In the last 30 days, I've seen 3 completely different events occur on 3 different engines owned by 3 different modelers that have reinforced a bit of wisdom I first had beaten into me about 5-6 years ago.
I'm putting this in the Gas Engine forum, since all 3 happen to have been gasoline engines...the wisdom holds for glow engines as well, however.
Our stories...
We'll begin with a 3W 157 in the nose of a Pilot RC Yak-54. The engine has probably 10 gallons or so through it, probably 9.5 of them burned hauling around an Extra 260. It's a reliable engine, and has been relatively trouble free since initial tuning and tweaking was accomplished.
Since installation in the Yak, something just hadn't quite been "right", according to the pilot. Much head scratching and debating ensued, eventually leading to the usual crowd of "helpful fellow modelers" pushing their way into the scene, and bangaing away on needles. This needle was opened, that needle was closed, those needles were twisted, these needles were reset to the ever popular 'factory settings", the cowl came off and went back on, the prop was changed, and we even ran and got new fuel.
None of this solved the problem. The solution was found approximately 30 minutes later, in the pits, by the owner.
Our next character was played by a ZDZ -80 in the nose of a WH Giles 202. This engine had performed flawlessly for 2 seasons in several different airplanes, and indeed, had flown the Giles several times without issue. On the fateful day, the engine started normally, but quickly developed a terrible "bounce" at higher throttle settings. Any time the trhottle was advanced beyond roughly 1/3, the motor would "bounce" rapidly, going from high to low RPM, back and forth, extremely rapidly. So rapidly, in fact, that most at the field simply presumed the pilot was "revving" the engine...until, for whatever reason, he wouldn't cut it out.
The engine was shut down, some settings fiddled with in the radio, then started again...same results. Pretty soon, our helpful crowd of fellow modelers arrived, the cowl came off, and the needle-tweedling began. Eventually, the poor little ZDZ was rich enough that it stopped bouncing...but only because it simply couldn't advance past 1/4 throttle without gagging. It was another 15 minutes before the solution was found.
We meet our final player in the form of a DLE-111 in the nose of anEF110" Yak. This engine has > 250 flights, most in a previous Edge 540. It, like its costars, had been a toruble free engine since initial break in.
Not long after the installation in the Yak, the little DLE did something strange...it sputtered to a stop after starting normally, as though it was a bit shy on fuel. "Odd", thought its owner, who restarted it, fiddled the choke a bit, and noted that once the engine had warmed up, things appeared fine.
This behaviour continued over a few weeks, prompting the owner to richen the low end just a bit, thinking perhaps the new environment (Yak vs Edge) was just a tiny bit different than before. He made note of the current needle settings, and set a maximum change of 1/4 turn on either needle. Politely declining the kind offers of assistance from the helpful fellow modeler crowd, he proceeded to follow his plan...richening the low end just a tiny amount each time.
Within just a couple of weeks, after ~ 3/16 of a turn on the low end, it was clear that the low speed performance was much farther out of whack than a new environment could explain, and was. indeed, getting worse by the day. The owner returned the needles to their original positions, and set about to resolving the issue. It was resolved later that same day.
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The problems and their solutions?
The 3W had a fuel pickup line in the tank that had split. Whether it had been split upon installation, or had developed the pslit later, nobody knows. It was, however, allowing air into the fuel system whenever any portion of the fuel tubing would find itself out of the fuel. Replacing the fuel line in the tank solved the issue.
The ZDZ had a throttle servo that had gone bad, and was "jumping" back and forth whenever it was moved past a certain point. A new throttle servo fixed things.
The DLE's fuel stopper had begun to wear, and bits of the rubber had made their way into the junk trap screen in the carb. Cleaning the screen and replacing the stopper solved the issue.
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So what's the wisdom here?
Well....none of this is meant to discredit the very kind offers by fellow modelers to assist with troubleshooting and engine tuning. Many of these folks really know their stuff, and genuinely have your best interest at heart. Indeed...please note that ALLTHREE motors were returned to peak performance, some with the assistance of those very same helpful folks. So...the wisdom being passed along here isn't to decline offers of help.
No...the wisdom is this:
If a previously reliable, sound motor begins heading south...THERE'SPROBABLYAREASON...a reason that no amount of needle tweedling in the world will fix.
(Ok...sure...changes in weather conditions (and thus, the density altitude for the day) might make SOME difference...but we're talking fractions of a turn so small you likely couldn't SEE them. Even in the most extreme of circumstances, 1/8 or so is going to account for any change in density altitude.)
Good, reliable, proven motors simply don't sudenly "go lean" or "richen up". When things go from"right" to "not right" in the span of 10 flights or less, your engine is trying to tell you something.
The wisdom here is:
Shut up and listen to it. 
Probably worth noting here that in our cases above, both of the first 2 owners had some significant "retuning" to do once the problem was solved. Why? Simple...the needles had been twisted SO far, by SO many people, in directions previously undocumented, that they simply had no CLUE where the needles had been before when things were right. Their only recourse was to return the needles back to the settings "most commonly used" or "in the manual"...and start the tuning process again. In one case, the owner even had to reporgram the choke servo travel in his radio, because someone had, at one point, decided that a partially open choke valve must be the issue.
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Point is, these critters just ain't that complicated. Sure...SOMETIMES you're the guy with the freaky circumstance that nobody's seen before, that gets fixed by a magic reed valve setting...
But more often than not, if an engine's getting fuel, spark, and air, it's probably running...and if it suddenly ain't, chances are pretty good one of those 3 has gone missing.
Don't try to force it to run anyway.