EricGorr
Super Power AssClown
- Aug 24, 2000
- 708
- 1
Safety Tips To Preventing “Stuck Throttle Condition STC”
By Eric Gorr
It’s a riding situation that most riders have experienced. The lack of control that makes riders panic when the engine runs wide open uncontrollably. This is often called “stuck throttle condition” or STC.
Recently Racer X online republished a news item http://www.racerxill.com/071502_tribune.cfm
originally printed in the Chicago Tribune. The article stated that a man had sued and won judgment against Yamaha regarding the injury he suffered on a 1996 YZ125. Since then I’ve received a lot of email, especially from kids concerned about the hazards of STC. Here are some notes that I put together on this important safety topic. Feel free to republish this post, or link to this article, for your local motorcycle web site or printed newsletter. Just please reference that it originated on www.dirtrider.net
Stuck Throttle Condition STC
STC means that the engine runs at high rpm without control from the throttle. STC can occur in three different forms; Mechanical, Air Pressure, and Auto-ignition
Mechanical
Mechanical STC occurs in the throttle grip, pulley, cable, carb slide, and or carb jet needle. Mechanical STC happens when crash damage or normal wear and tear cause one or more of these parts to stick in one position.
Examples: The bike falls on the right side and crushes the grip end causing it to bind on the handle bar. The cable wears and frays causing it to stick in the cable housing. The carb’s throttle slide and needle are exposed to dirt that passes by the air fliter, and gets wedged making the slide and needle inoperable.
Air-Pressure (Two-stroke engine only)
Air-Pressure STC occurs when there is an air pressure leak somewhere between the carburetor and the cylinder, and that includes the reed valve & manifold. crankcase, and crankshaft seals. The crankcase of a two-stroke engine is sealed and works as a pump. Leaks can occur at places like the rubber intake manifold and gasket surface, dry rotted cracks in the rubber, cracks in the reed valve or cylinder, gasket leaks at the reed valve and cylinder base, cracks or holes in the crankcase, and worn crankshaft seals.
Air leaking in can cause the engine to run lean and get hot. This is a common cause of piston seizures. Here are some other examples.
Examples: The engine idles erratic; rpm won’t fall to idle when the throttle is shut, engine bogs badly when accelerating abruptly, and the engine pings in mid range and pops on top end. Another common cause is when the engine runs out of fuel, causing a momentary lean condition followed by the engine shutting off.
Auto-Ignition
Auto-Ignition STC can occur on an engine that suffers from pre-ignition. That means a point (other that the spark plug) in the combustion chamber with a temperature high enough to ignite fuel air mixture. Once this STC starts it can’t be controlled with the throttle or kill-button.
Example: Air-pressure leaks can lead to auto-ignition STC. Low crankcase pressure causes the engine to run lean and hot. Raw fuel gets trapped in the bottom of the crankcase. If the piston crown hits the melting temperature of 1100F it could form an ignition source and fuel gets drawn up from the crankcases and the engine revs until a catastrophic mechanical failure stops the engine.
Why the Kill-Button Won’t Work?
If you’ve ever experienced a bike with STC your first reaction may have been to push the kill-button, and perhaps it worked. But were you ever surprised when it had no affect? Its possible and here’s why. The kill-button interrupts the ground to the igniter box. But when the engine is revving so high there is significant energy flowing through the box and electricity will arc to the point of least resistance. A common cause of an alternate ground path is when anti-freeze is spilled around the igniter box to the frame. The filler cap and overflow vent tube are located close to the igniter box on most bike. It’s easy for overspill to splash the initer box and frame.
Ways to Kill an Engine
When an engine suffers from STC, there are three ways to kill the engine with spark, fuel, and load. The kill button should end the spark but if it doesn’t work then the next easiest thing to do is lock up the rear and front brakes in order to apply a load to the engine. The third method works on engines with air-pressure and auto-ignition STC, engage the carb’s choke mechanism. Its like the song by David Bowie says, “putting out fire with gasoline”. A rich mixture will saturate the combustion chamber and lower the temperature, and reduce the available space for air to mix with in order to sustain ignition.
