I have a 2000 300ex. The last time I rode I noticed a lot of noise from my front sprocket. I took it home and put it on a jackstand, put it in gear, and looked a little closer. The chain seemed like it was catching real bad on the front sprocket. I looked at the front sprocket and it was somewhat worn. I went ahead and replaced the front sprocket and put a new RK X-ring chain on. The rear sprocket has very little wear. It seems better now but still does it. I put it on a jack stand again and put it in 1st (at idle), didn't really notice it (although the whole bike did seem to twitch a little - the chain didn't seem real smooth), 2nd... seemed ok, 3rd starting to jump, 4th .... started getting really bad, had to shut it back off. I tried a new 12 tooth (13 stock), and a 14 tooth (both without the case saver), no different. I tried having the chain loose ... didn't help. The alignment front to back looks ok. I tried putting a lot of chain lube on, no help. I did replace my axle (bent it when I rolled it) a little while ago, and the bearings. It was doing this a little before I rolled it though. Any ideas? thanks for any help

 

My xr80 did this and the rear sprocket was slightly to the left fromt the front one. So it was pulling the chain and actually broke one. It was only off the a very very small amount but it was enough to screw it up.

 

I looked again just now. Kinda hard to tell, it's buried in the garage now, but alignemnt front to back looked pretty good. I will try and look again tomorrow when I pull it out. What did you do to fix it? I don't think I can adjust it at all.

 

Put your quad back up on the stand so that the swing arm is free. First check to see if you rear bearing have play in them. Grab the rear wheel at about 12 o'clock and pull up and down. There should be no play. Second check the swing arm bearing. Grab both wheels and try to move the swing arm from side to side. A chain will come off if the bearing are bad.

 

I think I know what the problem is..... It looks like either my swingarm is slightly bent or I blew a bearing in the swingarm. It seems like my rear axle and swingarm are at a slight angle to the front sprocket. When I lift my quad up and set it back down, one wheel hits the ground first every time. I really hope it's a bearing in the swingarm and not bent. I also noticed that my front sprocket was really worn on the inside. The rear was also worn on the inside a little. How hard is it to replace the bearings in the swingarm? I replaced the axles bearings a couple weeks ago and it wasn't much fun.

oh, I did ride for about 4 hours this weekend and didn't notice any problems with the chain while riding. only when I jack it up and put it in about 4th gear does it start jumping

 

Take off the rear sprocket and set in a on a completely flat surface and see if all edges are flush. If they are the rear sprocket is bent. Replace the rear sprocket if u havnt. If you really didnt want to spend the money put the rear sprocket between to reall thick pieces of sheet metal and pound away this might flatten it. Since it one start to jump at high speed it is probaly a really small bend.
hope this help.