There are 3 spray tubes for the Amal carb, most come with a standard straight cut tip, there was also an angled tip for 2 strokes and finally a tube with the back half removed designed specificly for the 850 commando. I read on a Velocette and Vincent site that people had tried the cutback 850 tubes and it improved the idle and low speed range, so I thought I would give it a try.
The theory goes that on a big bore engine there is so much suction at low revs that a lot of the fuel goes straight up out of the jet and hits the underside of the slide causing the fuel to puddle and not break into fine droplets, by cutting the back away, the fuel has a better chance to hit the airstream and get drawn into the engine.
The spray tube is the little brass bit that is pressed into the throat of the carb not to be confused with the needle jet that sits in the main jet assembly which are available in either 2 and 4 stroke types
