[Farmall] K5

[Karl Olmstead]

Finally got the Zenith K5 carburetor on my F-20 back together.  Complicated 
unit; it took a lot more work than I expected.  I found a plugged-up 
economizer port, clogged compensator jet and an idle jet with dip tube split 
along its entire length.  The bushing on the driven end of the throttle 
shaft was badly worn.

Got the parts cleaned up last week and painted the castings.  Let them sit 
in the sun all week to dry.  Got the new parts from Gordon Rice, including 
gaskets, throttle shaft and bushings.

The throttle shaft bushing on the driven end of the throttle was blind; no 
way to get behind it and press it out.  So I set the throttle body up on my 
mill and squared it up relative to the mill table.  Used a small end mill to 
cut away most of the bronze bushing in two places and then pried the 
weakened bushing out with a knife.  While still set up, I cleaned up the 
remnants of the throttle limit post, the thing that (used to) keep the 
butterfly from being turned beyond wide open.  All that remained of this 
stop was a rusty nub, so I milled it away and center-punched the area.  Then 
I drilled and tapped it and replaced the stop with a 12-24 machine screw. 
This stop is important; it keeps the governor from turning the carb throttle 
shaft too far.

I pressed the new throttle shaft bushing into place, then used a 0.3125" 
reamer to enlarge the bushing to the proper size for the throttle shaft. 
With a new economizer valve (bushing) installed, the throttle shaft has no 
play any more.  Nice tight fit.  Since I wasn't quite sure about the 
orientation of the economizer valve, I used a cotter pin to fasten it to the 
throttle instead of the recommended tapered pin.

Since the original idle jet had a cracked tube, I borrowed one from a T-20 
carburetor.  The original jet was a #14 and the replacement is a #15, for 
what that's worth.  The orifice sizes looked pretty close.

Put the carb back together using mostly new fasteners and reinstalled it on 
the F-20.  All of the adjustment screws were open one turn, as recommended 
in the manual.  The engine started on the fourth or fifth crank.  After 
getting it good and warm, I adjusted idle speed, idle mixture and high idle 
mixture.

The exhaust is now clean at all speeds above idle.  At idle, it still blows 
black smoke.  Maybe because I changed the idle jet.  The engine now idles 
down nicely, which is one of the things it wouldn't do before.  I noticed 
that opening the idle mixture jet seems to lean out the engine; is this 
correct?

Spent more than an hour driving around, listening to that wonderful F-20 
sound.  I think we're ready for a parade.

-Karl