By Jason Ballamy 
Part twelve - Carburettor
M4. C4n

The carb is another tricky part carved out of a bronze casting. I started by holding the main body in the 4-jaw so I could skim the chucking spigot, this was then held in the 3-jaw and work started on the top of the body.

The hardest part is turning the tapered seat below the threaded portion as there is not a lot of room for the tool as it gets towards the bottom of the taper.
While the topslide was set over I machined the flow valve so the tapers would match.
It was then over to the mill to face back the various bosses before drilling and tapping these. There are also several passages that have to be drilled for the fuel, you can see one here that has been counter bored ready to take a soldered plug to cap off the end.
The final hole is for the needle valve and the 1/16" hole for the fuel has to come out in the tapered seat so not the easiest to line up.
Fortunately it came out just about right.
Two plugs were turned from the chucking spigot material and then silver soldered into position.
The hole for the fuel line did not come out very central on the cast boss so I decided to machine the boss concentric with the tapped hole. To maintain some draft angle I packed the rotary table up at one side. Then used a ball ended cutter to tidy things up.
The top was turned in the lathe before transfering to the mill to form the hexagonal part.
Before going back into the lathe to chamfer the edges.
It was then parted off and the make thread cut before once again returning to the mill to drill the air holes
And this is a rough assembly of parts so far.
The remaining bits are quite straight forward so not worth describing, this is all the parts that go into the carb.
And the assembled carb, just the spring and washer missing off the bottom. If you compare this shot of the body to the previous you can see where I have used an engraving tool to restore the "cast" texture around the fuel boss and the plugged holes.