induction can be thought of as a vacum cleaner hose. Your home vacum, if you were to put a hand over the hose, would develop a very low internal pressure; in the airplane this would be a manifold pressure drop, and it's exactly what happens when you close the throttle.
If you put your hand over the vacum cleaner hose and then slow down the motor, the pressure in the hose will rise, because the motor isn't drawing as hard. Close the throttle on your engine, and you see a low manifold pressure. The throttle is nothing more than an air valve; it lets more air in the wider you open it. If it's closed, it's the same as having your hand over the hose...when your engine is at idle, you have closed off most of the induction, and the pressure in the induction (your manifold pressure) becomes very low. If you could speed up the engine by without opening the throttle (which you can't)...you'd see the manifold pressure drop even more.
When you cycle the propeller and see the manifold pressure rise, it's doing so because the engine is slowing, and drawing less air. When it does this, the pressure in the induction, or induction manifold, increases, and you see a rise in manifold pressure. You probably think that an increase in manifold pressure is increasing the pressure the engine sees, and therefore means more power...this is wrong. What you're seeing is a measure of the suction from the engine, not the pressure feeding it, and certainly not the cylinder pressures. Manifold pressure in a normally aspirated engine is the pressure value to which the engine has reduced ambient pressure, and nothing more. Think about it this way...shut off your engine at sea level and what do you see? Nearly 30 inches of mercury (for those using that measurement)...your highest value with the engine shut off. Imagine that.
Cycling your propeller does not clear the engine. Doing so might make you feel better, but you're far more likely to draw up loose chips and damage your propeller than anything else. Cycling your propeller to clear a fouled plug is much akin to waving your arms to cure a wart on your foot. Makes no sense? Neither does cycling your propeller to clear a fouled spark plug.
Leaning is what is necessary. Your fuel has far too much lead...the consequence of a number of reasons but far too much tetraethyl lead, and this results in fouled plugs when not enough heat and combustion exists to scavenge it. If your engine is fouling on the ground, what you need is an idle mixture adjustment. Sometimes if your engine is fouled just a little, you can clear it by running it up a little...but this is a consequence of the higher power setting and operating temperatures...not cycling the prop. Any benifits that are perceived to come from moving the propeller are purely coincidental.
You can perform an idle mixture test by running the engine at runup speeds and power settings and agressively leaning to ensure that it's clear. Reduce to idle and let it sit for a moment, then from a full rich position on the mixture, very slowly reduce the mixture to the point of cutoff. Watch your RPM gauge. You should see a 10-25 rpm rise right before the engine dies, indicating that the idle mixture is adjusted correctly. If you see more than this, your engine is running rich. If you see less, your idle mixture is adjusted too lean. Readjustment for proper idle mixture should be done any time the aircraft base is changed to a different elevation or density altitude, and should be checked and adjusted seasonally as the idle mixture needs will vary with the seasonal base temperatures.
The governor is being over ridden by using the prop lever to change pitch. thats why the engine slows down when the prop is cycled. the engine will try to maintain the speed it was running at due to the manifold pressure being set and therefore the engine is required to do more work, resulting in a higher burn temprature as the piston is harder to push downwards and the combustion process is held in the cylinder longer before being released.
The governor isn't being overridden at all; you're changing the tension on the speeder spring and setting the governor...you're controlling the governor directly. The engine will not at all try to maintain the speed..the engine doesn't know anything about maintaining a speed. That's strictly a governor function...you set the governor, and the propeller controls engine speed. When you retard the propeller control toward coarse, slowing the prop down, the engine certainly isn't trying to increase the speed of the propeller. The engine is under a higher oad due to increased induced drag on the propeller blades, which prevents the propeller from spinning faster, and as the prop is attached directly to the engine, this slows the engine, too. See the previous discussion for why. The piston isn't suddenly trying to push harder...the engine has slowed, airflow through the induction has slowed and volume decrased and indeed less power is available...same effect has closing the throttle with respect to the amount of air flowing.
You have several options when controlling the engine; open the throttle to increase manifold pressure (by increasing the opening/decreasing the amount of blockage to the engine), increase RPM without touching the throttle (drops manifold pressure because the opening in the induction hasn't changed, but the engine speed has increased and suction therefore increased in the induction), or any combinaiton thereof. Of their own accord, these actions will not clean a fouled plug.
Aggressive leaning will.
Lean the airplane when you're taxiiing on the ground and you won't have these problems. Do a run at runup RPM just before shutdown after each flight, and clear the engine thoroughly, before returning to an idle that's leaned out nearly to the poitn of cutoff for several minutes, before shutting down. Most owners and operators don't even know to do this, though it's been recommended operating practice by engine manufacturers for decades. Fly safe.