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.
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
Now, tap the face for the mounting face as shown.
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.
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.
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.
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.
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
Here is the finished throttle plate and shaft assembled in the mixer throat.
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.
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: