Sprint 1050, Banging Noise While Accelerating

Hoping someone can shed a little light on this one. I've found many vaguely similar references on threads on forums etc but nothing exactly like this.

From around 100 miles ago my 2010 Sprint ST started making noises while accelerating which sound for all the world like chain slap. It initially started on my commute home from work, lasted the 10 miles to home but then disappeared for a couple of short journeys. It's now started again and on my last ride returning from Wales I doubted it would make it home. I've not ridden it since.

So as best as I can describe it.... Pulling away and accelerating up through the gears there is a loud and fairly regular heavy mechanical clank clank clank sound which I'd say was from the chain/gearbox area (impossible to be sure though whilst riding!). It's not related to how hard I accelerate; gently or aggressively it's still there. I can also feel the thumping through the bike, not dramatically but like the frame is being whacked with a rubber mallet. It's very pronounced in first gear, less so up through the gears though still there but seems not to be there in 6th (is that a clue?). Overrun is fine, cruising at speed is fine, but accelerating it's there. The frequency of the clanks ranges from around every half second to every two seconds though they don't seem to me to be absolutely mechanically regular ... hard to say for sure.

My first thought was a chain issue. The chain and chain rings are fairly new, they're in good condition, tension adjusted to about 32mm of slack on side stand. The rubbing strip on the swing arm is worn but bolt heads not exposed and it seems OK. Both chain rings are bolted tight and I've double-checked the front really is torqued up. Putting the bike on the centre stand and spinning the wheel the chain runs fine and the wheel bearing feels good.

So I then moved to the clutch. I drained the oil and checked for signs of debris in the old oil - nothing. I took the cover off the clutch, removed the clutch plates, inspected starter and drive but all looks sweet. Engine runs beautifully, clutch behaves fine and it rides great. One thing I do worry over though is missed gear changes. I believe it's common to this gearbox but occasionally I've missed up-shifts, I get a false neutral and the engine screams momentarily to red-line before it drops into gear with a bang. Could this have knocked a gear tooth off?

I guess I'm really hoping some wise guru on here will tell me 'oh, you just need to unscrew the furkle knuckle two coils, check the dabble plate resurfaces and return the glottal stop to home position'.

Help anyone???

 

The chainguard is off and there's no sign of interference from the chain. Given the severity of the noise I'd say it would have ripped it to shreds after a mile or two if that were the cause.

 

I have been thinking gear tooth but hoping it's not. So why would a missing gear tooth not show up on overrun? I'm saying only 6th is where it doesn't knock but I'd have to reassemble it all and go ride it again to say definitively. Certainly worse in lower gears.

Any idea how big a job it is for an enthusiastic amateur to split this gearbox? And would I be best dropping the sump off first to hopefully find the culprit tooth?

 

No, front sprocket is tight and good thanks.

 

Sooo, long story short.... Bit the bullet and dropped the engine out. With the sump off (again, absolutely no debris) I could see through to every gear. It's pristine, every tooth perfect like it's just left the factory, ... and this bike's done 47,000 miles. Splitting the crankcase looked like a job with jeopardy so with no broken teeth or debris I decided against. I cleaned and polished everything and rebuilt it. Again engine ran fine and I put it on the centre stand and snuck it into first, then up through the gears, spinning the rear wheel up. The noise and clunking from the chain/gearbox was horrible. Chain bouncing around as if gearbox was stop/starting rapidly.

I stopped the engine, rechecked the chain tension and it was tight. Odd, I'd only just set it up. I reset it and as I spun the back wheel and chain by hand noticed it went tight, slack, tight, slack. Bent final drive shaft? Wheel bearing gone? No. The most simple, basic fault; one chain link seized, and I mean really seized solid. Seized in the bent position to match the front cog. I'm guessing an O ring had failed and I'd parked it up a few days with that link on the cog.

Managed to free it up and work oil into it and get it mobile. Took it for a ride and the bike's fine again! Happy days!

So, the banging noise on acceleration must have been the bent link coming from the rear cog onto the straight towards front cog effectively shortens the chain which snaps up the hanging slack chain and whacks it onto the underside of the swing arm. On overrun the tension is in the underside and the slack top side is sliding along the runner on the top of the swing arm so it doesn't slap. I'm guessing at higher speeds it wasn't banging because of the higher chain inertia.

Lesson learned ...

BTW, I've never oiled the chain, I've only used Wurth dry lube, kept it clean and regularly deep cleaned it with Wurth chain cleaner. Perhaps I should oil in future? New chain on order.