Getting to Know Your Engine – Part 2

Aug 28, 2012 No Comments by

Throttle Adjustments – The throttle pushrod connects the throttle servo to throttle lever on the carburetor, and its adjustment is critical to the proper performance of the engine. The following steps will provide full power at full throttle, a reliable idle with the throttle sick at idle and the idle trim in its mid-range position, and the ability to stop the engine by then bringing the trim to its full low position.

Adjust the throttle lever on the carburetor so at mid throttle it’s perpendicular to a line from the carburetor to the throttle servo. Most throttle levers are held in place with a set screw or nut that can be used for this adjustment. With the radio on and the throttle stick at mid-throttle, place the servo arm on the servo so it is also perpendicular to the line from the carburetor to the servo. This will make the throttle lever and servo arm parallel with each other. Now adjust the length of the throttle pushrod so it can connect the servo arm to the throttle lever with each in its midrange position.

In your transmitter, check the throttle servo end points at 100%, and the throttle curve is a straight line from 0 to 100%.  Attach the pushrod to the throttle lever, but NOT to the servo arm. Bring the throttle and throttle trim to the full idle/low trim position, and rotate the throttle lever to the full closed (engine cutoff) position. Note where the throttle pushrod should be connected to the servo arm.   Now bring the throttle stick and throttle lever to their full open position, and again see where the throttle pushrod must be connected to the servo arm.

Hopefully one of the holes on the servo arm will meet both the cutoff and full throttle requirements, and you can attach the pushrod to that point. If that is not the case, attach the pushrod to the closest point on the servo arm that will provide full movement of the throttle lever. Adjust the servo end points in the transmitter to fine tune the servo throw to provide an engine cutoff at full idle and idle trim, and still provide full throttle. The throttle trim lever can then be moved to a higher position to obtain a suitable idle.


Paul Tradelius

About the author

A regular contributor to Model Airplane News, he is also the columnist for our “Rotor Speed” helicopter column. Paul has been flying RC helicopters since the early ‘80s and now enjoys all types of rotary machines, including scale and aerobatics, and he continues to experiment with modifications to improve performance.
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