Sport_Pilot,
hmmm....I'm talking about idle mixture setting. How do you explain if the engine just slows down and quit without increasing the rpm? Isn't it too lean? It still takes a few seconds. I'm not talking about the high speed mixture, in which case the result of pinching is instant.
Also in this "lean" condition, sudden opening of throttle results in engine quitting. And if this is "very rich", why would engine run better if it's further enriched?
I appreciate and value your comments/advice very much. So please don't take this as combative. I say this because when I read back what I wrote, it does look a little combative...but I don't mean that.
I usually pinch approximately 3/8" to 1/2" before the inlet. I wonder if we are talking about the same thing.
If the main needle is set properly, that is slightly rich, then it will be slightly rich when you slam the thottle down to idle. Thus if the idle mixture is too lean it will speed up before it quits, if it slows down without speeding up before it quits it is too rich. An overly rich idle mixture can also cause the engine to quit when throttled up just not as cleanly, however there is a point slightly leaner from that where it will not quit but the engine will have poor transition but not quit. It does not matter where you pinch the line, before the needle, after, close to the carb, or close to the tank, they will shut down the fuel flow just as fast at any point. There is a small amount of fuel that will drain from the line if you close it far from the carb, however this is small enough that the engine should respond withen a fraction of a second. Unless the idle needle is open way too much, which would be too rich in any event. I think you need to close that needle and observe how it responds to a lean setting.