It sounds to me like a governor spring has broken or is very weak. We need a full model number. Meanwhile, here is a picture of a governor spring and linkage. See if it's like yours.

Look at the tab where the spring hooks on the right side. Bend the tab to the left to decrease the high speed or to the right to increase it. Adjusting the engine high speed without the use of a tachometer can result in engine failure.

{{gwi:128760}}

Have u tried cleaning the carburator. Recently I had the same problem, i thought it was the governor spring. However after cleaning the carburator and reassembling. All is fine. I mowed the lawn recently and the mower fired up on first pull. You tube has some nice illustrations on how to clean the carburator.

Good Luck