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Discussion Starter #1
I've had my 1991 zx11 for 18 months and put 18,000 miles on it. Currently at 36,000 miles. About a week ago it started slipping out of second gear or so it would appear. At first I thought it was the chain slipping. I know it is just about at its limit and ready for replacement. Then I thought it might be the clutch, but it only appeared to slip out and back in and out again rapidly. I figured I would eventually lose second gear and just have to shift from first to third so I started riding it that way.
I was riding home at about 70 mph in 6th gear and suddenly I heard a loud popping noise. Being concerned something might lock up and throw me off the bike I pulled the clutch in and coasted to the hard shoulder. The front sprocket had ripped off and tore through the aluminum sprocket cover. The chain is intact. The splines on the shaft from the transmission look OK, but the threads that the nut goseon area bit damaged.
I never tried to start it again in case there is unwanted metal debris in the oil that could do further damage. I can shift through the gears And I can turn the output shaft by hand in neutral and in all gears when the clutch is engaged. When I do so there is not any noticeable binding or unusual resistance. If I disengage the clutch I can't turn the output shaft because of the resistance the engine adds. Is it possible that the sprocket came off for some reason other than the gears locking up. I was initially convinced the gears locked up and snapped the sprocket off since I thought 2nd gear was going out, but now I'm not so sure. What further tests can I do to determine the problem? I understand 1991 models had an issue with 2nd going out and they fixed it on the 1992 model.
 

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Without the sprocket replaced and running it not much . Dogs may be rounded and spitting it out of gear . You can pull the pan in bike and see somewhat of whats going on .
 

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Post some photos as well, so we can see what you are talking about. The only way I can think of for the front sprocket to come off is if the nut was loose/not locked in properly and came off first.
 

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Discussion Starter #4
Thanks for the advice. I was thinking of draining the oil and filtering it to see what comes out. Then I thought I might add oil and start the motor and see if it goes through the gears the way it is with no sprocket on it. It would perform differently with a load on it, but it would give me an idea what's going on before I go buy a sprocket. Do you think it would be unadvisable to just simply drain the oil instead of pulling the oil pan before starting it and running it through the gears? Am I unlikely to get all the metal fragments out by just draining and replacing the oil and filter? I don't want to ruin the engine if there is much chance of fragments circulating. I would think the pick up screen would catch most, but I would also assume most would be at the bottom of the pan ready to be flushed out. I suppose I should inspect the filter closely too for metal particles.
I'm thinking a sprocket either works or it doesn't and wouldn't cause intermittent slippage in only 2nd gear like i had for the last week or so. If it was coming loose it would likely affect more than one gear.
I'll drain the oil and report back and see if I should pull the pan afterwards. I put some strong magnets on the lowest drain plug.
 

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Discussion Starter #5
Without the sprocket replaced and running it not much . Dogs may be rounded and spitting it out of gear . You can pull the pan in bike and see somewhat of whats going on .
I ordered a new front/rear sprocket and chain. I ordered a new sprocket nut and washer. I drained the oil and only found a few shards of metal which are magnetic. I'd say 10-12 shards the thickness of a hair and 2-5 mm long. I filtered the oil and put it back in. I ran the bike and shifted it through the gears since no chain is on it. It shifted fine, but who knows under load when I get the chain and sprocket on.
I bought a 20mm 1.5 pitch die to rethread the output shaft and it threaded pretty good. It will hold a nut just fine even though some threads are still damaged.
It will be interesting to see if second gear is OK or not when I get the sprockets and chain on this weekend. Maybe I'll get lucky and it was just the sprocket and not the gears. Strange that it was only slipping in second though.
 

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Discussion Starter #6
Post some photos as well, so we can see what you are talking about. The only way I can think of for the front sprocket to come off is if the nut was loose/not locked in properly and came off first.
Here are pics. See how the sprocket ripped right through the cover and left the bolts and half of it in place! This is the threads of the shaft before I rethreaded it. The toothed washer is split in half and deformed and the nut has damaged threads and is chewed up. I found the nut lodged in and around the swing arm pivot area. The sprocket is on the freeway somewhere. After running it through the gears I feel comfortable putting a new sprocket and chain set on to see how it runs with a load. Parts coming Friday!
17094
17095
 

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Discussion Starter #8
OK. I guess the verdict is in.
I replaced my front and rear sprocket along with a new o-ring chain since the front sprocket came off on the freeway. I rode the bike and it still slips out of second gear sometimes, but only when shifting up. I'm now thinking the rapid random slipping of second gear may have contributed to the loosening of the front sprocket nut since it came off only a week or so after the gear started slipping. I have a replacement sprocket cover arriving tomorrow to finish off the project since the old one shattered. Having that sprocket come off has been quite an inconvenience, but I'm just glad I wasn't thrown off the bike.
My odometer has 36,000 miles on it and I'm sure these bikes will do well over 100,000 miles. I seen one advertised with 170,000, but the head finally had issues.
I won't get to fix this for a while, but I am up for the challenge when the time comes.
I've done all my own work on this bike and every vehicle I have owned for the last 34 yrs
What advice do you guys have for riding with the bike in this condition? Should I shift to 3rd even though 2nd is not completely out in order to prevent more wear?
Should I bypass it when downshifting?
 

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You definitely have a transmission issue. Could be a few different things but all will require the cases to be split. Engagement dogs, shift forks or shift drum.
 

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I have many parts transmissions and shafts. you need to replace that shaft with a good one. also split cases and repair 2nd gear dogs and the shift fork also.
 
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