[Farmall] Carbs

[Karl Olmstead]

As I've mentioned before, I've been pretty inactive on the list lately 
because most of my free time is going into home remodelling.  Now that the 
drywallers are working, I can at least work outside again.  Spent most of 
last weekend hauling pine needles around in a trailer pulled by my Farmall 
C, and running them through a 3-pt. shredder attached to my Farmall 350. 
Nasty, dirty work, but I did spend a lot of time working with red tractors.

Took a weekend off a couple of weeks ago and went to the California Antique 
Farm Equipment Show in Tulare, CA.  Sold one of my rebuilt F-4 magnetos 
there, and bought a nice '34 F-20 from Bob Currie.  The F-20, which came 
from South Dakota, was restored by its original owner back in the 80's.  It 
starts beautifully, but I'm not satisfied with the way it runs; too much 
black smoke out the exhaust, and noticeable missing.  Bob said that it also 
fouls plugs fairly quickly.  Carbon fouling, not oil.

So I pulled the Zenith K5 carb last weekend, and the pieces are now soaking 
in my carb cleaner bucket.  I found a plugged bowl vent and a badly worn 
throttle shaft.

I ordered replacement throttle shaft and bushings from Rice Equipment as 
well as a gasket set, but I got to thinking about carb repairs in general.

I know that machinists routinely bore out the old throttle shaft holes and 
press in bronze bushings to restore the tight, leak-free action required to 
give a good idle.  My question is how you figure out where to bore the 
holes.  By the time we decide to repair a worn throttle shaft, the holes in 
the cast carburetor body are grossly oversized, and probably egg-shaped as 
well.

I'm not much of a machinist, but I do know that setup is the most difficult 
part of most jobs.  Once you get the work and the tool properly aligned, the 
job is nearly done.  My machinist buddy wasn't much help; he suggested that 
I try using reamers starting at just barely over original hole size and 
working up from there.  He said reamers tend to follow the original hole 
pretty well.  But that didn't satisfy my concern... that the original hole 
is gone, and you are probably dealing with something that is no longer 
centered on the carburetor bore.

Then there's throttle plates.  They aren't round discs; they are designed to 
close off the carburetor bore before they reach the 90 degree position.  How 
are they made?

Anyone have any experience with the machining involved in fixing 
carburetors?

-karl