How to Tune Your Model Airplane Engine

Tuning is simply adjusting your engine for peakLeaner: reduced smoke, sound starts changing to
performance. The factory-fresh settings may be ahigher pitch
good starting point. However, you may need toLeaner still: little smoke, max rpm reached
compensate for higher altitudes, or for temperatureToo lean: very little smoke, max rpm can be heard
changes with the seasons. Changes in fuel type orfalling off. Open the needle valve up 20 or 30
engine wear, may also require a tune-up.degrees from this point. If you have a tachometer,
Relatively more fuel to air is known as a "rich"open the needle about 300 rpm less than max.
mixture, while the opposite is known as a "lean"5) Still at high throttle, perform the pinch test. Briefly
mixture. A lean mixture generates more power, butpinch then release the fuel tube going into the carb.
also generates more heat. Too much heat couldIf the engine momentarily speeds up, then slows
damage the engine. A rich mixture generates lessback down, you're done. If the engine does not
power and less heat. Too rich and the engine mightspeed up, or quits, then it is too lean; richen the
quit while you're flying.needle some more.
The popular twin-needle carburetor has two needle6) Now throttle back to idle; let the engine idle for a
valves. The high speed needle controls the total fuelcouple minutes.
flow into the engine, and affects the engine's7) Now perform another pinch test. This time pinch
behavior from half to full throttle. The low speedand hold the line. The engine will want to speed up
needle tends to have more of an effect from halfand die. If it speeds up immediately, it's too lean,
throttle down to idle. Both the low and high speedrichen the low speed needle. It should take 4-6
needles can be richened (counterclockwise turning),seconds to speed up and begin to die. Any longer
or leaned (clockwise turning).and it's too rich.
Tuning Procedure:8) Also check the throttle transition. With the aircraft
1) Start with the factory recommended needle valvesecured and engine idling for a couple minutes,
settings.advance the throttle quickly to fully open. If the
2) Start the engine.engine hesitates, the low speed mixture is too rich.
3) With the engine or aircraft secured, open theFor more information about Model Airplane Engines,
throttle to full.visit the RC Airplane Advisor - Model Airplane Engines.
4) Lean the high speed needle slowly and watch andThis article may be reprinted if proper credit is given
listen:and all links left intact.
Too rich: lots of smoke, low pitched sound