[Farmall] Bad K-5

[Karl Olmstead]

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