How to Troubleshoot a Ryobi 31cc String Trimmer

With the development of its+ system of battery-powered tools, Ryobi includes de-emphasized gasoline motors in its lineup of series trimmers — and the 2-cycle, 31cc versions are vestiges of their past. Contemporary Ryobi gas trimmers have 4-cycle engines with a maximum displacement of 30cc. You can still purchase used 31cc versions, though. They share several attributes with newer Ryobi trimmers, as well as models from other producers, and troubleshooting processes are alike.

Assess the Gas/Oil Mixture

The Ryobi 705r and 725r trimmers, the two versions with 31cc motors, require a 32-to-1 gas-to-oil combination, which is more oil-rich than that for most newer 2-cycle engines. You may not notice some problems while running the trimmer during the initial two tankfuls of the 40- or 50-to-1 oil mix most modern motors require. But eventually, you might start hearing rough engine noises, including scraping and grinding from badly lubricated metal parts. When you start hearing these sounds, stop the engine immediately, or it might seize. Drain the fuel and then replace it with the appropriate mixture, which you make by adding 4 ounces of 2-cycle engine oil to every gallon of gasoline.

Starting Procedure

The 705r and 725r trimmers possess an on-off switch that you need to set to the “On” position before starting the engine. In addition, you should set the choke to “Complete,” pump the primer bulb five to seven days and then engage the throttle before pulling the starting cord. If you are having trouble starting the engine, it might be because you didn’t follow this process. After five pulls, then you should set the choke to “Partial” and pull up to three additional times. If the engine won’t start, and you smell gas, then it might be flooded. In that case, turn the choke to the “Off” position; depress the throttle, and keep pulling. The engine should start.

Hard Starting and Poor Performance

Remove and clean the air filter after every 10 hours of service. If you do not, airflow to the carburetor is limited; combustion is partial, and the engine may smoke, run badly or fail to start. You need to set the choke to the “Partial” position before unscrewing and removing the air filter cover. Clean the filter by squeezing the dirt out at a bowl of soapy water. Let it dry, and then gently coat it with SAE 30 motor oil. If the engine has been running or smoking poorly, you should also remove the spark plug and replace or clean it. The spark plug gap should be 0.020 inches.

Engine Stalls While Cutting

If the engine stalls while you are using it, it might be because weeds and grass have collected round the trimmer head. Remove the spark plug and clear the debris. Stalling might also be linked to fuel grade; in case you are likely to use the trimmer after a long period of storage, then you should always replace and replace the old fuel. It’s also likely that the carburetor needs alteration. To fix the idle speed, then start the engine; insert a small flat-head screwdriver into the hole just above the air filter cover, and then turn the screw clockwise in 1/8-turn increments until the engine idles smoothly. The trimmer head shouldn’t spin while the engine is idling — if it does, then back off the screw until it ceases.

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