It sounds like what you are describing is a condition often called "run on". It can have several specific causes, but in general is a result of excessive cylinder temperature that causes spontaneous ignition. It is often accompanied by pinging ( sometimes described as sounding like marbles in the cylinder). A common cause is an overly lean fuel mixture, but it can also be the result of heavy carbon build up.
What do the spark plugs look like when you pull them out?
Does the engine use oil?
How many miles on the bike?
Any modifications?
Compression numbers?
The more information you provide the more likely you are to get useful feedback. There are some extremely knowledgeable, and helpful, people here, but they need information to help you.