I thread the holes in the manifold with a 1/4" NPT pipe thread and make bushings for the heat riser shaft from brass fittings. Then I fabricate a shaft and a flapper valve from stainless steel. The thing will last about three days past forever that way. It's not easy or cheap- - - -it just works! If the air leak around the shaft is big enough and the fuel mixture is slightly rich, there will be a backfire while coasting.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!