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"set on the ground so it is just *barely* in a 2-stroke, as rich as it will go and still be two-stroking. When you release the airplane, it *should* go into a constant 4-stroke in level flight, just "rich" enough to stay in a 4 all the time. in the maneuvers, say, a round loop, it should go about 1/4 of the way around, then break into a 2-stroke, stay until it is about 3/4 of the way around, and then go back into a 4 at the bottom. That ought to repeat in consecutive loops. Same thing on outside loops. It should return to a 4-stroke exactly as it was within 1/4 lap, if it takes a lot longer, like an entire lap, then it needs more run time.
If, when you release it, it stays in a 2-stroke or gets leaner, then, there is something wrong with it, or you have to big a prop. If, when you release it, it slows down to blubbery rich, there is something wrong with it, or you have to large a prop.
Brett"
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Does this apply to smaller engines as well? Say .15's to .35's?
Frank was specifically asking about a McCoy 40, but it generally applies to 35-40 4-2 break baffle piston engines running 6" of pitch in stunt. That seems narrow but it covers a lot of the classic stunt engines.
I would also note that there are two things going on here - "breaking in" an engine (which for some reason is being done to a engine that has already been run), and picking a venturi or prop. Frank was/is trying to use a large venturi to get a particular, probably unrealistic, RPM on a particular prop. I was trying to give him a way to determine whether it is excessive for practical use, after having one problem with grossly excessive venturis after another, on a wide variety of engines, while also having low power very consistently.
There is some factor we are not seeing here. I very strongly suspect that a competent expert pilot could probably figure out the missing element, have an "A Ha!" moment and at least diagnose the issue in about 10 minutes. That's the problem with trying to figure out everything at once by yourself, no one knows this stuff naturally. What is going on here is a bunch of people trying to land an airplane over the radio like "Airport 75", with people yelling "give it up, give it down!" at Frank over the internet, while we attempt to put the peices together remotely as to what the real problem might be.
The venturi sizes that will work are in a very narrow range, there is no real debate about what the range is, or what venturi needs to be on particular common engines. If you are just going to experimentally discover the right range, you need to evaluate what you see accurately, and make appropriate decisions. In general, when you take off, the engine should "go rich" - that is, sounds the same way it sounds if you opened the needle one click on the bench. In reality, the actual mixture might get richer or leaner depending on the tank setup, the effect is really due to a change in the load (it goes down...) but it will be universally be interpreted as "richer". Same in the maneuvers, it should "lean out" - that is, sound as if you closed the needle a click or two to "lean it out".
Excessive version of these changes are probably due to too much prop or (in this case) excessive venturi choke area. If you are *way* over the top on either, the effect will go backwards to the above description, and a very excessive venturi will cause it to "go hard lean", i.e. the dreaded runaway.
You are *much better off* if you do not fully grasp these effects *starting with a smaller venturi* and working up to larger ones. At least it will needle properly and you will begin to understand how various things are supposed to work. If, after a lot of experimentation, you are having to set it too lean to get adequate speed, *then*, you start trying to bump up the venturi *in small amounts*. .005" of diameter is a pretty good change. What usually happens, however, is that people start too big, and then start "diagnosing" the problem as too much compression or some other thing *that is best left alone*, and never work their way down because everyone know that You Can Never Have Too Much Power!(tm).
Note that some of the effects can also be break-in effects, particularly, going lean in-flight (due to the engine tightening up when it gets hot and increasing the load, which tends to feed on itself) is a classic "I need more run time". If you don't know exactly what you are doing - or even if you do - it can be pretty easy to misinerpret the signs.
This is also why I would suggest running engines *stock* first - because at least you know it was "right" in some sense. When you get modified engines from who-knows-where-random-local-expert, it may be pre-screwed-up because many of the local experts don't understand it any better than the customer, and have just enough knowledge to be dangerous.
Brett