Little else brings joy to a rider on a dirt bike than the green light to pin it. Little else terrifies a rider on a dirt bike when that pin gets stuck.
A stuck throttle causes all sorts of havoc to rider and machine because it's unpredictable and when it happens, panic usually sets in. The first thing most riders do is try to unstick the throttle in order to retain control. Your window of opportunity is small here because you're gaining speed - fast. If you get the throttle loose again you can power down quite easily.
Otherwise, options are limited.
What Causes a Stuck Throttle?
Most commonly, a stuck throttle results from debris getting caught between the throttle tube and handlebar that makes it sticky. This debris can be from a past ride you failed to remove or even during the current ride from roost or from a crash.
Another common cause is when the throttle cable gets stuck in the open position and doesn't release when you let go of the throttle. When this happens the grip throttle moves back into position causing you to believe you've let up on the gas but the stuck cable essentially keeps it "pinned." In a carbureted dirt bike, a dirty carburetor can also cause a stuck throttle.
If you've recently put new grips on, excessive glue can seep between the throttle tube and handlebar which can cause stuck throttle. However, this is more likely to delay your riding day rather than present as a problem later.
Finally, a less common but a most alarming reason is when you get arm pump. A MotoSport employee once crashed through the whoops after he rolled the throttle and arm pump essentially paralyzed his arm in place. He had little choice but to hang on for dear life until the bike huckabucked and he crashed.
How to Prevent Stuck Throttle
The best way to prevent a stuck throttle is maintenance. The pre-ride check should always include looking for debris between the handlebars and throttle tube and ensuring the throttle grip snaps back when rolled.
Inspect the throttle cable that it's routed properly and doesn't get pinched by the handlebars turning motion, and not intertwined with the brake or clutch cables and electric start cable, if applicable. Lube the throttle cable regularly which prevents it from getting stuck in the throttle housing and aging prematurely. A damaged throttle cable is also a recipe for throttle problems. If you're running a carburetor make sure it's clean.
Just to be safe, it's always a good idea to have a spare cable on hand to replace a worn one or if one breaks while out riding.
As for arm pump, most riders know if it's becoming an issue and stop riding. It's not worth it. If arm pump is routinely an issue, then take the appropriate steps to prevent it from happening either through exercise, nutrition or seeing a doctor. A number of riders dealing with arm pump have found help using the HemoFlo supplement.
What to do When Riding With a Stuck Throttle
OK, back to handling the stuck throttle situation. Obviously if you can't correct the problem in the moment you need to revert to Plan B. First grab the clutch, pull it in and apply the brakes. Shut the engine off. This method is equivalent to putting a car in neutral and stepping on the brakes. This should take care of the problem every time. Of course, this doesn't stop the bike in its tracks therefore engage Plan C if you're headed towards a cliff or immobile object like a tree that's going to inflict severe pain if you're still seated when the bike hits it.
In all seriousness, stuck throttle happens but it's not that common and can be avoided entirely with proper maintenance and a pre-ride check. Most riders can tell immediately that the throttle is sticking prior to riding and clean out the debris under the throttle tube. General maintenance keeps the carburetor clean, the throttle lubricated and alerts you to address any fraying or rust.