LOBO PUP TWIN

FUEL SYSTEM -2

By Gail Graham

The needle for the fuel control needle valve is about as easy as it gets.

I started off with a length of 1/16 inch diameter straightened music wire from the local hobby shop. Marked off 2 inches and cut it off by clamping in my small drill press vice and cutting with a abrasive cut off wheel in a Dremel tool. The ends were cleaned up on the bench grinder and one end was rounded and polished for the handle end.


The wire was transferred to the lathe where the taper on the end was ground with a fine grit wheel in the Dremel. Before grinding, the grinding wheel was trued up using the same diamond dressing tool I use on the bench grinder. It was trued up  hand so while it is round the working face is not very flat. By keeping the grinding wheel moving, a reasonable taper is generated. Since I ground the needle by hand the
dimensions are eyeball approximations. After grinding to shape, I dressed the needle to a finer finish and more even taper using a EZ-Lap 1/4 inch square 600 grit diamond lap.


Then it was polished with 1200 grit abrasive paper. When finished I could not feel any roughness with my fingernail. Finally I polished the entire shank of the needle with abrasive paper to remove any surface contamination so it can be soldered into the barrel (to be made next).


Next up is the barrel that holds the needle.

A few operations, but all are easy. 3/16 diameter brass stock was cut to length and it turned down for a 1/4 inch so the spring will slide over it. Centre drill, drill to tap 5-40 and then tap. Drill through with 1/16 drill bit for the needle. Reverse in the lathe and file the end to round it off just for looks. Where the 1/16 hole broke through, the hole is deburred using a small centre drill and counter sunk a little bit (5 to 10 thou) to help guide solder
at assembly. I just hold the centre drill in my fingers to do this.















































































Last item is the spring. It could probably come out of one of the junk drawers or be rescued from a ball point pen, but I have found that I can make three up in the time it takes to find one.


The purpose of the spring is to keep the needle valve from changing position from engine vibration.


I used a long 1/8 inch dowel pin to form the spring on. When the spring relaxes from being wound the ID will be close to the 0.141 dimension that is desired.


Starting off with about 2 feet of 0.018 music wire, I bent a hook on one end and
gripped the other end in a locking set of pliers. With the dowel pin gripped in the collet of the lathe, I inserted the bent end in one of the collet splits and put tension on the wire. My lathe goes down to about 40 RPM, so setting at low speed I ran it in reverse to wind the wire on the mandrel. If you are trying this, and your lathe will not go below 100 RPM, I would suggest that you use regular pliers and not locking pliers. This is so you can just
let go of the wire. When you let go of the wound spring, it will unwind and the loose end can whip around and cut you. Be careful!


As I close wound the spring, I then stretched it out so 10 turns filled half an inch. Using heavy duty cutters, I snipped off a 10 turn length. Put the spring on the mandrel to provide support and a place to grip and then ground the end off a little bit to flatten it
so it would turn smoothly on surfaces it will be bearing on.































On to the assembly of the fuel system.

First the spray bar was installed in the venturi. The cross hole should be pointed at the side of the tube, but it is not critical. If you can not see the hole looking in the air inlet side then it is OK. Drilling the hole the way I did places the hole in line with one of the points of the hex portion of the spray bar, so alignment is easy.


The barrel is threaded on
the spray bar until it bottoms and then it is backed off one turn. This leaves one turn to make sure the valve can be shut off all the way. The needle is inserted into the barrel and pushed all the way down. Then it is retracted a little bit, about an 1/8 inch and flux applied to the needle next to the barrel. Working the needle in and out of the barrel a little bit to distribute the flux in the hole, the needle is reseated all the way down and the excess
flux is wiped off.















































































The needle was removed from the spraybar and the end of the needle is bent to about a 45 degree angle to form a handle. The needle was left out at this point as it will not clear the mounting lugs on the crankcase when installing the venturi on the crankcase.







The 1/4-40 nut was installed on the venturi and then the venturi was screwed into the crankcase and turned to the desired position and the nut tightened. After sliding the spring on the needle, the needle was reinstalled.



Part 1 here  Part 2 here Part 3 here Part 4 here  Part 5 here  Part 6 here 

Part 7 here    Part 8 here    Part 9 here    Part 10 here    Part 11 here

Part 12 here   Part 13 here    Part 14 here  

Part 15 here

 



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