A steer on gears
I like a puzzle – and Chris Taylor’s Yamaha XS650B provided a good one. It’s started ‘shunting’ when changing into first and fourth gear and Chris suspected failed shock springs in the clutch basket. But all was fine there; no loose sprockets and no nasties in the oil strainer – but in-gear and on the stand, he finds the rear wheel has double the backlash in first and fourth as the other gears. “It’s a real head-scratcher and investigating means a complete engine strip – any suggestions?” he pleads.
Well, Chris knows much more about XS650S than I do and has checked the most likely issues – but looking at the xs650.com website, I discovered that broken gearbox dogs are not unknown.
The obvious answer is ‘check for wear and replace worn components’, but come on – it’s a 40 year-old gearbox. It’s worn, but how much wear is too much? The xs650.com site provided some useful info, including a photo showing the gears are laid out 1-4-3-5-2, making 1 and 4 neighbours – coincidence? I then found a parts diagram showing which gears are splined to shafts, which have dogs, etc. By expanding the principle explained in this month’s ‘How to’ (p81), I was able to map out which pinions do what in each gear. That seemed a good way to pinpoint the pinions most likely to be faulty.
The only shared feature of both gears is that drive to the sprocket relies on the splined fit of the fourth gear pinion to the output shaft. It’s an unlikely failure, but at least it’s a start. I think Chris needs to strip it anyway; that ‘shunt’ will put a heavy shock loading through the gearbox – and gearbox failure usually results in a locked wheel and an unscheduled landing.