After a bit longer of a break between posts than I intended, I'm back to try to document the carburetor build today. My excuse is that I've helping my brother with the rebuilt of an old house he's going to move into. We just finished the kitchen cabinets installation. That turns out to be tricky in an old house!
In any case, on to the carb build!
I followed JEH's drawings exactly and I found them to be completely accurate and clear, so no need to build my own model of the carb. As he suggested, i started some some PM Research castings. In this case, I purchased a 5/16" casting tree and here I've just sawn off one of the 90 degree elbows to start working on it.
I fabricated a quick and dirty jig to hold the fitting, but this turned out to be far from optimal. The biggest issue is that it turned out to be too long so turning it in the mill vise didn't work out so well. Plus it was just a waste of material. The pins are 1/8" dowels secured with Loctite in one half of the jig and the holes were a close fit in the other half.
And here's the fitting secured in the jig after I drilled two intersecting holes just under the fitting body diameter. No pix of that as that process was straightforward.
And here's the first process, just milling each end of the fitting flat using a 3/8" end mill. Here's the problem with this long fixture. It's too long so that when I tightened the mill vise, it tended to spread the top so I secured the top with a Kant-Twist clamp before tightening the vise.
Then as per the drawing, I drilled each side with a 'K' drill to .410". This worked fine and came out exactly as per JEH's drawing.
And here's the elbow after a bit of filing and work with a Scotch Brite wheel.
I don't show the lathe work on the head/pipe elbow as it's simple lathe work. I secured the pipe into the elbow using some high temp Loctite 620 retaining compound. Here's the 2 parts prior to securing them together.
And after Loctiting them together. Not terribly exciting!
Here's the needle knob. I don't show the lathe work on this as it's pretty simple. I started with some 5/16" brass rod, knurled a small section, drilled and tapped the 2-56 hole in the center and turned the small OD and parted it off.
Here's the first delicate operation, making the fuel needle. I started with some 2-56 all thread, set the compound to 4 degrees and started taking VERY gentle cuts using a very sharp ccmt insert. I think I ended up doing a bit of filing on the part when I got close.
And here's the fuel needle and needle knob assembled. They were secured together with a drop of Loctite 242 Threadlocker.
The next few pix are of the process of making the mixer body. I started with a small piece of 1/2" brass hex bar and chucked it up in a hex 5c collet. The two OD's were quickly turned with a ccmt tool and then I used a tool holder as shown that holds a ccmt insert so that one of the 40 degree sides can be presented to the work. That makes as nice rounded feature as shown.
Next was drilling and reaming through the center of the stock and here I'm putting a small chamfer on the ID as per the drawing.
Parted off the work.
I reinserted the part into the collet and using the 40 degree tool and a small radius tool, I formed the end feature, not exactly as JEH's drawing calls for but close enough as this is all for appearance.
Then over to the mill after installing the part into the 1/2" hex collet in a hex collet in the milling vise. This way I was able to easily index the part to do the operations on the hex faces as per the drawing. Here I'm reaming 1/8" thru as per the drawing.
After indexing the part using the hex collet block, I spot faced, drilled #43 and tapped the part as per the drawing.
Finally, here's the finished mixer body with the set screw installed in the tapped hole. This all came out well.
The last, and most scary part to me, is the mixer spray bar. It's scary just because of the very small holes to be drilled.
Just to give everyone a sense of perspective of the drill sizes involved, here's a pix of my 1/2" drill index with my newest drill index laid in the lid. The small drill index ranges from #61 to #80 (.040" to .135").
Here's the brass round stock and drills to be used. Stock is 1/8" and the drills range from #50 (the largest) to the #76 (the smallest)
I took the approach of drilling the #76 cross hole first as I was scared of drilling each a small hole that would cross the interior hole. I thought it would be better to drill thru the solid material vs fighting the drill starting on the 2nd interior surface. I did touch the stock with a small center drill prior to starting the small drill. Note that I DID use a sensitive drill attachment here, that I'll show in a subsequent pix.
And here's the sensitive drill attachment that I used.
And here's how it's used in the mill. It's simply held using a 1/2" R8 collet.
Then over to the lathe with the work held in a 1/8" collet. Here I've just drilled #50 and am tapping 2-56 using a spring loaded center and a very small tap holder. This gives a good feel while tapping so that snapping off the tap can hopefully be avoided!
Then I turned the part around in the collet and first drilled .370" deep with a #55 drill. Here I'm using the sensitive drill attachment to drill thru the part with a #65 drill. Luckily the operation worked and the drill didn't snap off!!
And finally, here's the assembled carb! Everything fit together well but we'll have to wait to see if it actually works!!
So this completes several days of work on one of the more complex parts on this engine.
Enjoy!
Mike