The carburetor on a Ryobi 31CC lawn trimmer comes properly set from the factory. Over time, as the trimmer ages, the carburetor may need a few minor adjustments to achieve its original factory settings. Even so, over-adjustment of the carburetor can cause serious and permanent engine damage. Leave all major carburetor adjustments to a service professional.
The Carburetor Screws
When the throttle opens the intake valve on the carburetor, fuel flows into the carburetor in an amount that matches the engine's speed. If the valve opens too far or not far enough, the engine will get either not enough or too much gas, and performance will suffer as a result. Two screws regulate these settings. The idle screw regulates the idle and low-speed amount of fuel flowing into the carburetor. The high-speed screw regulates the amount of fuel flowing into the carburetor at its wide-open position.
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To achieve the proper speed, the high- and low-speed screws need proper setting. This can only accurately be done with a tachometer, which counts the amount of sparks and measures the rpms of the engine. The idle speed for a 31 CC Ryobi trimmer needs to run at a speed of 2,800 to 3,300 rpm. The high speed, or wide-open throttle, for the trimmer needs to run at a speed of 7,000 to 8,300 rpm.
Before Adjusting Carburetor
Several common occurrences can quickly prevent the trimmer from operating in the normal ranges of speeds. Before the carburetor can be reset, the operator should check and make sure the air filter is thoroughly cleaned or replaced if it's too dirty. The operator will also need to dump out any old fuel left over in the tank and mix up a fresh batch, using a 50-to-1 ratio of gasoline to two-cycle engine oil. This amount will ensure the carburetor can suck in air and fuel at the proper rate.
Setting the Carburetor
If the speed falls above or below these specifications, the trimmer will not operate properly, and the engine may become damaged. The operator will need to adjust the high- and low-speed screws clockwise to increase speed or counterclockwise to decrease speed. Since damage can occur quickly if done improperly, the operator will need to use a tachometer. The carburetor screws are usually located on the side of the carburetor nearest the fuel tank. A small screwdriver will be required to adjust these screws without damaging them.