Pawl the other one
RICK ANSWERS YOUR QUERIES
My mate John Sayer asks why his BSA B40 trail bike kickstart has suddenly stopped working. I would have got him to drop it round, but he’s in France, so we had to do a bit of remote diagnosis.
Most British kickstarts drive one end of the gearbox mainshaft through a ratchet, with the other end attached by splines (or taper and key) to the clutch centre.
Kicking turns the clutch centre and (via the clutch plates) the clutch chain wheel and chain to the engine. With a connection from kickstart to gearbox shaft, shaft to clutch centre, clutch centre to clutch outer, and clutch outer to crankshaft, there are several suspects. So which is John’s problem?
I asked him whether the bike would bump-start in gear? Yes; that suggests there is nothing wrong at the clutch end, like slipping plates or a detached centre.
My next question was what happens if you kick the bike over in gear? Because the kickstart turns the gearbox mainshaft, when in gear it has to turn the output sprocket, too, clutch in or out. John reported that nothing happens, so it looks like the ratchet has failed.
There are two types of ratchet; one is a gear with ratchet teeth on its face, driven by a matching part, splined to the mainshaft. For this type to fail suddenly, I’d guess the retaining nut had come off, allowing the ratchet out of engagement. The other type has ratchet teeth machined inside the layshaft first gear pinion and the kick shaft has a springloaded pawl. Once the pawl’s edge wears, it will slip and wear quickly. John’s bike has this type, so I advised him to order a new pawl and a spring; if that’s broken it would cause the same trouble.