[Farmall] Zenith K5 mystery

[Karl Olmstead]

In the midst of rebuilding the Zenith K5 carburetor on the F-20 that I got 
from Bob Currie, I was browsing a rebuild sheet that Gordon Rice sent me a 
couple of years ago.  Reading the parts list, I came across something called 
an "economizer valve".  When I looked up the part on the drawing, it was the 
big brass bushing on the undriven end of the carb throttle shaft.  Caught my 
attention; I didn't know why a throttle shaft bushing would be called a 
valve.

Did some inspecting last night.  The bushing is pinned to the throttle shaft 
and turns with it, an unusual configuration.  There's a slot in the bushing 
which is hidden inside the throttle shaft bore in the carburetor.  And 
there's a passageway from the carb bowl area, thru a jet and into the 
throttle shaft bore.

That still didn't make any sense; what good is a valve with only one port? 
Upon closer inspection, I found another port in the throttle shaft bore, but 
this one was completely plugged by rust.  I looked inside another K5 that  I 
was using as a reference, and it had the same port, also plugged up.  So I 
bent a piece of #18 fencing wire and dug the rust out of the ports.  It 
turns out that the port connects to the carburetor main air passage, 
somewhere in the venturi area.

I have been having problems with my T-20 running rich, not responding to the 
idle adjustment, etc.  I'll bet money at this point that its carburetor has 
the same problem... plugged economizer valve passage.  It appears that at 
low throttle settings, the economizer valve equalizes pressure between the 
carburetor bowl and the venturi, reducing the amount of fuel pulled through 
the idle or main jet.

Who'd even look for an air passage in the bore of the throttle shaft?  I 
never did before.  And since it's difficult to see, nobody ever cleans out 
the port when rebuilding the carburetor.  Soaking in carb cleaner won't do 
it, only mechanical removal of the rust works.

I'll bet that there are plenty of T-20s and F-20s (and Cat 22s) out there 
running overrich because their economizer valves aren't working.  Another 
symptom is that the engine doesn't respond to adjustment of the idle mixture 
screw; it can be closed down completely and the engine will still idle. 
Unfortunately, that forces the engine to run off the main jet, which 
provides far too much fuel at idle.

Might be worth checking when your K5 equipped tractor starts blowing black 
smoke!

-karl