Comments on the Yamaha Case
In the recent case where a judgment was made against Yamaha on a 1996 YZ125, the plaintiff claimed that the throttle stuck due to peeling of the hard chrome coating on the carb’s throttle slide. This is a mechanical STC with a slide damaged from abrasives causing seizure to the carb’s throttle slide bore. The plaintiff claimed that it was an adhesion failure of the hard chrome, which violated Japanese industrial standards.
If you think about it, there are two possible ways that the hard chrome plating can suffer from peeling adhesion. Peeling is a shearing action that causes an adhesion failure of a coating. Two ways to for the plating to peel are from abrasive media like dirt passing through the filter and a faulty pre-treatment during the electrolytic plating process.
Regarding this case and the problem of STC, the lesson for us is that the throttle responded in a certain way prior to the crash. If you can distinguish the three different STCs and their symptoms, you can possibly avoid a crash by shutting your bike off and fixing the problem.
Tips for Checking Your Bike’s Throttle Slide
If you’re concerned that the throttle slide in your bike may be sticking, check it like this. Remove the throttle slide from the top of the carb. Visually examine it for dirt or sand. This is the most common cause of a stuck slide. Look for vertical scratches in the slide face. Those scratches are wear patterns from dirt that became wedged between the slide and slide bore. To examine the slide for chrome peeling, look for jagged spots of discoloration in the surface of the slide. When the chrome chips, the base metal aluminum is more gray and dull than the chrome finish. Some slides are coated with a dry film lubricant that is black in color. When the color changes from black to gray, that indicates wear. If any slide has peeling from dirt of chrome damage, replace it rather that try to repair it. Also take care to polish and clean the carb’s throttle slide bore. If the new slide still binds or rattles with too much clearance, then replace the carb too.
Conclusion
If you have any questions about STC or you want to correct, clarify, or comment on anything in this article, please post it under this thread on DRN.
By Eric Gorr
It’s a riding situation that most riders have experienced. The lack of control that makes riders panic when the engine runs wide open uncontrollably. This is often called “stuck throttle condition” or STC.
Recently Racer X online republished a news item http://www.racerxill.com/071502_tribune.cfm
originally printed in the Chicago Tribune. The article stated that a man had sued and won judgment against Yamaha regarding the injury he suffered on a 1996 YZ125. Since then I’ve received a lot of email, especially from kids concerned about the hazards of STC. Here are some notes that I put together on this important safety topic. Feel free to republish this post, or link to this article, for your local motorcycle web site or printed newsletter. Just please reference that it originated on www.dirtrider.net
Stuck Throttle Condition STC
STC means that the engine runs at high rpm without control from the throttle. STC can occur in three different forms; Mechanical, Air Pressure, and Auto-ignition
Mechanical
Mechanical STC occurs in the throttle grip, pulley, cable, carb slide, and or carb jet needle. Mechanical STC happens when crash damage or normal wear and tear cause one or more of these parts to stick in one position.
Examples: The bike falls on the right side and crushes the grip end causing it to bind on the handle bar. The cable wears and frays causing it to stick in the cable housing. The carb’s throttle slide and needle are exposed to dirt that passes by the air fliter, and gets wedged making the slide and needle inoperable.
Air-Pressure (Two-stroke engine only)
Air-Pressure STC occurs when there is an air pressure leak somewhere between the carburetor and the cylinder, and that includes the reed valve & manifold. crankcase, and crankshaft seals. The crankcase of a two-stroke engine is sealed and works as a pump. Leaks can occur at places like the rubber intake manifold and gasket surface, dry rotted cracks in the rubber, cracks in the reed valve or cylinder, gasket leaks at the reed valve and cylinder base, cracks or holes in the crankcase, and worn crankshaft seals.
