Ryobi string trimmers run on a single-piston combustion engine. Pressure within the engine is a necessary part of any piston-run machinery. When your Ryobi string trimmer's compression readings are low, the trimmer's engine will fail to start or will cut out during trimming.
A Ryobi string trimmer's two-cycle engine in good working order should have a compression gauge reading between 90 and 110 pounds per square inch. A cool engine's reading is close to 90 while a warmed up engine has a higher reading. Any number below that range indicates a loss of compression, a problem generally caused by air leaks. Leaks can occur along the crankcase, the piston seals or the intake manifold. A pressure below 70 psi usually is the result of a catastrophic leak, one that is probably more expensive and time-consuming to repair than the cost and time necessary to buy a new string trimmer.
Compression is measured much like tire pressure, with a gauge. Turn off the string trimmer's ignition switch, and remove the trimmer's spark plug by using a ratchet. Insert a compression gauge in the spark plug's place. An adapter may be needed to make the gauge fit into place, depending on the gauge used. The seal between the gauge's O-ring and the trimmer should be tight. Ensure that the ignition switch is still off, and open the throttle. Pull the starter rope a few times to get an accurate reading, and continue to pull the rope until the compression gauge stops moving.
Combustion Engine Basics
All kinds of combustion engines function the same basic way. A piston moves up and down in a shaft. That movement drives a crankshaft, which in turn creates vacuum pressure. The vacuum draws fuel into a carburetor, which spins a flywheel and trips the ignition. The ignition fires, and the electrical charge produced causes combustion, or the burning of fuel that creates heat and energy.
Two Cycles, One Piston
Like most basic yard tools, a string trimmer has a two-stroke, or two-cycle, engine. During the piston's first stroke, fuel compresses on one end of the piston, and previously compressed gases are drawn to the other side of the piston. The piston's second stroke draws a charge on one side while releasing burned gases on the other side. Compression is the amount of pressure needed for this process to function properly.
- Jupiterimages/liquidlibrary/Getty Images