What could cause the timing chain to snap - Twice?

[mfallavol]

I picked up a 2003 KLF-250 for my son's first ATV a few months ago.  It had no compression and overall needed a lot of work.

To make a long story shorter, the major problems were in the head; mostly a bent valve.  We got it back and working and he was enjoying it until it just died.  Started back in to it only to find a broken timing chain.  I figured it had gotten stressed due to whatever caused the bent valve so no big deal...so I thought.  The problem is he was back riding it this weekend; probably has less than 4 hours on it since the timing chain was replaced and the same thing happened.  He'd been riding it down the trail very gingerly, we stopped to chat and he went to give it gas and it gave a little 'pop' and died.  When he hit the starter it was obvious it needed to be towed back.

Got it home to find the timing chain is broken again.  I really have no idea why.  It runs fine when running but now I'm gun shy.

Any thoughts and ideas would be appreciated.  He's getting pretty bummed so I'd like to get this figured out.  Just ordered another timing chain so we'll get back into it this weekend.

[Ajmboy]

That's interesting and the first thing that comes to mind is that it's too tight and stressed. Bad tensioner? Check out the service manual below and look at page 75 to inspect that tensioner. Also make sure you timing is correct:

 

[skidooer_3]

On 1/31/2017 at 9:17 PM, mfallavol said:

I picked up a 2003 KLF-250 for my son's first ATV a few months ago.  It had no compression and overall needed a lot of work.

To make a long story shorter, the major problems were in the head; mostly a bent valve.  We got it back and working and he was enjoying it until it just died.  Started back in to it only to find a broken timing chain.  I figured it had gotten stressed due to whatever caused the bent valve so no big deal...so I thought.  The problem is he was back riding it this weekend; probably has less than 4 hours on it since the timing chain was replaced and the same thing happened.  He'd been riding it down the trail very gingerly, we stopped to chat and he went to give it gas and it gave a little 'pop' and died.  When he hit the starter it was obvious it needed to be towed back.

Got it home to find the timing chain is broken again.  I really have no idea why.  It runs fine when running but now I'm gun shy.

Any thoughts and ideas would be appreciated.  He's getting pretty bummed so I'd like to get this figured out.  Just ordered another timing chain so we'll get back into it this weekend.

also check the gears to make sure they aren't worn to a point that the chain will jump over the teeth causing excessive stress and snapping the chain.  Pay particular attention to the gear on the crankend.  Make sure it's not slipping on the crank.  Good luck