10-11-18, 02:03 AM
No problem 
I didn't draw it just found it on a google search. I'll explain it a bit more. With the butterfly valve closed the vacuum of the engine is drawing fuel and air through the idle circuit as I explained in the last post. Once you start to open the butterfly valve by opening the throttle the effect of engine vacuum moves past the butterfly valve and moves further back along the carb to the bottom of the slide. See the hole in the bottom of the slide marked SVP in the diagram well the vacuum from the engine goes up through that hole, up the centre of the slide and it acts on the top of the rubber diaphragm. So now there is vacuum acting on the top of the diaphragm. It's not shown in that diagram but there's a slot cut in the body of the carb so that atmospheric pressure acts on the bottom of the rubber diaphragm. The top and the bottom of the diaphragm are sealed from each other so with low pressure acting on top of the diaphragm and relatively high pressure from the atmosphere acting on the bottom of the diaphragm the diaphragm deflects up. As it deflects up it lifts the jet needle out of the main jet. So now the main fuel circuit can come on. Air can now flow through the main air jet (maj), through the holes in the emulsion tube and fuel gets draw up through the centre of the main fuel jet and mixes with the air from the main air jet and the mixture flows past the jet needle and is drawn into the engine. And as you open the throttle more, more vacuum can act on the top of the diaphragm so the diaphragm lifts more and the jet needle lifts more and more fuel flow. That's my understanding of it anyway
I didn't draw it just found it on a google search. I'll explain it a bit more. With the butterfly valve closed the vacuum of the engine is drawing fuel and air through the idle circuit as I explained in the last post. Once you start to open the butterfly valve by opening the throttle the effect of engine vacuum moves past the butterfly valve and moves further back along the carb to the bottom of the slide. See the hole in the bottom of the slide marked SVP in the diagram well the vacuum from the engine goes up through that hole, up the centre of the slide and it acts on the top of the rubber diaphragm. So now there is vacuum acting on the top of the diaphragm. It's not shown in that diagram but there's a slot cut in the body of the carb so that atmospheric pressure acts on the bottom of the rubber diaphragm. The top and the bottom of the diaphragm are sealed from each other so with low pressure acting on top of the diaphragm and relatively high pressure from the atmosphere acting on the bottom of the diaphragm the diaphragm deflects up. As it deflects up it lifts the jet needle out of the main jet. So now the main fuel circuit can come on. Air can now flow through the main air jet (maj), through the holes in the emulsion tube and fuel gets draw up through the centre of the main fuel jet and mixes with the air from the main air jet and the mixture flows past the jet needle and is drawn into the engine. And as you open the throttle more, more vacuum can act on the top of the diaphragm so the diaphragm lifts more and the jet needle lifts more and more fuel flow. That's my understanding of it anyway