The needle arm is forked at both ends; one fits the groove in the needle, the other fits the groove in the diaphragm plate pin. The walbro tool is used to set the distance of the diaphragm end of the arm below the surface of the carb body with gasket fitted (diaphragm is last before bottom plate). Slot the short end of the arm into the needle groove, place spring under arm and tighten down the pivot arm. The free end of the arm should be just touching a straight edge placed across the carb body (rough guide as a starting point). Pop off can be set at this point assuming all other diaphragms /plates are fitted 12/8 is a good starting point but 8 to 12 /5 to 8 is Ok providing they are holding. use a bit of WD40 on the needle tip. I usually pump up to about 10, then release by pressing free end of arm gently a couple of times before checking pop off (careful not to get an eyeful of WD!!) Fit gasket, then slot diaphragm pin onto arm whilst positioning diaphragm, then fit bottom plate. You always need to 'prime' the carb when fitting to ensure fuel line is full of fuel - this is done by turning engine over whilst covering carb air inlet. If you have a top feed, cover end of fuel pipe with finger and blow (don't suck) down overflow pipe so fuel is forced up fuel line; this makes priming easier. Prime until fuel is right to carb and a couple of extra turns if doing by hand as opposed to a starter. If you have overflow tank, this should have a hole in the cap to allow air in, otherwise you get a vacuum and fuel starvation. The tank should have it's inner pipe just short of touching the bottom to allow it to follow fuel when cornering, fitted either with brass intake or brass in tank filter. Too long and it gets stuck = fuel starvation on certain corners. Too short and it runs fine for a bit then fuel starvation. There's some decent picks of a WB32 here http://www.aerocorsair.com/id27.htm similar to the WB19 and shows parts in order of assembly. Note that the WB19 check valve gasket has holes at both ends of the crescent slot.
Hope this helps.
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