I'm now down to the last major item to build for this engine: the carburetor. I spent a considerable amount of time trying to learn the basics of carburetion for small model engines and what the alternatives are. In the end, I decided to attempt to build the carb first documented by Chuck Fellows, with well documented builds by Brian Rupnow and Andrew Wakefield and I'm sure others as well. I liked this carb as it seemed relatively simple to build and it offers a throttle option. I did end up building a 3d CAD model of this carb for my version of this engine and generated 2d drawing which I used to build the engine.
So to start I milled a small piece of Al to the required dimensions for the body of the carb and then spotted the off-center hole on the body using the mill. I drilled a small center hole at the spec'd location and moved to the lathe.
On the lathe, I put my spring loaded centering rod in the tail stock and then used a dial indicator to center the rod which in turn puts the to be drilled hole in the correct location. As you can see, I used my 4J chuck and had to use some thicker packing under the jaws in order to actually grip the material. My 4J is an 8" model and that's really too big for this sort of work, but I did manage it. One of these days I'd like to get a 4" or 5' 4J for work like this.
Then drilling thru at the just located point.
Then turning down the OD to round. This is on the intake end of the carb.
Then, I changed the 4J back to the collet setup and turning down the outlet end to .25"
Threading the outlet side 1/4-32 with my tailstock mounted die holder. Rather than using a slip fit and Loctite, I decided to thread all the ports on the head the same as the spark plug since I have the 1/4-32 tap and die on hand. On the Webster I build, I noticed that the Loctite wouldn't necessarily hold well when the head got hot.
Next I moved back to the mill and drilled thru the body as called out on the drawings and also tapped 10-32 in what will become the throttle hole. Here's the body with the work completed on it as compared with the print.
Next part to be made was the jet. I started with a piece of 3/8" hex brass stock and turned the first end down and then drilled .052" to start forming the jet. I was careful as this hole has to be drilled to .625" so i was careful to clear the chips often while drilling. Shown is the drilling operation with a 20P coin for size comparison.
Threading 8-32 on the longer end of the jet. This was done with my tail stock die holder and rotating the holder by hand.
Back over the mill with the jet held in a collet block. After finding center of the block and the end of the part, shown is drilling a .040" hole thru the side of the jet, being careful NOT to drill into the other side of the ID. The Queen is observing the operation.
Back to the lathe, holding the part in a collet by the just finished end and turning down the opposite end to .125" OD. This is what becomes the fuel input port.
After drilling the initial .063" hole, shown is shown drilling thru the remaining material in the jet with an .040" drill. The 20P coin is there for size comparison.
And here's the jet after it was finished laid on the drawing.
The next part to be built is the needle assembly. The first problem I ran into was that I had no idea what a '#18 darning needle' was. So I went over to a local sewing store and ask the question. They came up with 2 items, both packages only about $2 so I bought them both, just in case. On the left is what is known as a '#18 chenille needle' and on the right is a '#18 darning needle'. It turns out they are both the same diameter that being .049" to .050" and the #18 refers to AWG. As you can see the differences are the overall length and the eye, but either works fine to make a carb needle from. Hopefully this info helps someone out!
Shown is how the needle fits within the jet. It seemed like it fit properly so I decided to proceed with what I had.
I knurled a short piece of 3/8" brass just to give the needle adjuster just a bit of grip.
Next was turning down the small end to .22" and parting off the adjuster.
I didn't show the lathe operations, but they were pretty simple, just drilling and tapping from one side, then turning the part around and drilling thru for the needle. Shown is the finished part on the print, and you can see where I needed to hand-correct a mistake I made when doing the CAD model.
Here's the needle after cutting it to length, ready to be soldering into the adjuster. I used Harris lead-free plumber's solder. This is the stuff with 4% silver content that melts at a low temp, typically about 430F or so. I find it quite easy to work with. I also used the matching Harris flux. The needle is gently inserted as deep as it can go into the jet and the adjuster is backed out about 2 full turn. Both parts had flux applied and a very small piece of solder was laid right in the center of the adjuster where the needle passes thru it. I made a very small countersink on the hole in order to give a place for the solder to flow to. A small plumber's torch was used to apply heat.
And finally here's the finished carb on top of a 123 block. I ended up NOT using the slits on the adjuster to apply tension, but made an additional spring to apply tension to it. The issue I had when I tried to use the slits for tension is that they simply didn't' work all that smoothly and tended to bind at different point on the thread. I made both of the springs from .022" music wire using one of my spring winding mandrels. I have enough experience making springs now that I find it fairly easy.
So now all the major parts are finished! Nothing left to do but to do the final assembly and then try to start it up!
All for now.
Enjoy!
Mike