# Carburetor assembly jig



## Brian Rupnow (Aug 15, 2013)

I posted plans for the small bore carburetor which I used on my "Rupnow Engine" and Gus has asked me how the heck to you align the parts for soldering. Well, as in all things which require alignment, a jig is necessary. The following post will show what I used for an alignment jig.--Firstly, I will post the carburetor in question---


 

View attachment RUPNOW SMALL BORE CARB ASSEMBLY.PDF


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## Brian Rupnow (Aug 15, 2013)

Next up is a solid model of the carburetor held in the jig, ready for soldering--


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## Brian Rupnow (Aug 15, 2013)

And lastly, a detail drawing of the jig itself--there are two sheets to the drawing, you will have to open the pdf to see them both

 

View attachment CARBURETOR ASSEMBLY JIG.PDF


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## Brian Rupnow (Aug 15, 2013)

One thing I didn't mention---Use soft solder, not silver solder. It requires much less heat, and is less apt to stick to the aluminum jig. I use a bit of soldering paste on the brass parts being soldered. The roll of solder I have for "fine work" is only about 1/16" in diameter.


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## Brian Rupnow (Aug 15, 2013)

The really important part of the drilled holes in the two smaller carburetor parts (See pdf in very first post) is that the #2-56 thread in the needle valve side MUST be concentric with the 0.031" diameter hole in the fuel inlet side. However, its not that critical that the .031" diameter hole be perfectly concentric with the .063" dia. hole in the same part.  So---After everything was soldered together I put the longest side (the fuel inlet side) into the 3 jaw chuck in my lathe, and first drilled the .069" dia. hole thru the needle valve side, let the drill continue thru the air gap between the two parts, and  about .010" into the fuel inlet side. Then I changed drills to .031" dia. and continued drilling all the way through the fuel inlet side. By the time the .031 drill broke thru the far end of the fuel inlet side, it had wandered off center a bit, but I wasn't worried about that, because I knew it was concentric in the critical area close to the center of the carburetor. I then tapped the .069" hole for a 2-56 thread, and flipped the part around and held the very short needle valve side in the chuck, and ran a .063" drill in to the specified depth. Doing it in this manner guarantees that the needle valve will be concentric with the fuel orifice.  You will see in the carburetor details that both the fuel inlet side and the needle valve side protrude into the carburetor bore by about .015". This is very important, because this obstruction in the main air passage is what sets up the venturi effect to draw fuel up from the gas tank.


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