Timing chain riding up on gears

[Ski Whiz]

Wondering if it’s normal for the timing chain to ride up on cam sprockets as the cam lobes pass over center on valves?
As I rotate engine, the valve spring pressure is enough to force the chain up out of the gear teeth and it actually depresses the timing chain tensioner.
At first I thought chain was stretched, but after head was off I could wrap it all the way around sprocket and it fits perfectly.

 

[Kurt J]

Which way are you rotating the engine? If backwards, it WILL jump time on the camshafts. Rotating forward it should NOT ride off the cam teeth.

 

[smokingcrater]

The only way you could be depressing the tensioner is if you are rotating backwards...

 

[Ski Whiz]

The only way you could be depressing the tensioner is if you are rotating backwards...

No, I’m rotating it counter clockwise facing the clutch. When rotating slowly, as soon as the cam lobes pass over centre of the valves, you hear a click- tensioner depresses and timing chain pops up on teeth. I’d have to make a video to show it but it’s all apart at the moment

 

[Mooseman]

I had the same problem when I was doing the head on mine. Had to fiddle with the chain tensioner. I zip tied the two cams together pointed properly to prevent them from moving while putting on the chain. Then installed the tensioner and undid the clip so it would extend. Maybe your tensioner isn't extended or you may need a new one.

 

[grizztracks]

The problem is that the Yamaha engines run two overhead cams, exhaust and intake. With a four stroke engine the normal progression is to open the exhaust valve first then on the following stroke open the intake valve but before the exhaust fully closes the intake is starting to open so there is "Overlap" between the two. So as the exhaust cam comes off a lobe the valve spring pressure causes the exhaust cam to rotate toward the intake cam but the intake cam overlap is already opening the valve and the valve spring is resisting the rotation. With the exhaust cam wanting to rotate and the intake resisting the rotation the cam chain develops slack between the two cam gears and skips.

When rotating the engine in the proper direction with the valve cover off you should tie strap the chain to the gears. you'll have to keep cutting and replacing ties as you rotate. Also, The valve cover has a guide that sits just above the cam gears so with the cover installed the chain doesn't have enough clearance to skip over teeth.

 

[Ski Whiz]

The problem is that the Yamaha engines run two overhead cams, exhaust and intake. With a four stroke engine the normal progression is to open the exhaust valve first then on the following stroke open the intake valve but before the exhaust fully closes the intake is starting to open so there is "Overlap" between the two. So as the exhaust cam comes off a lobe the valve spring pressure causes the exhaust cam to rotate toward the intake cam but the intake cam overlap is already opening the valve and the valve spring is resisting the rotation. With the exhaust cam wanting to rotate and the intake resisting the rotation the cam chain develops slack between the two cam gears and skips.

When rotating the engine in the proper direction with the valve cover off you should tie strap the chain to the gears. you'll have to keep cutting and replacing ties as you rotate. Also, The valve cover has a guide that sits just above the cam gears so with the cover installed the chain doesn't have enough clearance to skip over teeth.

Ok, thanks.. that’s a much better description of what I was observing. It seemed odd watching the chain behave that way and I was trying to convince myself that under operating conditions the situation would probably never arise.
I very much appreciate the feedback for reassurance.
On another note relating to the project…
When getting a head resurfaced, at what point do I worry about needing a shim for head gasket? My head had up to .006 in warpage and Yamaha’s tolerance was .0039in.
Depending on how much material the machinist removes, when do I worry about changing the compression?

 

[Big_Phil]

Ok, thanks.. that’s a much better description of what I was observing. It seemed odd watching the chain behave that way and I was trying to convince myself that under operating conditions the situation would probably never arise.
I very much appreciate the feedback for reassurance.
On another note relating to the project…
When getting a head resurfaced, at what point do I worry about needing a shim for head gasket? My head had up to .006 in warpage and Yamaha’s tolerance was .0039in.
Depending on how much material the machinist removes, when do I worry about changing the compression?

I had a head on my old Nytro that was .007 warped, had it machined and ran 15000kms with no issues til I sold the sled. You’re good.

 

[Srxspec]

I have thicker head gaskets on hand if you need to. I wouldn't worry about up to .010" or so machined off and you should still be fine, just run premium if you can.

 

[craigwsrx]

Have one doing very similar thing. Tensioner snaps and resets tension on chain as you roll over engine. Installed new tensioner but didn't correct issue. Sounds like a rod knock when running, not sure where to go next. Any advice?