Air leaking in can cause the engine to run lean and get hot. This is a common cause of piston seizures. Here are some other examples.
Examples: The engine idles erratic; rpm won’t fall to idle when the throttle is shut, engine bogs badly when accelerating abruptly, and the engine pings in mid range and pops on top end. Another common cause is when the engine runs out of fuel, causing a momentary lean condition followed by the engine shutting off.
Auto-Ignition
Auto-Ignition STC can occur on an engine that suffers from pre-ignition. That means a point (other that the spark plug) in the combustion chamber with a temperature high enough to ignite fuel air mixture. Once this STC starts it can’t be controlled with the throttle or kill-button.
Example: Air-pressure leaks can lead to auto-ignition STC. Low crankcase pressure causes the engine to run lean and hot. Raw fuel gets trapped in the bottom of the crankcase. If the piston crown hits the melting temperature of 1100F it could form an ignition source and fuel gets drawn up from the crankcases and the engine revs until a catastrophic mechanical failure stops the engine.
Why the Kill-Button Won’t Work?
If you’ve ever experienced a bike with STC your first reaction may have been to push the kill-button, and perhaps it worked. But were you ever surprised when it had no affect? Its possible and here’s why. The kill-button interrupts the ground to the igniter box. But when the engine is revving so high there is significant energy flowing through the box and electricity will arc to the point of least resistance. A common cause of an alternate ground path is when anti-freeze is spilled around the igniter box to the frame. The filler cap and overflow vent tube are located close to the igniter box on most bike. It’s easy for overspill to splash the initer box and frame.
Ways to Kill an Engine
When an engine suffers from STC, there are three ways to kill the engine with spark, fuel, and load. The kill button should end the spark but if it doesn’t work then the next easiest thing to do is lock up the rear and front brakes in order to apply a load to the engine. The third method works on engines with air-pressure and auto-ignition STC, engage the carb’s choke mechanism. Its like the song by David Bowie says, “putting out fire with gasoline”. A rich mixture will saturate the combustion chamber and lower the temperature, and reduce the available space for air to mix with in order to sustain ignition.
Comments on the Yamaha Case
In the recent case where a judgment was made against Yamaha on a 1996 YZ125, the plaintiff claimed that the throttle stuck due to peeling of the hard chrome coating on the carb’s throttle slide. This is a mechanical STC with a slide damaged from abrasives causing seizure to the carb’s throttle slide bore. The plaintiff claimed that it was an adhesion failure of the hard chrome, which violated Japanese industrial standards.
If you think about it, there are two possible ways that the hard chrome plating can suffer from peeling adhesion. Peeling is a shearing action that causes an adhesion failure of a coating. Two ways to for the plating to peel are from abrasive media like dirt passing through the filter and a faulty pre-treatment during the electrolytic plating process.
Regarding this case and the problem of STC, the lesson for us is that the throttle responded in a certain way prior to the crash. If you can distinguish the three different STCs and their symptoms, you can possibly avoid a crash by shutting your bike off and fixing the problem.
Tips for Checking Your Bike’s Throttle Slide
If you’re concerned that the throttle slide in your bike may be sticking, check it like this. Remove the throttle slide from the top of the carb. Visually examine it for dirt or sand. This is the most common cause of a stuck slide. Look for vertical scratches in the slide face. Those scratches are wear patterns from dirt that became wedged between the slide and slide bore. To examine the slide for chrome peeling, look for jagged spots of discoloration in the surface of the slide. When the chrome chips, the base metal aluminum is more gray and dull than the chrome finish. Some slides are coated with a dry film lubricant that is black in color. When the color changes from black to gray, that indicates wear. If any slide has peeling from dirt of chrome damage, replace it rather that try to repair it. Also take care to polish and clean the carb’s throttle slide bore. If the new slide still binds or rattles with too much clearance, then replace the carb too.
Conclusion
If you have any questions about STC or you want to correct, clarify, or comment on anything in this article, please post it under this thread on DRN.
