Throttle Assembly

Throttle

You’ll find that most riders of all levels are very cautious about throttle assembly performance. Walking through the pits during an mx / offroad race it’s not uncommon to find factory mechanics or riders themselves disassembling throttle assemblies for cleaning between races. Having a sticky throttle assembly can be very dangerous for the rider.

Basic Inspection And Cleaning

Start by removing the rubber dust cover and the plastic throttle housing assembly; inspect the throttle pulley for any damage or wear. Next inspect the cable and make sure there are not any frays in it. Pay close attention to dirt or any other foreign matter that could be inside the throttle tube assembly. Examine the other sleeve of the throttle cable for any cracks or damage; replace throttle cable if any issues are found.

Throttle pulleys are made of nylon and don’t require lube, but a light lube doesn’t hurt anything but could attract dirt. The throttle cable does require lube for smooth operation. Remove the carburetor slide from the carb; remove the throttle cable from the slide. Using a cable lubing tool will allow you to force cable lube down inside the cable housing. The throttle pulley, throttle housing need to be cleaned with contact cleaner. Lube the throttle tube and handle bar area with light oil (silicone spray works pretty good).


Carburetor Slide Inspection

Newer slides are chrome plated: you may find that older carburetor slides could be anodized or even Teflon coated. Teflon coated slides could appear to be dull gray in color or even black. When inspecting the carburetor slide you’re looking for wear marks or coating that has be worn away. Most wear found on slides is due to dirt being drawn into the carb throat. Damaged or worn slides can cause the throttle to stick; don’t take any chances replace the damaged or worn slide.


Throttle Tube And Grips

Most bikes come with plastic throttle tubes that are susceptible to damage from a crash and can cause sticky throttle. The ideal throttle tube is the aftermarket aluminum ones. They are two kinds of aluminum throttle tubes; closed end tubes are used mostly for motocross and open end tubes are used for off-road riders. The open end tubes allow the rider to install wrap around hand guards. You’ll find that throttle tubes can have a knurled pattern to help improve the bond to the grip when using grip glue. Most grips have grooves that are designed for safe wire to help hold the grip in place.  

Runaway Throttle

Runaway throttle can cause serious injury or death to a rider. Runaway throttle is a situation where the engine runs wide open uncontrollably. This is often called stuck throttle condition; or you may see it referred to as STC. STC means the engine is running at a high rpm with out any input or control from the throttle. STC can occur in three different forms; auto ignition, mechanical, and air pressure (Air Leak).

Auto ignition STC will occur on two stroke engines that are experiencing pre ignition; which occurs when there’s a point (not the spark plug) in the combustin chamber with a temperature high enough to ignite the fuel/air charge. This type of STC can not be controlled with the engine stop button or the throttle. The carburetors choke is the best thing to use to help kill the engine before catastrophic failure occurs.
Mechanical STC occurs at the throttle assembly; which includes the throttle pulley, cable, carb jet needle, and carburetor slide. Mechanical STC can be caused by crash damage or normal wear and tear of the throttle assembly.

Air pressure (Air Leak) occurs when there is an air leak between the carburetor and cylinder, which includes reeds and intake manifold. A two stroke engine could also have crank case seals or crank case halves leaking.