I finally tore down the top end of my Bear 350. Everything looks good except the piston. Bad scoring on one side, from the top edge to the bottom of the skirt. According to Mr Google, it's a result of running too hot. I can see that because just prior to the break down I was having a bear if a job getting her to idle. One trip at dusk I could see the front exhaust was glowing orange. I got it back down on the idle but I guess the damage was done. Stoopid ol bugger I am.

 

Sounds like you found the problem. Good time to upgrade the piston for a little more power since you have it apart.

 

For sure on the new piston and rings. I'm thinking valve set too and timing chain and sprocket, while I got it tore down. Looking for opinions on that too. I was surprised how simple this teardown was, the only thing I'm worried about is getting the chain tensioners back in right. Again, opinion and Help appreciated, please.
Thanks for looking in.

 

"the only thing I'm worried about is getting the chain tensioners back in right"

I managed to get it wrong on a Bruin, you need to be sure the "fixed" one is in its slot at the lower end. Fitting a new chain is a right pain in the bum, don't, if the adjuster sprung out when you took it off, indicating some more movement left. Grind the valves in, this isn't a racing engine, new valves and springs won't make a blind bit of difference, unless the stems are badly worn, or the valves burnt.

 

Ok , so no to new valves unless stems are bent -- sounds good to me. The tensioner on the right-hand side pretty much fell out , the left hand one towards the front on the bike , I pulled out. Damn . I was thinking the chain may have stretched a bit because when I lined up the timing marks, the cam mark was off about one tooth. is that "normal" ?
Thanks for you help, I really appreciate it.

 

There is only one tensioner, two guides, the moving guide is held against the chain by the tensioner, which is a spring loaded ratchet. You take the tensioner off first to take the strain off the chain and guides as you strip down. If it springs away from the cylinder as you loosen it, there must be some adjustment left. I would say a min of 1/8" of adjustment left, otherwise do fit a new chain. Never heard of anyone fitting new sprockets, on most quads the lower sprocket is part of the crankshaft. On the 350 Yam you need to take the flywheel and balancer gears off to fit a new chain, there are 5 different wrong ways of re fitting the gear and one right way to re-fit it, on all of which the timing marks line up.

 

My mistake, again I meant to refer to the two guides that I was worried about getting back in place. The tensioner did spring back when I removed it, quite a bit actually. I was told the Cam sprocket should be changed when the chain is replaced, no real explanation as to why, I'm Supposing just because it's tore apart. Should I be concerned that the timing mark seems to be off by one tooth on the cam sprocket ? Will the chain guides just drop down into place or is there a special procedure ?
Thanks again for the help, this is just what I need

 

Timing does move a bit as the chain wears but you are going to put the chain back with timing marks lining up so, why worry about it not being right before? As I wrote earlier, I managed to get it wrong with the fixed guide when I did a Bruin, only way to be sure, certain, positive is to pull the alternator and take a peek. Moving guide simply flaps about until the tensioner is fitted so I don't think you can go wrong with that.

 

Ok no more worries on the timing chain or guides. Everything seems straight forward now, thanks to your help. I plan to take the the head and piston to a shop on the weekend and have cylinder and valves checked, get new rings etc.
I noticed quite a bit of oil under the bike yesterday, so my next project is pull the skid plate and check the oil pan, while I wait for the top to get fixed up.
Thanks Merryman 👍