Engine: Vibrates Excessively

Bearings allow a rotating shaft to turn within a stationary part. When a bearing becomes worn, it will rotate with too much play, and this will lead to vibrations. Crank shaft bearings are extremely durable and will usually only require replacement from a severe lack of maintenance. Replacing crankshaft bearings is a relatively difficult repair, and is usually not recommended for a DIY-er, although the process is more labour intensive on some models than on others. To replace a bearing, start by draining all the oil from the engine. Next, remove the recoil assembly and the flywheel If your your engine is equipped with a starter clutch, remove the starter clutch with a clutch removal tool or a pry bar. Lock the fly wheel in place as you use channel locks to loosen the starter clutch. Using a pry bar, apply a little pressure behind the flywheel and tap the center shaft with a hammer (or air hammer) and remove the flywheel. Remove the crankcase bolts and tap the crankcase off with a rubber mallet if necessary. Then, remove both bolts that hold the connecting rod in with the oil dipper. It’s a good idea to mark both sides of the connecting rod to assure proper re-assembly. Next, remove the bottom part of the connecting rod and pull the crank shaft out. Proceed to remove the bearing using a bearing removal tool. Once you have removed the bearing it’s time to install the new one. An easy way to do this is by placing the crankshaft in a freezer overnight, then 30 minutes before you are ready to go, place the bearing in an oven at 175 to 225 degrees F. Using protective gloves, remove the crankshaft from the freezer and the bearing from the oven, and the bearing should slide over the crank shaft and into place. Next, re-install the crankshaft onto the engine and re-install the bottom part of the connecting rod, lining up the markings with the oil dipper. Clean the joining surfaces where the crank case gaskets are, to ensure a tight seal. Then, re-install the crankcase and tighten the bolts in an alternating pattern (to the manufacturer’s specifications) to prevent warping. Re-install the flywheel with the key way, and re-install the starter clutch if you took one off. Finally, re-install the recoil and add fresh oil.

Guards protect you from hazardous areas of the engine, as well as anything that is attached to the engine. Although a guard will not cause a vibration, a lot of guards vibrate from normal equipment use. This can usually be resolved by tightening the bolts or screws securing them in place.

Although the housing is a stationary part and will not cause a vibration on its own, on some equipment the engine is bolted to the housing. If these bolts are loose, the housing can vibrate. To solve this issue, check the engine mount bolts for tightness and tighten if needed.

The crankshaft rotates inside the engine and is attached to the piston by a connecting rod. If the shaft becomes out of round, it can cause a very serious vibration and may even want to shake itself apart. If the shaft is out of round it will need to be replaced, but keep in mind that certain models may require more labor-intensive repairs than the one we describe here. To replace the shaft, start by draining all the oil from the engine, pulling off the recoil assembly, and removing the flywheel. If the engine is equipped with a starter clutch, either remove it with a clutch removal tool or use a pry bar and lock the flywheel in place as you use channel locks to loosen the starter clutch. Using a pry bar, apply a little pressure behind the flywheel and tap the center shaft with a hammer or an air hammer, and remove the flywheel. Remove the crank case bolts and tap the crankcase off with a rubber mallet (if necessary). Next, remove both bolts that hold the connecting rod in with the oil dipper. It is a good idea to mark both sides of the connecting rod to ensure proper re-assembly. Remove the bottom part of the connecting rod and pull the crankshaft out. It is important at this point to apply a little clean engine oil to lubricate the new crank shaft. Some crank shafts will slide into bushings rather than using a bearing, and this will make the job much easier. If you must install a bearing, an easy way to do so is by placing the crank shaft in a freezer over night, then 30 minutes before you are ready, place the bearing in an oven at 175 to 225 degrees F. Using protective gloves, remove the crankshaft from the freezer and the bearing from the oven and the bearing should slide right over the crank shaft and into place. Next, re-install the crank shaft back into the engine and re-install the bottom part of the connecting rod, being sure to line up the markings with the oil dipper. Clean the joining surfaces where the crank case gaskets sit. Then, re-install the crank case, tightening the bolts in an alternating pattern to the manufacturer’s specifications to prevent warping. Now, re-install the flywheel with the key way. Finally, re-install the starter clutch (if you took one off), re-install the recoil and add fresh oil.

The connecting rod attaches the piston to the crankshaft, and causes the crankshaft to rotate. If the connecting rod is bent, it can cause excessive vibration, and will need to be replaced. To do this, first remove the air filter and the air filter housing. Next, remove the spark plug wire and the spark plug. Remove the muffler and carburetor from the cylinder head, disconnecting the linkages and the fuel line from the carburetor. Remove the engine mounts and the engine from the unit and place it onto a bench. Pull the valve cover off and rotate the piston to the bottom. Next, loosen the rocker arm nuts, slide the rockers over, and remove the push rods. It is important to remember which push rod is which for re-installation. Remove all the bolts holding the cylinder head to the engine block in an alternating pattern, to prevent warping the cylinder head. With the cylinder head off, remove the old head gasket. Clean both surfaces of the engine block and the cylinder head. You do not want to damage this area, as you will need a tight seal during re-installation. Next, remove the crankcase bolts and wiggle the crankcase off. You may have to tap it off lightly with a hammer and punch where ever you can make contact on the case. There may even be a keyway on the crankshaft that will have to be removed as well. This process will vary depending on your model. Next, simply pull the camshaft out and remove it. Remove the camshaft tappets, connecting rod bolts, and the bottom part of the rod bearing. Then, rotate the crankshaft to the top and reach in and push the piston out with your fingers. To remove the piston from the connecting rod, remove the clips with pliers, holding the connecting rod pin in. If there are no clips, then the pin is pressed in and you will have to take it to a machine shop to have it removed. With the clips removed, hold the piston in boiling water to get it to expand for a couple of minutes (do not let the piston touch the bottom of the pot). The piston will expand at a much faster rate than the pin. Insert a plastic punch or something else that is not metal (to prevent damage), and hammer out the pin. Once the pin is out, remove and discard the old connecting rod and install a new one facing the reverse way, and re-install the E-clips on both sides. Next, position the ring gaps at 1/3 intervals around the piston to prevent blow by, and make sure oil ring is on the bottom. Using a piston ring installer to line everything up, gently tap the piston into the cylinder. Lubricate the cylinder with fresh motor oil beforehand, and be sure that all bolts are torqued to the manufacturer’s specifications. With the piston at top dead center, and contacting the crankshaft, rotate together by pushing down on the piston so you can re-attach the connecting rod bearing. Next, lubricate the camshaft tappet, re-install them, and install the camshaft by making sure the timing marks are in line. Be sure to lubricate all moving parts with motor oil or white grease first. Re-install the crank case cover with a new gasket and tighten down to the manufacturer’s specifications. Next, re-install the cylinder head with a new gasket and tighten the bolts in an alternating pattern to the manufacturer’s specifications, to prevent warping of the cylinder head. Then, re-install the connecting rods, and slide the rocker arms back into place. Set the gap between the rocker arm and valve by adjusting the pivot. The gap should be 3 to 5 thousandths of an inch. Tighten the jam nut down while holding the pivot stationary to prevent movement, then re-install the valve cover with a new gasket, and tighten the bolts in an alternating pattern to prevent warping of the cover. Re-install the engine back into the unit, re-attach the carburetor, hook up the linkages, and re-attach the fuel line. Finally, re-install the air filter housing and filter, and install the muffler.