Hey Mate
Mixture screw on carby won't change gas mixture. Normally there is an adjuster for the gas, where the gas hose joins the top of the carby. It's a pretty agricultural adjuster. Well is on mine at least. Mine has a cable running into the cabin which is pulled to open mixer to let more air in when running on petrol. When doing some work on the motor some time ago, I reconnected this cable. Didn't get it quite right, when shutting it mixer wasn't closing properly, letting to much air through and car cutting out when switching to gas. Don't know if this helps worth alook.