It sounds like a screaming demon and if you're sitting on your dirt bike don't put it in gear.
A stuck throttle doesn't occur often and usually happens when you're already in motion. That requires some quick thinking to prevent a well-talked about crash. Sometimes though, the throttle sticks upon start-up.
If the bike is on the stand and you've simply started the engine for a warm-up after routine maintenance it might scare the heck out of you but no harm no foul. Simply turn the engine off and figure out a solution. Same rule applies if you're sitting on the bike for the start of ride day. Kill the engine and take a deep breath, you're not going anywhere despite the engine's eager longing to hang it all out.
So what happened?
A number of things cause a stuck throttle while riding as we discussed for "Stuck Throttle! Now What?" but what would cause the throttle to stick from the start? It worked just fine last time out, right?
Throttle Cable Issues
A damaged throttle cable, perhaps from the crash that ended your day early a few weeks ago, causes throttles to stick. So does a frayed or damaged cable. In these instances, it was probably a matter of time so instead of the throttle sticking through the whoops you caught it on the stand or on start-up.If you recently replaced the throttle cable it's possible it was routed incorrectly. By the way, it's probably a good idea to always start your bike on the stand after intake or ignition work.
Throttle Slide Stuck in Carb
On 2-strokes and non-fuel injected 4-strokes, the slide in the carb sometimes sticks resulting in a stuck throttle even though the actual throttle moves freely. Rolling the throttle opens the slide in the carb that allows fuel in. If it sticks it's essentially the same action as if you pinned the throttle. Not much you can do here except kill the engine and address the problem.
As for unsticking the stuck slide, remove it and clean with carburetor cleaner or similar type solvent. Slides usually remove pretty easily without disassembling the entire carburetor by a cap or screws at the top of the carb that when loosened allows removal of the slide. In the future, replace the air filter more frequently. A sticky slide is often the result of poor air filter maintenance.
For additional help on carb related problems check out "A Simple Guide to Jetting Your Carb" and "How to Adjust Float Height."
This is the hardest to figure out because an air leak anywhere allowing more air into the engine causes a lean out and consequentially a rev out. The air boot, engine case and, on 2-strokes, the reed valves all pose a potential problem for air leaks.
Air leaks like hiding so unless it's a large hole don't expect to find it simply by visual inspection. (If it is a large hole you probably won't have an issue with a pinned engine on start-up.) Therefore, grab a bottle of carb cleaner or brake cleaner and spray in the area of the potential leak. The other option is to work from the top at the air box to the air boot and work your way down through the various gaskets to a damaged head. Issues with the following all contribute to air leaks:
- Unseated air filter
- Broken or cracked reeds (2-stroke)
- Hose clamps
- Damaged hoses
- Loose spark plug
- Carb insulator
- Bad gaskets
- Bad crank seal
- Cracked or damaged head, cylinder or crank case
Finding the vacuum leak works similar to the soap and water test when locating a pinhole leak in an inner tube except in reverse. The carb cleaner gets sucked in to the engine and you'll notice an obvious change in RPMs or the engine chokes a bit. When that happens, you've found the leak.
Finally, it's always possible a bit of debris remained between the handlebars and throttle tube from the last ride which lodged into an unfortunate spot when you rolled the throttle during the pre-ride check but unlikely. However, it's worth a check.