Mud in My Blood Forum banner

1 - 3 of 3 Posts

·
Registered
Joined
·
2 Posts
Discussion Starter #1
The timing chain on the Prarie decided to grenade on me today. I had previously rebuilt the engine with new piston+rings, seals, and new intake/exhaust valves. After the rebuild, I made sure everything was spec. but still heard almost a sound like somebody blowing in a tube, coming from the exhaust valve. x2 checked that it was in spec and had it running with the sound gone away for the most part. After 5 minutes of riding, i heard a snap and the machine died and would not start. Timing chain grenaded and did a ton of damage. Could this be cuased by valves being too tight? What is the likelyhood that my new piston slammed the valve? Am i looking at just a new timing chain or a new rebuild? Thanks for the help.
 

·
Moderator
Joined
·
7,353 Posts
First let me say it is likely the piston did hit the valves. Depending on how bad will determine weather you can use it again or not. Most times not. Bank on replacing the valves. Now lets talk about cam chains and valves. Lets say on the last rebuild you didn't replace the chain, bar and upper sprocket. As chains and sprockets wear and stretch the slack is taken up bu the tensioner and bar on the back side. as this happens the distance on the front side increases. This causes the cam to retard or trail behind slightly...as much as 1.5 degrees in some cases. Now let's talk about "overlap". Overlap is during TDC of the exhaust stroke where while the exhaust valves are on there way to closing the intakes start opening. So there is a point where both are slightly open and it is during this time that there is damn little room between the partly open valves and the piston. Something like .010 as I recall. Now consider the exhaust valves are late in their cycle because of that warn out cam chain. That means that it/they are farther open when the piston gets to the top then they are supposed to be. That .010 may not be there at all. And all it takes is a little bit too much touch to send enough force through the cam to snap that old chain.

Ok, let's say you did replace the chain, guide bar and the upper sprocket like you should have. That sound you were hearing may have been a valve binding in the guide and at some point it bound enough to get hit by that piston..and pretty hard too..enough to take out the chain. I'll bet when you pull it down it will have a story to tell. Post pics. We love to see stuff like that.
 

·
Registered
Joined
·
2 Posts
Discussion Starter #3
Looks like I got lucky. Pulled engine last night and removed valves. They are not bent by some miracle! What looks like happened is that crank bearing on bottom end let go and what I think happened is that this caused chain to hop off of the gear and hence the broken chain.
Still waiting on a flywheel puller to take off the magneto to see how bad it is. ( I can see the remainder of the bearing race) Ultiamtely the chain took out the KACR weights and made a cut in the oil line going up to the cam. Will post pics when parts come in with a before and after. Thanks for the reply.
 
1 - 3 of 3 Posts
Top