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Barrette's Small Engines Inc. - SmallEngines.ca FAQ's
Why does my starter rope jerk backwards when I attempt to pull it?
The above question is a common inquiry resulting from a blade which may have suffered an impact and may have caused damage to the engine crankshaft.
Engine crankshafts are often damaged when rotation of an engine is abruptly impeded by a stationary object. Rocks, metal survey markers, chunks of wood, insect mounds, tree roots, toys (as well as other debris that should not be in the work area) are common objects that can cause this failure. Crankshafts are manufactured from hardened steel. Once they are damaged, there is no approved repair available. Damaged crankshafts need to be replaced.
The most common form of this service is what is referred to as "short blocking" the engine. Since the crankshafts and bearings have to be assembled using specific processes to ensure proper fit and operation. It is generally less expensive to purchase the crankshaft and block assembly (known in the industry as a short block) and install this new assembly.
Since this type of failure only occurs during operation, prevention of this type of engine failure is the operators responsibility. As a result, every major engine manufacturer denies warranty repairs to any crankshaft damage created by impact as explained above.
Every engine is thoroughly factory tested. If the engine ran properly at testing, then the crankshaft was not damaged or defective when purchased new.