Throttle Spring Modification – Top 80 engine and others

by Had Robinson
update December 3, 2021

An annoying problem with the external and extra main throttle return spring on the Top 80 is its tendency to fail often.  The stock throttle return spring system also puts unnecessary wear on the throttle shaft bores.  Here is a way to permanently fix the problem for less than $1 and in about 20 minutes.

First check to see if your throttle cable tends to stick.  If so, go to the throttle cable section for instructions on how to correctly clean it.  Do not make this modification unless your throttle cable moves freely and smoothly.  You may have to replace the cable or the sheath or both.

If you fly in cold weather, have a left handed throttle installation, or have any dust in your throttle cable, you will have experienced problems with the throttle returning quickly and firmly to idle.

Below, the original screw and throttle return spring.  The original spring had broken after less than ten hours in flight and was likely due to harmonic vibration of the spring.  Newer Top 80's now come with a dampener for the spring.  Note the distortion of the original throttle spring (photo below) caused by sharp bending around the throttle plate shaft. 

This fix that will give a more positive return of the throttle to idle.  Additionally, heavy users will note that throttle shaft wear in the bushings is greatly reduced, extending the life of the carburetor.

Parts needed

(2) compression springs 7/32" x 1-1/2" or (1) compression spring 7/32" x 3".  These are available at most hardware stores including Harbor Freight.

Modification

1.  Remove the original throttle spring on top of the carburetor that goes from the throttle shaft tab to the crankcase.  Do NOT remove the secondary throttle shaft return spring (arrows) that is located under the tab.  In case of a failure of the throttle cable, there must be some mechanism to close the throttle.

Below, the top throttle shaft return spring tab – now disconnected from the original spring.

2.  Carefully remove the knarp (cable stop nut) from the throttle cable at the base of the carburetor.  Pull the throttle cable out of the tab on the throttle shaft.

3.  Place the compression spring(s) on the throttle cable.  Optional: A 1/2" long piece of 1/4" ID copper tubing can be split and placed around the spring and ferrule to help keep the throttle cable from rubbing against the spring.

4.  Re-connect the throttle cable to the throttle shaft.  You're done.

This modification ensures that the throttle cable fully returns the throttle to the idle position.  Note that the exact details of your throttle shaft tab may be different.  Pilots may also want to change how the throttle cable attaches to the throttle shaft tab.  The modification below puts the wear point on a small cotter pin rather than on the strands of the cable.  This will extend the life of the throttle cable to the maximum, especially if you have one that is Teflon coated.  It will also give smoother throttle operation.

Be sure to thoroughly test your new spring installation before flying.