* Sticking throttle cable. The throttle cable is responsible for opening the throttle valve on the carburetor. If the throttle cable is sticking, it will not close completely when you release the throttle, causing the engine to continue to run. To check the throttle cable, try moving it by hand. If it feels stiff or doesn't move smoothly, it is likely sticking and needs to be replaced.
* Fuel mixture too rich. A rich fuel mixture can cause the engine to run rough and stall. It can also cause the engine to continue to run even when you release the throttle. To check the fuel mixture, remove the spark plug and inspect it. If the spark plug is black and oily, the mixture is too rich. If the spark plug is white or light gray, the mixture is too lean. You can adjust the fuel mixture by turning the mixture adjustment screw on the carburetor. Turning the screw clockwise will make the mixture leaner, while turning it counterclockwise will make the mixture richer.
* Air filter clogged. A clogged air filter can restrict the flow of air into the engine. This can cause the engine to run lean, which can lead to the engine continuing to run even when you release the throttle. To check the air filter, remove it and inspect it for dirt and debris. If the filter is dirty, replace it.
* Vacuum leak. A vacuum leak can cause the engine to run lean, which can lead to the engine continuing to run even when you release the throttle. To check for a vacuum leak, spray some carburetor cleaner around the intake manifold and carburetor. If the engine RPMs increase when you spray the carburetor cleaner, there is a vacuum leak. To fix a vacuum leak, replace the damaged gasket or O-ring.
