This discussion reminds me that diagnostics need to be addressed from a logical starting point and progress thru to a logical conclusion. Some basics were skipped as assumptions were adopted without proof.
Example:
Stuck valves can be detected by simply "pulling the prop thru" and listening to the exhaust/intakes hissing in conjuction with loss of compression on the affected cylinders. This would likely have brought us to the diagnosis very early. (I often fail to acknowlege some owner's level of experience and then fall into this trap.)
As for leaning to prevent lead build-up: Yes, leaning aggressively is important to raise EGTs to the point that bromide additives (already included in 100LL) can do the job intended of lead-scavenging. This is generally recognized as being above 1200F and many power settings on carb engines run cooler than that when rich. (TCP also requires high EGTs to be effective.) Frequent experience with stuck valves is likely contributed-to by lack of aggressive leaning, especially in non-mixture-equipped Cubs run occasionally on avgas.
Advancing the throttle during shut-down is NOT a good practice for several reasons. The most important reason NOT to advance the throttle during shut-down is that the carburetor incorporates an "accelerator pump" which is a plunger designed to give a "shot" of raw fuel into the carb-throat to assist in sudden acceleration, and avoid backfiring due to a suddenly-opened throttle-plate and the associated overly-lean condition. By advancing the throttle duiring shut-down one is actually injecting additional fuel into the intakes. (The accelerator pump by-passes the mixture, and the shot of fuel is, needless to say, VERY rich...and therefore full of lead, delivered while the engine actually has low EGT...so the lead fouling is increased with each shut-down so-performed.) Early carbs did not have enriched/accelerator pumps and the technique of advancing the throttle was a method of suddenly subjecting such a carb/engine combination with a lean condition during shut-down. Old-timers who learned the technique have passed it along without realizing it does not apply to subsequent designs which are now prevalent.
"Note: Do not open throttle after stopping. Opening the throttle actuates the accelerator pump." -Operator's Handbook, C-Series, 6-Cylinder Continental Motors Corp., page 10.
Leaning during taxy is a good technique as long as the pilot knows the proper technique. Although it varies in effectiveness in different installations, the desired result is much less fuel being fed to the engine during low-power operation. Less fuel thru the engine (especially during lower EGT-operation such as taxying) means less lead on the plugs and valves during the period maximum lead may accumulate due to the low temps. I personally do not lean during taxy on this carb/engine combination but that doesn't mean it's not a good idea as long as the pilot doesn't forget before takeoff. (Proponents of the technique recommend taxying SO lean as to cause the engine to cough/die if much power is asked of the engine such as during takeoff.) When I fly fuel-injected engines I always taxy aggressively leaned, but this is a personal choice for me on this carburetor. (I'm one who's never experienced a stuck valve on my own airplane.......so far.)
I use only avgas, no snake oil, aggressively lean in-flight, and allow engine temps to stabilze before shutdown. (It's my personal opinion that many stuck valves are caused by coked oil (carbon) caused by hot engines being shut down too soon after flight. It's always a good idea IMO to taxy at low power (less than 1000 RPM) for a couple minutes and/or face the wind and idle a couple minutes before shutdown. Idling at 800 RPM and pulling the mixture without advancing throttle is my preference.
Snake oil/Marvel Mystery oil is a favorite among some operators who believe it's claim to lubricate upper cylinders.
The presumption for most is that it softens carbon and helps prevent valve sticking. Many C-190/195 owners swear by it.
I believe jet fuel or diesel is cheaper, no red dye or perfume needed, and just as legal to add to avgas and just as effectively negates the legality of mogas STCs.
(I can't help but wonder if it doesn't contain an anti-scuffing, lubricity-enhancer known as tri-cresyl phosphate as one of it's "mysteries".) Hmmmmn....
(Bruce and I were both typing simultaneously, and after I posted I had opportunity to read his excellent remarks.
I'd like to express a different opinion on a couple things:
It's not a good idea to "lap" valves which are not leaking, and it's never a good idea to do it with the cylinder on the engine. I believe Mark is seeking a solution that does not include removing cylinders.
Also, the rockers which incorporate oil passages are required on all exhaust valves.
Here's the SB:
I have been lucky so far not to have experienced a stuck value and hope this continues. One of our now deceased members experienced multiple stuck valves and he almost always burned mogas. Last year at one of our fly ins another member, who almost always burns mogas, experienced a stuck valve. SO, it remains a mystery whether mogas or 100LL is more prone to sticking valves. Sure would be nice if there was a simple solution that would fit all C-O300/145 engines.