dj - I actually don't know how to remove that. Based on the pics below, do you have any suggestions?
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Yes - that's a riveted head onto that washer. they definitely don't sell that part separately but they should be able to sell that assembly arm although the parts catalog doesn't show these parts so you likely would have to buy a whole carburetor.
To remove that piece, I'd use a jewelers saw and cut the head off running the saw cut right down the face of that washer. It actually should be fairly easy to do although probably a bit time consuming. You might have to use a needle file to clean up that cut face.
You should then be able to pop that part out - don't loose that plastic or metal washer. There may be a very thin washer on the other side also - don't loose that one either.
Take some measurements now - especially how much head is sticking out at the moment - you don't want your replacement part to stick out any further, it would probably bind up on something. You also want to know how far out those holes are from the arm.
Then to make the replacement part, you'll need the diameter of where the rod and set screw go. Then the diameter of the shaft going through that assembly. You can make the length a bit more than you need for making the rivet head, but if you can measure that also, it would make it easier rather than fitting, marking and then cutting to about 1/8" bigger than you your current head protrudes - I just can almost never get it right the first time so making it a bit longer just gives me peace of mind...
The next trick to making that rivet, is to drill into the back side which will become the rivet head. OK, what size drill to use... This one gets a bit tricky. You've measured how far that head sticks up in the back. The drill size you select will leave approximately the same amount of material around that diameter as that head was sticking up. You can drill to a bit below the interface where the bottom of the head will be against that washer on the back side. You can then use a center punch to flare out the your new rivet head working it carefully so that the rivet creates just the right amount of friction to allow that part to spin freely - one of the functions of that plastic washer under the washer where your rivet sits. Once the "feel" is right, you stop riveting and any excess metal you simply file off to give the proper clearance. That whole solid head is absolutely not needed.... It's simply done that way at the manufacturer because they have a completely controlled system where that part is mass manufactured.
The other side where the rod and set screw go, you will want to drill pilot holes only until after this riveting process is finished. Then do the finish hole sizes and threading once riveted.
That should give you a better part than OEM....
dj
p.s. As an FYI - You probably didn't really break that part. In production, they would have done those two cross holes in one operation, then done the riveting. It's very likely that created a crack during manufacture that finally manifested itself with you tightening down that set screw...