The throttle wire, or throttle rod, in a chainsaw connects the interlocking trigger with the carburettor. When you squeeze the trigger, the throttle rod opens a vent on the carburettor, allowing gas into the mixing chamber; the further you squeeze, the further the vent opens, and the faster the engine runs. This rod can get detached from either the trigger or the carburettor if the saw is dropped, bumped too hard or damaged in some way.
Close the choke lever by moving it to the closed position to prevent dirt from entering the carburettor. Unscrew the air filter's protective cover and lift the filter cover away. Pop off the grommet, if installed on your Stihl model, from the carburettor box.
Unscrew either the two slotted nuts or retaining wing nut, depending on the model of Stihl, to remove the air filter from on top of the carburettor. Unhook the throttle rod (the metal rod that runs from the carburettor to the trigger) from the side of the carburettor.
Unscrew the handle moulding screw from the underside of the rear handle. Lift away the handle moulding cover from the trigger. Move the master control lever to the "RUN" position, which looks like an "I" on the side of the handle. Lift up the throttle interlock lever from its seat inside the handle. Pull the throttle rod out of its seat on the trigger. Remove the old throttle rod.
Push the new throttle rod into the seat on the trigger. Push the throttle interlock lever back down. Move the master control lever back to the "STOP" position. Reattach the handle moulding cover. Fit the screw and tighten against the handle. Push the throttle rod into its seat on the side of the carburettor.
Refit the grommet over the carburettor adjusting screws and push the grommet into place over the screws with your thumb. Refit the air filter and attach the slotted or wing nuts to secure the filter to the carburettor. Reattach the air filter's cover over the air filter.