Settings: Despite all the complex timing and extra parts, the model pilot operates the four-stroke exactly as if it were a two-stroke. The carburetor has the same low- and high-speed needle valves that work the same way. Adjust the high-speed needle valve until the engine runs 400-500 rpm less than maximum. Adjust the slow-speed needle valve until the engine maintains a constant 2,200-2,400 rpm idle.

If the idle slows, the idle mixture is too rich; there is too much fuel and too little air. If the idle speeds up, the mixture is too lean; there is too much air and too little fuel. If the engine quits when the throttle is quickly opened, the idle mixture is too lean. If it stumbles during acceleration, the idle mixture is too rich. A too-lean idle can also lead to detonation during throttle-up that could cause propeller throwing.

Because model four-strokes do not have accelerator pumps, the idle must be set slightly rich. The same is true of a two-stroke but nowhere as critical. They are simple, easy adjustments to make, just as they are on any two-stroke.

However, the four-stroke engine is intolerant of lean high-speed mixtures. Although two-strokes may run with a slightly lean mixture, four-strokes will not. A lean mixture usually causes the engine to experience detonation; the piston actually stops its upward travel because combustion occurs too soon.

This sudden reversal can cause the propeller to loosen or even separate from the aircraft. Just one such detonation can be expensive. Never lean a four-stroke to peak rpm, and always operate at least 400 rpm less than peak—more if the weather is dry and cool.

Even when run at normal mixture settings, four-strokes tend to loosen propellers. Four-stroke acceleration is not always smooth. There is much change in the amount of torque the engine delivers during speed-up and slow-down. This happens because the ignition and valve timing is mechanically fixed—not variable as in a car engine.

Timing can only be optimized for one rpm range. Therefore, the engine torque varies, as does its power output, as its speeds change. These sudden changes in the amount of acceleration or deceleration eventually cause the propeller to loosen.

It is a good idea to tighten the propeller before flying each day. Eventually the engine's thrust washer will wear out and need replaced. Most four-strokes are supplied with two propeller nuts; one tightens against the propeller and the other locks the first in place. Never use just one propeller nut on a four-stroke. If you do, detonation will cause the propeller to leave the aircraft while still rotating. Anything or anyone it hits will come out on the losing end.

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