How To Make a (not very efective) Throttle
This idea seems to have originated with the AE Rivers company of Middlesex, England, circa 1960. It was available as an "option" for their 3.5cc "Silver Arrow" diesel, screwing into the crankcase backplate. The idea is to reduce the pumping capability of the engine by progressively bleeding crankcase pressure (primary compression) via a sliding plunger that slid in a flute-like tube. This would decrease the engine's ability to draw fuel and to transfer what it does draw from crankcase to cylinder. In theory, it sounds good.
David Owen decided to give it a try and made a replacement backplate for a PAW, as seen here. He reports that it had no throttling effect and was entirely unsatisfactory as a reliable cut-out. David says not having anything else but the description of the Rivers unit, he had to make an informed guess at the bleed holes, using ten 1mm holes drilled in one side only of the musical instrument, figuring he could reasonably expect some usable response. This was not the case. For the curious, the Aeromodeller engine review appears below, with the throttle illustration from Model Aircraft. Interesting that the "throttle" was interchangable with a pressure feed nipple.