Is this advice for all engines, including old iron piston Foxes, etc, or “newer” ABC engines like OS?
These are the specific instructions cut/pasted from the (innumerable and tedious) threads about how to run a 20FP and "new" 25LA, at least a few dozen of them, they are all the same.
As a general rule, virtually ALL, repeat, ALL consumer glow engines are intended to be set and run as described by default. Being able to do this and not sag from overheating or getting too tight is the indication that it is broken in. Peak them out lean, back off until it is as rich as possible without losing RPM, good luck and happy flying.
Nobody (or at least, nobody at a major commercial engine manufacturer) ever designed 2-stroke glow engines to be run blubbering rich in a 4-stroke. For stunt, people have found that this is the best way to run *some* of them, trading off power for in-flight response to load. The 25LA is certainly in the former category.
In this case, a specific case for the 25LA, it needs to run in a 2-stroke. The problem Motorman ran into is the prototypical example of why you can't run engines this large on such a small airplane - although it's closer to working than a lot of other cases. However, having done this experiment multiple times and witnessed this particular set of circumstances dozens of times, I know exactly how it is going to go, and was hoping to head off any more of the "drill this and grind that" that nearly always follows.
The problem is to get it to run properly (that is, in a 2-stroke) while still being slow enough to be visible in flight. The proposed plan (slightly increasing the venturi size to move the 4-2 break point to a higher RPM) and running it in a 4-stroke could possibly "work", but makes it extremely likely to be unstable due to decreased fuel draw. This is the old "use the needle as a throttle" approach.
My alternate solution (since we have already disregarded the best solution , which is to use a 15FP) is to reduce the venturi, which both makes the engine more stable and will slow down the 2-stroke to a workable degree. The goal is to get the engine running in a "medium" 2-stroke in level flight at a reasonable speed. He tried that as is, it was (as predicted) far too fast. The most straightforward solution if it is too fast at an otherwise good "setting" is to reduce the throttle, e.i. use a smaller venturi. Do that, and it is *extremely stable* and you can adjust the power variation in flight by slight mixture changes up or down.
I will note also that he already modified the engine in such a way that it reduced the power by drilling out the crankcase to install an aftermarket spraybar (probably ST or clone) , and it's still too much. This tells you that it would be even further over the top if it was stock - again, because the *engine is far too powerful for the airplane*. Stock, and set properly, the engine will pull the airplane at about 3.6-3.8 seconds a lap on 60' lines - I know exactly, lots of people have done exactly that, we know he result within a gnat's ass.
BTW, before anyone asks, other engines:
20FP = 3.8 seconds+-
Veco 19bb = 4.2 seconds
"Old" 25LA = 4.2-4.3 seconds
"New" 25LA = 3.6-3.8 seconds
25FP - 3.4-3.6(!)
15FP (8.5-4.5 Circus Hobbies)- 4.5-4.8 seconds
If nothing else, given it is patently obvious that the engine is too powerful for the airplane, *enlarging the venturi and making it even more powerful* and hoping you can needle it down isn't a very good idea.
As always, this ws just (in this case, highly informed) advice, and I am quite well aware that it's probably going to be disregarded. I get nothing from it either way. I just feel I need to do what I can to steer people in the right direction.
Brett