Riding lawn mower engines need to be tuned and maintained in order for them to perform at peak level. Although riding lawn mower engines may be viewed as "just a lawn mower engine," riding mower engines are much larger and much more powerful than their push mower counterparts. Because of their increased complexity, they are a little more work to maintain. The biggest single issue facing carburetors is their low idle speed getting out of whack somehow. You will know when your low idle speed needs to be adjusted because your low idle speed either runs too high for the engines design, which wastes a lot of fuel; or the low idle speed runs too low and the engine will not idle without supervision. High speed mixtures are preset at the factory and typically cannot be changed. The low idle speed, however, can be adjusted by playing with the low idle fuel adjusting screw.
Start the engine and allow the engine to warm up to its normal operating temperature (this may take 10 to 20 minutes). Shut the engine off.
Remove the air cleaner cover and the air filter to allow you access to the air cleaner base.
Locate the low idle fuel adjusting screw which is about where the air cleaner element sits and screw it into place (clockwise) until it slightly bottoms out. Be very careful when adjusting this screw as it is machined to tight tolerances and if it is forced, it may be damaged and will have to be replaced before the engine will run properly.
Turn the screw out (counterclockwise) 2 1/4 turns to achieve a preliminary setting. Start the engine and let it run at half-throttle for 5 to 10 minutes to let it warm up again.
Attach a tachometer to a spark plug wire. Turn the throttle control down to the idle or slow speed setting. Then adjust the low idle speed adjustment screw in or out so the low idle speed is at 1200 RPM (+/- 75 RPM) on the tachometer.