[Farmall] Bad K-5
Karl Olmstead
olmstead at ridgenet.net
Thu Jun 9 09:09:41 PDT 2005
Some of you will remember the fun I had a year or so ago when I took my '31
T-20 crawler to the Tulare, CA antique tractor show. Basically I never got
more than ten feet from the trailer, despite many hours of trying. Water in
the gas and a major flooding problem kept the engine from running properly.
Now that I have successfully rebuilt the Zenith K-5 carb on my F-20, I
decided to re-visit the carb on the T-20.
When I rebuilt the carburetor about five years ago, I failed to completely
disassemble it. I couldn't figure out how to pull the cast zinc venturi out
of the cast iron carb body, and I didn't pull the throttle shaft because it
is secured with tapered pins through collars.
The venturi was easy this time. I sprayed penetrating oil around it and
pushed it out using the hydraulic press and a rounded brass rod. The
penetrating oil had wetted nearly the whole interface of the venturi in a
minute or two; I don't think it took more than 50 pounds of force to push
out the casting.
The throttle shaft took longer, but drilling out the tapered pins went very
well using my mill.
As expected, the economizer valve port was completely plugged with rust.
Worked away at it for fifteen or twenty minutes before I got the port opened
up. All by itself, that undoubtedly made the engine run rich throughout its
midrange.
As I was cleaning up the bore in the main carb body where the venturi had
been, I spotted some pits. Closer exam revealed that they were probably a
combination of casting porosity and rust pits. And when I squirted carb
cleaner into the pits, it ran out the fuel intake port of the carburetor.
So what I had was a passage that bypassed the needle and seat entirely, and
let raw fuel drain into the passages surrounding the venturi, some of which
dump right into the airstream. I was puzzled before by the very high
leakage rate I had observed. I couldn't understand how most of the one
gallon starting tank managed to run through the carburetor and down into the
air inlet pipe in about fifteen minutes. There's no way that the fuel
should have gone past the (brand new) float bowl needle and seat that
quickly.
I dabbed some epoxy on the 'pits' and sanded it flush after it cured.
Hopefully this weekend I'll get a chance to finish rebuilding the carb and
try out the T-20. Somehow, I think it'll run a LOT better.
I've pulled the throttle shaft on four K-5's now. Every one of them had a
plugged economizer valve port. I'm convinced that is is pretty much a
universal problem. And now I know that you MUST remove the venturi and
check out the main carb body behind it.
-karl
More information about the AT
mailing list