Making A Throttle Plate
For
A Mixer or Carburetor

After making several throttle plates or "butterflies" with mixed results, I've come up with a way to make it so it contours to the bore of the mixer or catburetor.

First, get out a piece of bar stock end that is larger in diameter than the carburetor bore and securely clamp it in the mill vise at an angle.  The angle is not critical but remember that if the trhottle plate closes too close to diagonal to the bore of the carburetor, it could stick when closed.

Then, after milling the top of the dummy shaft, find the center of the elliptical end of the dummy shaft.  Spot drill and then drill the hole for the screw that will secure the throttle plate to the dummy shaft.

Now, tap the face for the mounting face as shown.

Mill a land on the end of the shaft for a replica of the throttle shaft.  The land is centered on the long axis of the ellipse and only needs to be a few thousandths high. The width of the land should be just a shade narrorer than the diameter of the throttle shaft.  This land is used to locate the throttle plate and keep it in place while turning it.

Cut a piece of the material you are going to use for the throttle plate.  Find the approximate center of it.  Now drill a clearance hole for the mounting screw and mill a slot 0ne half the thickness of the plate.  Make sure the slot is deeper than the land where the throttle plate will locate on the shaft.
Firmly screw the throttle plate blank to the dummy shaft, making sure that the land in the dummy shaft is engaged in the slot in the throttle shaft.
     
Mount the dummy shaft with the throttle plate blank mounted on it in the lathe and turn the dummy and throttle plate to the diameter of the bore it will work in.  It may be necessary to turn the diameter one or two thousandths of an inch smaller to aid in assembly and final fit.

When the throttle plat or butterfly is mounted on the throttle shaft, the center of it's thickness should be along the centerline of the throttle shaft.  It is shown assembled to test the fit on the throttle shaft.

Here is the finished throttle plate and shaft assembled in the mixer throat.  

To aid in assembling it, I drilled a hole in the mixer bore that lines up with the mounting screw.  This hole is for a standard machine screw and is just large enough for a screwdriver or Allen wrench to fit through.  It is tapped and, after assembly, a short screw is used to plug the hole.

For your information, the throttle shown is for The Upside Down Engine.  It runs on propane so there is no jet.  The "venturi" is a restriction on the air inlet that makes the engine create a vacuum that works on a diaphragm to supply gas.  The design I've come up with for the demand regulator isn't optimum but it does work.

If you have any comments, I'd be happy to hear from you.  I can be reached via email at this address:
[email protected]