F15 low speed circuit throttle screw

[Sam]

Looking at a SELCO diagram oN how the different fuel circuits work (P3-13 1995-04 manual), the throttle plate seems to be completely closed at the low speed circuit. If this is the case then it the throttle screw could be all the way out - not touch the at all. This is not the case on my motor where I do need a little opening of the throttle to run at all. The question is how do you know if the low speed circuit is working or if you are actually using the mid range circuit?

Yamaha 9.9 High Thrust 323 cc

[boats.net]

The way you would know if working is if the engine runs ok at low speed and will accelerate, if doesn't carb need to be cleaned. the trottle plate opening is control by idle speed screw and throttle lever(or grip if tiller handle model).