Hi Paul and Bill,
   I have seen this idea before, years ago, on a  two stroke M/C engine,
   Puch, as I recall. I think the T/B had just the hole into the throttle
   bore and the sliding block had the tapered slot, this makes mixture
   adjustment easier by just  exchanging the sliding block with one with a
   different slot size or slope. Same idea as having a range of needles
   for the Bing;  you have a range of tapered slots.

Dave the Lectron carb I mentioned is still in business. They use a fixed
orifice and a tapered needle BEHIND the slide about 1/4 inch so it's
always in vacuum. Good vorking carb too I helped built a Yamaha TZ 750
for a guy that like them with 4 38mm versions. worked well and got mega
horsepower on the dyno.
Bill Jepson



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