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Adjusting or Repairing a Carburetor’s Throttle
Often a problem such as a lack of power with an engine is confusing. You look for a complex cause instead of a simple solution. A carburetor’s throttle operation is a good example of this.
Every engine has a means to operate the throttle. With a carburetor, it will be the throttle plate that is opening. A fuel-injected engine will have a throttle body; a diesel uses an injection pump. If the throttle cannot achieve full motion, the engine never produces maximum power.
Checking the throttle movement is quite simple, but it requires two people (one to move the throttle; one to confirm movement). With the engine off, have your helper fully open the throttle. See if the throttle body, carburetor, or injection pump is going to its stop. To confirm this, gently grab the connection to see if you can obtain more travel. If you can, then there is an issue.
The first thing to check is whether the foot pedal movement is being restricted by a mat or rug bunching up under the pedal. Then check for a stretched cable from the throttle to the engine. Over time (from use and thermal cycling), the cable will grow and give up movement.
Any part of the throttle-control system that swings in an arc presents a problem if the pivot point is worn and lateral movement is induced instead of an arc. This could be the pedal assembly or, on a small engine, it could be the control on the handle. If you work with your helper and study the movement, you can see where the deflection is.
Keep in mind that when an engine loses control of the throttle, the degradation of the movement is linear across the range of operation. Thus, as you command movement, the angle of rotation is less than desired. You think you are inducing one third of the throttle, but you’re only moving one quarter of the range.
Over the last few years, the simple but reliable throttle cable has been replaced with a drive-by-wire system. With this system, an electric motor is connected to the throttle butterfly and controlled by a series of sensors under the pedal.
When the throttle is operated, there are two sensors that work on a 5-volt signal, each with the reverse output. This redundancy is used as a safety factor.
The two sensor output voltages are compared by the engine control unit (ECU). It then sends a signal to the electric motor on the throttle plate and moves it the required amount based on an advanced algorithm. Much engineering time was invested to make the throttle feel normal to the driver as a cable would.
Your family car and any gas- or diesel-powered light-duty farm truck uses the ECU electric motor system. To check this device, pop the hood, find the throttle body, and look for a small rotary motor. If such a drive-by-wire engine suddenly loses all power, the rotary motor or its electrical connection is usually the problem.
On a farm truck, keep in mind that dirt under the accelerator pedal can impact the sensors there and cause the same result.