I consider a visible exhaust trail to be the normal condition, absence of which makes me suspect the engine is too lean. Most of my crashes have been a result of engine failure, and most of those have been a result of running too lean. I am happy to compromise a few hundred rpm at the top for no flame-outs. In spite of my vigilance in this regard, my first attempted flight of this season ended in flaming out on take-off at about thirty feet of altitude on what I thought was a trusty old HB .61 hauling a vintage Lanier Dart. As soon as it lost power, it tip-stalled and came straight down. I didn't like that plane anyway.

Optimum tuning of the low end needle to me is a.) via pinch test, in which you pinch the fuel tube closed and hold it there until the engine does something different--revs up a little (or a lot), which means you can lean it out some more, or sags and dies, which means you can richen it just a bit, or just keeps running in spite of the pinch, in which case I leave it where it is OR b.) a smooth, non-sputtering run-up from stable idle below 3000 rpm to full bore when you punch the throttle. I like to see an idle down around 2500 rpm, but I usually settle for whatever revs do not quit and do not pull the plane forward on our grass (and dirt) field.

Optimum setting on the high-end needle is a.) I can see the exhaust stream when the plane is flying toward or away from me (but not always when it is flying with its wingtip pointing at me); AND b.) I can hold the plane with nose straight up and less than half a tank of fuel, at full throttle for 30 seconds, and there is no sign of sagging. If I can't pass the nose-up test, I put it on the ground and go through my pinch test for the high end. This is pretty much the same as for low end, which is pinch the fuel line firmly (usually only a brief pinch of one second or so) and listen for an increase, decrease or engine stops. If rpm increases markedly, I lean the n.v. a click or two; if rpm increases only slightly, I leave it where it is; if rpm sags or engine dies, I richen a few clicks.

Anytime anything changes in the combustion scenario (fuel mix, glow plug, prop size, change in temperature or humidity, etc.) it's worth going through at least the high-end pinch test. But if I am in a hurry and want to fly real bad, I rely on the nose-up test. When I don't (like with my Dart the other day), I often regret it.