it will affect the rpms to a point, but u dont use them to udjust the idle, just to smooth it out, yes, 2 1/2 turns out is the best starting point for most carbs...but, its mostly only gonna be smooth when the engine is warm...thats why auto chokes where invented, cause its almost impossible to make a carbed motor run smooth when cold without a choke or throttle control to keep the rpms over 1200 until its warm...plus when cold carbs tend to "ice over" because the fast moving air freezes any water vapor inside the idle circuits of the carb, hence the reason for choke stoves (tube coming from the exhaust manifold) on stock air cleaners...at our shop the way we remedy rough running motors is to ditch the holleys, edelbrocks, carters and stock carbs, and replace them with demons.....demons need a little tuning after install, and then they run great with improved throttle response and power...but even those carbs run rough until the motor is warmed up...EFI engines have an advantage and run smoother cold because they adjust accordingly to what the sensors tell the computer...