The Sprag Clutch and the Starter Motor

If you're trying to start your motorcycle with the electric starter and the starter motor is spinning but it looks like it not 'catch' the engine. So your motorcycle engine will not start, the problem is likely the sprag clutch.

A sprag clutch is a mechanism that only allows rotation in one direction between two concentric components. Most electric-start motorcycle engines use a sprag somewhere between the starter and the crankshaft, and even the slipper clutches that are being incorporated on a growing number of sporty motorcycles use a variation on the sprag clutch concept.

Sprag clutches generally look kind of like roller bearings, with an outer 'race' built onto one of the two affected components (the crankshaft, for instance, in an electric-starter system) and an inner race connected to the other (such as the electric-starter motor). Instead of round rollers, seen in most classic roller-bearings, most sprag clutches these days have dogbone- or figure-eight-shaped elements held in place by light springs. The elements fit between the races at a slight angle so that when the outer race is turning and the inner race is not (which is the case when the engine is running), the tilt of the elements lets them slip harmlessly across both races surfaces, having no effect on the inner race.
But when the inner race is turning and the outer one is not (such as when the engine is stopped and the starter motor engages), the spring-loaded elements 'stand-up' and wedge themselves between the races. This makes both races turn at the same speed until the rpm of the engine (outer race) exceeds the rpm of the electric-starter motor.

Today I found that something on a Yamaha motorcycle is causing the elements to skid along both races rather than wedging between them when the electric-starter motor is engaged. The spring that hold the elements is place could have broken, or perhaps the sprag assembly is in the process of disassembling itself. Whatever the problem, we will need to either repaired or replaced the sprag clutch.
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