By Gail Graham


The last group of parts are for the fuel system. The most visible of these parts is the Venturi that everything else mounts to, either directly or indirectly.

The Venturi has no critical dimensions. The 1/4-40 thread has to fit the crankcase and the axial hole in it should be reasonable close. Most of the Venturi action is created by the reduced cross section caused by the spraybar passing through the axial hole. The air intake or bell portion of the Venturi can be any shape or even left as just the axial hole. On a higher performance engine all these things get far more critical but this engine will probably only reach about 10,000 RPM.

I started the Venturi with length of 3/8 diameter aluminium bar. One end was turned down and threaded with a 1/40 die. I use the front of my tail stock chuck with the jaws retracted to start the die square.

After centre drilling and drilling, the part is cut off a little long to allow cleaning up to length.

After switching ends, the part is faced off to length. The compound slide is set to 12 degrees for an included angle of 24 degrees and the exterior of the bell is cut using the compound slide. Nothing critical about the angle. It is only for looks.

The end was opened up with a 60 degree countersink. The shallow angle is just for looks. A standard 87 degree would work just as well. A small boring bar could also be used to generate an angle, or the hole could be left plain but it would not look very good to my eye. I then rounded off the sharp edges and did a light polish with abrasive paper.

After transferring to a collet block in the mill the two sides are faced off to provide flat mounting surfaces for the spraybar. The collet block was positioned by a stop on the vice to make it easy to flip the block 180 degrees to do the second side. The centre flat surface is centre drilled and drilled for the spraybar. After removal from the collet, the axial hole is de-burred on the inside using the same drill that was used to drill it.

Two of the parts left are the nuts to lock the Venturi to the crankcase and the nut for the spraybar mounting. They are easy, and the operations are the same for both so I will just roll them into one post. Although I used hex stock for these, they could just a well be made of round stock with a flat milled on two opposing sides. I normally make up a few extra as it is very little extra work.

Starting off, the stock is faced off, centre drilled and drilled with a tap drill. Then tapped. If making extras, the tap can be run in deep enough for all of them. Then part off the individual nuts using a narrow parting tool. The one I am using in the photo is ground to 0.018 wide.

The nuts will have a nasty burr on the cutoff side. There is no way to avoid this as the final part of the cut is going into the internal thread which will catch to tool at the final stage. I use a hobby knife to slice off most of this burr. On small nuts, like these, it is helpful to stand it upright against a scrap of metal rather than try to hold it with your fingers. Easier on fingers also.

Finally I clean up the remaining burrs on both sides of the nut by sliding the nut along a fine file. I set the file on a flat surface and hang the handle over the edge. Slide the nut toward the handle while putting light downward pressure on the nut with your finger tip. On the side that had the nasty burr it will probably take two strokes and only one on the other side. I am using a 6 inch single cut mill bastard file in the photo.

The last complicated part is the spraybar. It is not really complicated, but it does have a lot of operations on a small part. Three of the operations are a little bit difficult, but fortunately dimensions are not critical. They are the drilling of the hole lengthwise through the spraybar with two different size drills and drilling the cross hole through one side of the spraybar.

Starting off with 3/16 hex bar stock, it was turned to 0.125 diameter for 0.625 length, and then threaded for about 7/16 of an inch with a 5-40 die. Thats 16 to 18 turns of the die.

The section that will be centred up in the Venturi is reduced to 0.093 inch using a square nose parting tool. I used a 0.040 tool. Too wide a tool will probably grab and bend the part. Then the part is centre drilled and drilled to a depth of 5/8 inch with a 0.063 drill. After the first 0.2 inch or so it was necessary to peck drill in about 0.05 increments to keep the flutes of the drill bit from filling up. Then the part was extended from the collet some
additional length and parted off a little bit long to allow clean up of the end.

After changing collets in the lathe, the other end of the part was cleaned up to length and turned down to 0.093. A shallow groove was put in to help retain thee fuel line and the end was bevelled a little bit with a file to make it easier to get the fuel line on.

The final lathe work is to centre drill and drill with a 0.040 drill through to meet the hole from the other end.

A nut was put on the spraybar and then the spraybar was lightly gripped in a small drill press vice with the hex portion of the nut and spraybar slightly above the top of the vice. Turning the vice over, it was pressed down on a hard surface to bring the points on the hex level with the top of the vice and the vice tightened. A parallel could have been used under the spraybar, but this method is a quick way to get the spraybar level in the vice.
Finally, the cross hole through one side of the spray bar needs to be drilled. Fortunately, the exact position is not critical and neither is the size. The size should be some where around 0.032 diameter, but anything under 0.040 down to about 0.025 should work well. I used a #68 drill because I have a small pile of carbide circuit board drills that size. I put the vice on the drill press table at about a 45 degree angle so by
moving my head left and right I could look either length wise of the spraybar or perpendicular to it. I centred the drill on the width of the spray bar and in the centre of the reduced diameter section by eye and drilled through one side of the spraybar. I was using a 5 power visor for this. Some people wish for 20 year old eyes. I would settle for 60 year old eyes. So if I can do it most anyone can. Any way, the position if the hole is
really not very critical so if it looks about right it will work.

Finally, using the 0.063 drill, I deburred the inside of the spraybar to remove the burrs left by drilling from the other end and from the cross drilling. I held the drill bit in my fingers, and first turned the drill backwards to break off the burrs and then forward. If you turn forward at first, the drill will try to pull into the hole at the far end when it first engages the burr. Clean out the spraybar with compressed air or blow through it. Any crud in it can really make getting a smooth needle valve setting very difficult.