The words "It went lean on me" has to be one if the most used, least understood phrases in modeling today. It seems that any time a model motor loses power, the accusation is "it went lean on me." There are several reasons why a motor could slow down and, possibly, quit. A properly broken-in model motor, running on the right fuel and plug, and properly cooled, does not sag and heat up when the fuel level is dropped too far or the needle is tweaked in too far. What should happen is that the motor just slows very slightly and then quits firing. In the cases when the motor sags down and cooks, there are three probable causes: (1) the crankshaft bearings are not getting enough cooling fuel and heat up and start dragging, (2) the cylinder and piston don't have enough clearance and start binding, or (3) (and the most usual cause if all) is that the motor starts pre-igniting.
When the motor is leaned in and makes more power, it also makes more heat. When it makes more heat, the plug element gets hotter and the ignition starts occurring sooner. if the motor has a little extra heat added because if being new and creating friction, or because the weather is hot, or because the fuel has too much nitro, or because the glow plug retains too much heat, the motor can come up to a critical point at which the ignition starts too soon, and with the over-advanced ignition, the combustion pressure presses the piston down on part if the up stroke, and this in turn generates more friction and more heat, which again further aggravates the pre-ignition problem. Most model motors are set up so that they will run very close to the pre-ignition point, because that is the way that we get the most power. Once a motor goes over the power peak, the only way to get it back is to throttle back and cool it off. Remember, if you start to lean your motor in to get maximum power, and as the motor warms up it seems to slow down, you are suffering from pre-ignition. A slight bit if pre-ignition on a dead lean setting may not be too bad because that is the way you usually get the most power out if your motor. However, if there is much sagging at a dead lean setting, then you are giving away too much if the rich lean tolerance if the motor and something should be done.
Switching to a cooler running plug helps. Reducing the nitro in your fuel can make a lot if difference. Adding a little more oil to your fuel can help some. And if all if these put together in reasonable quantities isn't enough, then reducing the compression ratio is in order.
Unfortunately, we see a lot if cases of preignition these days. One of the reasons is that nitro is not as expensive as it was a few years back, and people, thinking that more nitro means more power, will order the more expensive fuel. It is not a bad idea to carry around a can if FAI type fuel (4 parts straight alcohol, 1 part straight castor). The next time you or one of your flying buddies experiencs a "leaning out" tendency, try cutting your fuel with the FAI mixture and see if it doesn't help.
Extracted from a Fox Manufacturing infomercial, Model Airplane News, January 1991, p49.