Check the fuel level to make sure the trencher's gas tank is filled up and, if it is not, fill it with gasoline and then try to start it. Also, make sure the fuel shut-off valve is open when trying to start the trencher.
Check to make sure the throttle lever linkage is firmly attached to the carburetor control arm. If it is not, slip the linkage through the hole in the control arm with a pair of pliers.
Check the engine oil level by removing the dipstick, wiping it clean and then inserting it again and removing it. If the oil is low, add oil to the tank.
Check the spark plug and replace it with a fresh plug if the old one looks dirty or has any carbon scoring on it. Use a standard spark plug wrench to remove the old plug and install the new one.
Lubricate the chain with chain oil if the chain runs noisier than usual or vibrates excessively. Access the chain through the hole at the front of the engine.
Check the air filter elements if the trencher runs rough or emits smoke. The foam element can be removed by hand and washed with a high flash-point cleaning solvent. Remove the paper filter by hand and replace it with a fresh one if the old one is dirty.