The MS 180 saws have a Zama BIS (balanced idle system) carburetor. The idle speed is controlled by the “LD” adjustment screw that is an air bleed. It is not a traditional idle speed screw that opens the throttle plate, but bleeds air into the venturi. Don’t overthink this, it’s a really simple system that just works well. Not high performance, not a fine-tunable carb... just a simple carb for a basic saw.
If your saw won’t idle down by turning the LD screw counterclockwise, it probably has an air leak. A pressure and vacuum test will confirm or deny this problem. If it has an air leak, fix it. The 180 saws (1130 series) aren’t problematic at all, and are a dependable platform. One common issue is the plastic ring seal between the carb and the intake manifold. I’ve seen more than a few of them installed cockeyed and leaking air.
The 180 is a great little saw with few problems. Take the time to accurately diagnose this problem and keep it running for years to come.
