Trimmer: Engine Rotates But Trimmer Head Doesn't

The most common issue with gear boxes, aka gear cases, is the failure of the teeth on the gears themselves. This can happen over time due to normal wear or can happen quickly if the trimmer is overloaded or abused. A lack of lubrication doesn’t help the gear box either. If the shaft spins but the head does not, or if you hear a grinding noise coming from the gear box area, then you need a new gear box. To remove the gear box, remove any covers on the head that stop debris from flying if they are in the way. Insert a screwdriver into the hole on the gear box located just above the trimmer head. Rotate the trimmer head until the screwdriver locks into place. On some models, you might have to lock the screwdriver in place against the washer inside after it is locked inside the hole. Then remove the trimmer head by rotating it counter clockwise. Trimmer heads are reversed-thread. Unbolt the gear box and remove it. Install the new gear box and rotate the spindle until it lines up with the inner shaft, then tighten down the bolts. Re-install the flange and the trimmer head, keeping in mind the threads are reversed.

The drive shaft housing supports the drive shaft and connects the trimmer head housing to the engine housing. If the drive shaft housing becomes damaged and applies pressure on the drive shaft inside the housing, it can keep the drive shaft from spinning. The fastest and easiest fix for this is to replace the drive shaft unit. Detach the drive shaft from the engine by removing the screws attaching the unit to the engine. If needed, detach the throttle and ignition cables and pull apart. Remove the shaft from the gear box housing by unbolting it and pulling it apart. Transfer any handles, guards, or anything else off the old drive shaft housing to the new drive shaft housing. Install the new drive shaft housing assembly by re-attaching it to both ends and tightening down the bolts and screws and re-attaching the ignition wires and throttle cable.

The shaft connects the crankshaft from the engine to the trimmer head. The crankshaft attaches to the drive shaft and connects to a gear box, which allows the head of the trimmer to spin. If the drive shaft is broken or detached, the trimmer head will not spin. To inspect the drive shaft, remove the engine assembly from the shaft and look to see if the shaft is in its proper place. Re-install the shaft if it has slipped from its place. If the shaft is damaged or broken, it must be replaced. Separate the drive shaft housing from the engine housing. You may have to disconnect the ignition wires and throttle cable on some models. With the shaft housing separated from the unit, remove the drive shaft with a pair of needle-nose pliers. Lubricate the new shaft from top to bottom with some grease and re-install the shaft into the housing. Rotate the shaft around till you feel it lock into place in the trimmer head coupler. (This is a trial and error process, and can take some time.) Once the shaft locks into place, the end of the shaft should be flush with the shaft housing. Re-assemble the engine assembly and drive shaft assembly back together. Re-attach the ignition wires and throttle cables.

The clutch prompts the drive shaft to start rotating once a certain speed is achieved and allows the engine to run without always rotating the drive shaft. If the clutch is worn out or the springs are not tensioned properly, the clutch will have issues engaging. To replace the clutch, separate the drive shaft from the engine. Remove any engine covers, allowing you to access the clutch and the spark plug. Disconnect the ignition wires and throttle cable if needed. Remove the spark plug boot and spark plug, and insert a small piece of rope inside the cylinder to prevent the piston from moving as you remove the clutch. Remember to keep a small portion of the rope out so you can remove it later. Use a clutch removal tool to un-thread the old clutch. Install the new clutch by threading it onto the shaft and tighten it down. Remove the rope from the cylinder and re-install the spark plug and boot. Re-install any engine covers and re-attach the drive shaft to the engine housing.