broken cam chain

[floridajoe]

on my 79 xs11s as i was driving cam chain broke. pulled cam cover off no marks on sprockets or cams. my question is what kind of internal damage can i aspect ? thanks for any info before i start to tear down .

[crazy steve]

At the very least you can expect some bent valves, so you'll need to look for a good used head (the valves are long discontinued...). You may have damaged the pistons as well, you'll see that with the head off. Worst case, the cam chain may have 'balled up' around the crank sprocket and broke the cases.

This will require careful checking and maybe a major teardown.... sorry...

[James England]

Oh dear! That's a bad thing to happen and it's 99.9% likely that you'll have bent valves, if not holed pistons and possibly damaged cylinder bores too if any bits of piston have broken off. It's a kind of 'everyone's nightmare' type incident. Sorry about that. As Steve says, you'll need to do a careful stripdown with the head off.

Complete cylinder heads, with valves and camshaft and everything, can be had on eBay for a good price. I've seen them go for $50 in the UK once or twice. Get one from a low mileage bike. If your pistons and bores are OK, you'll not need to take the barrels off (can be tricky). All you'd need is a new head gasket. It's not a difficult job, so all is not necessarily bad.

[bikerphil]

If you check your valve clearances, the ones that have huge numbers will indicate the bent valves.

[TopCatGr58]

Hey Phil,

I would think it would be difficult to check the valve clearances with the cam chain BROKEN!? And trying to rotate the cams or crank to get the lobes to their lowest points could drive pistons into stuck valves as well!

I'm assuming that when you pulled the valve cover off you found NO chain running across/along the cam sprockets...just bare cams/sprockets!?

SCARY!! Yep, a good look at the bottom end for damage to the crankcase for sure, but once the cams stopped turning when the chain slipped off, you may have only bent 1 or 2 valves?? Good Luck, let us know what else you find!

T.C.

[trbig]

I thought the '78 and '79 motors had the same valves as the 750/850 motors and were still available? Even if they're not making them any more, I'm betting you could score what you needed off the triples site if all you find (Very hopefully) is a broke chain and some bent valves. Lap 'em, shim 'em, install new chain and go. If the head gasket isn't delaminated and comes up cleanly, I've even re-used those with absolutely no problems. I know that drives some real mechanics crazy since they've had it pumped into their heads all along that you HAVE to replace a head gasket, but the only difference I've seen between a good used one and a new one, was that I didn't have to go back and re-torque all the head nuts after the first heating cycle. It's already pre-squished. You aren't sealing any water jackets with these head gaskets.

[bikerphil]

Hey Phil,

I would think it would be difficult to check the valve clearances with the cam chain BROKEN!? And trying to rotate the cams or crank to get the lobes to their lowest points could drive pistons into stuck valves as well! T.C.

Yep, TC you're right.

[Poprock]

Check with Geezer for valves. He gets 'em in polished stainless steel. Price is right too. They work great. You'll want to change all 4 if any on a side are bent, because they flow better than stockers. He only sells sets of 4.

[crazy steve]

Tod is right, the '78-79 valves can still be bought new from Yamaha, but you'll choke at the price; $52 each for the exhausts, and $64 for the intakes... Cheaper to buy another motor...

Geezer is out of the valve business, so that source is gone....

[bikerphil]

This place says they have the 78-79 intake valves, but not the exhaust...

http://www.newmotorcycleparts.com/mo...ts/valves.html

[crazy steve]

That's a lot better than $64!

[JasonRedbeard]

I would think it would be difficult to check the valve clearances with the cam chain BROKEN!? And trying to rotate the cams or crank to get the lobes to their lowest points could drive pistons into stuck valves as well!

Gonna have to argue you on this one TC. Turn the crankshaft 90 degrees off TDC and all four pistons will be halfway down the bores. At that point you could turn the cam by the hex machined into it without worry of piston-valve contact. This is a method I commonly use on car engines...I see no reason it won't work here. Just don't force the crank....you don't want to bind the chain at the bottom and crack the case, as mentioned earlier.

Excessive valve clearance will indicate warped/bent valves. Looking in the cylinder with a borescope using a right-angle adapter works well too, if you have access to that sort of thing.

[cywelchjr]

Gonna have to argue you on this one TC. Turn the crankshaft 90 degrees off TDC and all four pistons will be halfway down the bores. At that point you could turn the cam by the hex machined into it without worry of piston-valve contact. This is a method I commonly use on car engines...I see no reason it won't work here. Just don't force the crank....you don't want to bind the chain at the bottom and crack the case, as mentioned earlier.

Excessive valve clearance will indicate warped/bent valves. Looking in the cylinder with a borescope using a right-angle adapter works well too, if you have access to that sort of thing.

Now there is a good point and would let you know how many valves got killed, which would tell you how much damage you may have. Your still going to have to pull the head, but if you only have one bent valve (if all but one valve is in spec you most likely only have one bent valve (and would feel really really lucky)) you would likely only look for one valve rather than a head, but if you had 4 bent valves I know I wouldn't bother looking for valves at all, just go for a head right off.

[petejw]

Gonna have to argue you on this one TC. Turn the crankshaft 90 degrees off TDC and all four pistons will be halfway down the bores. At that point you could turn the cam by the hex machined into it without worry of piston-valve contact. This is a method I commonly use on car engines...I see no reason it won't work here. Just don't force the crank....you don't want to bind the chain at the bottom and crack the case, as mentioned earlier.

Excessive valve clearance will indicate warped/bent valves. Looking in the cylinder with a borescope using a right-angle adapter works well too, if you have access to that sort of thing.

the only problem with that is that if both cams are still attached to the
head and u only turn 1 cam at a time u run the risk of bending more valves
as the cams arent being turned together at the same time.

[natemoen]

the only problem with that is that if both cams are still attached to the
head and u only turn 1 cam at a time u run the risk of bending more valves
as the cams arent being turned together at the same time.

BTDT.............dont try this one at home kids.