The carburetor's throttle which includes the spray bar assembly and rotating throttle barrel were tackled next. First up was the spray bar assembly. In addition to a couple Delrin spacers it consists of a spray bar which screws into the carb body and a needle subassembly that screws into the spray bar.
The spray bar's delicate features and ultra fine threads made it one of the more challenging parts of the carburetor. After starting out with 303 stainless, a switch was made to 12L14 when the threading dies turned their noses up at stainless. Since the features needed to be concentric as possible, all lathe operations were indicated in a four jaw chuck.
A .022" fuel passage drilled through the .040" spray bar nozzle was done using a sensitive drill feed and an accurately aligned tailstock. The 10-56 external threads on one end of the spray bar were simple enough, but the 8-80 external threads on the opposite end were a real headache. The 8-80 threading operation was the last step in the spray bar's machining, and after scrapping three nearly finished parts I started practicing on less precious stock.
An 8-80 thread is only .007" deep, and a clean start with the die in my tailstock die holder seemed impossible even with the help of a couple lathe starting passes. In nearly every case the threads cleaned up only after the entire die had engaged the workpiece. It seemed the clearance stack-up among the die, die holder, and tailstock was just too great for an 80 TPI thread.
Initially the part was made longer and the bad threads cut off, but then cross-threading became a problem. What finally worked was turning a slight starting taper (approximately the width of the die) on the workpiece. After a couple frustrating days I finally had a spray bar I was happy with.
The needle assembly was designed around a common sewing needle, and its 8-80 internal threads went without a hitch. A Delrin bushing keeps the needle centered inside the spray bar as it enters the nozzle, and a spacer protects the whole assembly from damage by limiting the fully closed position of the needle. For best accuracy the needle was JB-Welded in its holder while the whole assembly was temporarily assembled and the needle fully closed.
The throttle barrel was machined from black Delrin for a snug sliding fit inside the carburetor. It's captured in the carb's body with a bolt-on retainer. The head of a judiciously placed SHCS serves both as both an idle and WOT stop. (The idle is adjustable with a screw in the throttle.)
The fuel loop was set up once more. Sanity checks were made on the throttle by blowing air through the carburetor throat and verifying the needle had some semblance of control over the spray pattern below the carburetor. A final tweak was also made to the standpipe height to prevent startup transients from forcing fuel out the bowl vents - Terry