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Throttle Spring Modification – Top 80 Engine and Others

by Had Robinson

An annoying problem with the main throttle return spring on the Top 80 is its tendency to break at any time. The stock throttle return spring system also puts unnecessary wear on the throttle shaft bores, wearing them out long before the engine wears out.  Here is a way to permanently fix the problem for less than $1 and in about 20 minutes.  An added bonus is that this modification is very unlikely to break or fail like the OEM setup.

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.  Furthermore, the small size of the original spring results in overstretch and frequent breakage of the spring. 

Below, the original screw and throttle return spring.  The original spring had broken after less than ten hours in flight.  Was it a defective spring?  Replacing it with a new factory spring does not fix the design problem.  Note the distortion of the original throttle spring (photo below) caused by sharp bending around the throttle plate shaft.

Throttle spring screw on the Top 80 engine 

This fix that will give a more positive return of the throttle to idle.  Additionally, heavy users of the Top 80 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.

Top view of Walbro WG8 carburetor 

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.

throttle cable attachment modification 

Be sure to thoroughly test your new spring installation before flying your Top 80. 

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