ORIGINAL: HighPlains
I've been at this for a very long time too, 36 years of RC with another 10 of U-control before than. Most trainers have the tank too low for a good needle setting, so one needs to set richer than what you would if the tank was in the correct positon. I see far more sport fliers cook their engines than I ever see operate them correctly. Most instructors are clueless too, since they only do what they were taught.
I guess you can count me as one of those clueless types, since I do what I was taught. Of course one of my instructors was flying masters at the time, and continues to be active at high levels of competition. What he, and MANY others taught me is that the "pinch test" is a very inaccurate method of checking for proper high speed mixture. The first reason is nobody's going to pinch the fuel line when the plane is in the air. Second, what constitutes a proper pinch? Is it .1 second, .5 second, or what? Bottom line, it's inaccurate, and teaching this method is a disservice to others. A far better test is pointing the nose of the plane vertical to see how the engine reacts. This simulates something that the plane is actually going to do in the air and will yield far more accurate results than a pinch.
You might re-read where I was talking about IDLE and the effects of varish. Plus, from some of the posts here, some are beginners for a very long time.
I've got engines that are over 25 years old. They continue to work as well today as when they were new. They've never been subjected to an antifreeze treatment, and probably never will. There are many reasons an engine could have idle problems, and it's hard to imagine jumping from plug replacement to an antifreeze bath without looking at other causes. In fact, it's a bit hard to imagine such a suggestion in response to a simple request for information on tuning an engine.
One other observation. Since today's fuels contain much less, or no, castor, varnish buildup is much less common than it was 25-30 years ago.