Last year, we attended a neighbor's auction on a beautiful November day, and I ended up buying an old Snapper lawnmower. I thought it might be good project to get running again, and if I could, then my Kiddo #2 could start to learn to mow the yard. I bought it for $7.50.
After a little on-and-off-again tinkering through the months, I found some carburetor problems. Here's a picture of the carburetor all stripped down and cleaned up:
It turns out the throttle shaft was pretty well worn out. There were some significant gaps between the shaft and the throttle stop. Perhaps either the shaft or the stop were worn out, or maybe even both. Doing some careful measurements, I found the following:
Taking off the stop, it's easy to see it's all egg'd out and worn unevenly. That little 0.100 roll pin sure was difficult to remove! Here's a closeup of these 2 little parts:
The throttle shaft itself really tells a better story. You can see how badly it was worn out. The slight bend in the middle is simply from me trying to remove it.
I bought a new throttle shaft and throttle stop.
Here's a comparison of old to new:
Now the tricky part. I had to make some bushings to insert into the body of the carburetor to take up that big gap between body and shaft. Not a problem with my little machine shop at work. I stayed late after hours and made up a couple. I started with some 1/4" NPT plugs that had been tumbling around in my toolbox for the last decade. This is my raw material:
I'm a little more skilled on my mill rather than the lathe. It's a little unorthodox to make round parts in a mill, but it can be done. I'm sure a better machinist could do it faster and easier and more precisely in a lathe, but that's not me. It looks pretty small in the jaws of my vice on the mill:
Here's my interface to my CNC Mill. It's a Tormach PCNC-770.
This program shows that I will helix down into the material
Halfway finished:
My design is a stepped bushing, essentially. That step will keep me from pushing the bushing in too far.
I removed the excess material in the lathe, and had a finished bushing thereafter:
Two custom-made bushings on a brand new throttle shaft. Very nice and smooth:
Now it gets even trickier. I had to make the mating holes for those new bushings in the body of the carburetor. Back to my manual non-CNC mill for that job. It took about 30 seconds to mill and drill the holes, but about 30 minutes to locate vertical and find the center.
First, I found vertical in relation to the X-plane using
my indicator and a straight piece of material
Then I did the same for finding vertical relative to the Y-plane.
This was all pretty tricky.
Finally, all the pieces are coming together.
The bushings fit snugly in their mating holes:
And the real moment of truth: Will the throttle shaft slide through my new bushings once they are installed? Well, no. It didn't. I had to run a reamer through the assembly. I was close, but it wasn't happening. Nonetheless, a reamer fixed all misalignment issues from machining the body. (Thanks for the reamer, Toby!). Once reamed, that shaft moved smooth as silk.
Carburetor installed. It is dazzingly white compared to the rest of the engine.
Amazingly, it fired right up, and after a little adjustment, we tried it out. My Kiddo #2 has been super anxious to try it out and do some actual lawn mowing, so I was happy to give him a lesson and let him go. He's had a couple of summers of lawnmower driving, but not actual lawn mowing. So this is his first time actual cutting grass. He did pretty well, and I was pretty proud of him!
Here's a little video clip of him puttering along.
It looks like he's really having some fun:
