A Little Help Please - Timing Chain Jumping

[erlrich]

Hey guys,

Was putting the motor back together this afternoon - got the degree wheel set, the top end all bolted on and torqued down, and the timing chain/gear in - and then noticed as I was checking the cam timing that if I turned the crank at any speed the timing chain would occasionally jump, as if it were trying to jump a tooth or something. So, I pulled the rockers and tried turning the cam with no load - that was fine, so I decided the problem must be in the rockers/rocker arms. The wierd thing was, the head hadn't been touched in the rebuild - it was nice a straight and had been reconed at the start of last season.

Then I finally decided to see what would happen if I tried just turning the cam with the rockers installed. I set the crank so all the cylinders were at mid-stroke, pulled the timing chain, and tried just turning the cam by itself. When I did, I quickly found the problem - the cam would literally jump ahead as the rockers under load passed the top of the lobe. The only thing I can figure is that all of the rockers bled down while sitting around (confirmed by checking the slack lifters for free movement), and the resulting lack of pressure on the cam by the slack rockers is allowing the ones under load to slam the cam forward as they travel down the backside of the lobe.

All of this brings me to two questions; first, has anyone else ever encountered this phenomenon, and second, assuming that is the issue, is there any way to pump up the lifters before installing them?

Thanks for any help.

[Joe Harlan]

Hey guys,

Was putting the motor back together this afternoon - got the degree wheel set, the top end all bolted on and torqued down, and the timing chain/gear in - and then noticed as I was checking the cam timing that if I turned the crank at any speed the timing chain would occasionally jump, as if it were trying to jump a tooth or something. So, I pulled the rockers and tried turning the cam with no load - that was fine, so I decided the problem must be in the rockers/rocker arms. The wierd thing was, the head hadn't been touched in the rebuild - it was nice a straight and had been reconed at the start of last season.

Then I finally decided to see what would happen if I tried just turning the cam with the rockers installed. I set the crank so all the cylinders were at mid-stroke, pulled the timing chain, and tried just turning the cam by itself. When I did, I quickly found the problem - the cam would literally jump ahead as the rockers under load passed the top of the lobe. The only thing I can figure is that all of the rockers bled down while sitting around (confirmed by checking the slack lifters for free movement), and the resulting lack of pressure on the cam by the slack rockers is allowing the ones under load to slam the cam forward as they travel down the backside of the lobe.

All of this brings me to two questions; first, has anyone else ever encountered this phenomenon, and second, assuming that is the issue, is there any way to pump up the lifters before installing them?

Thanks for any help.
[/b]

When you have oil pressure you chain tensioner will not let this happen. Right now you have nothing hiolding constant tension on the chain.

[erlrich]

When you have oil pressure you chain tensioner will not let this happen. Right now you have nothing hiolding constant tension on the chain.
[/b] Aha, didn't think of that. Thanks Joe.

[kthomas]

I don't know 240SX motors, but on a Z motor we don't depend on the tensioner to take up slack in the chain. Because of the phenomenon you noticed- ratcheting of the cam by virtue of the geometry and springs- it will do the same ratcheting to some degree at speed when oil pressure is relied on to take up slack. On a Z motor we set the chain up very tight without any oil pressure on the system.

[erlrich]

I also wondered if the fact that the head has been shaved .025, in conjunction with the block having been decked, had created enough extra slack in the chain to let this happen. But as Joe pointed out, this motor relies on the tensioner to keep the chain taught, and I think maybe that along with some slightly deflated lifters may be leaving enough slack that this can happen. I hope so anyway. I should have it all back together by Tuesday, so I should know soon.

[Joe Harlan]

I also wondered if the fact that the head has been shaved .025, in conjunction with the block having been decked, had created enough extra slack in the chain to let this happen. But as Joe pointed out, this motor relies on the tensioner to keep the chain taught, and I think maybe that along with some slightly deflated lifters may be leaving enough slack that this can happen. I hope so anyway. I should have it all back together by Tuesday, so I should know soon.
[/b]

K this tensioner does not lend itself to the l.series fix but it can be helped with some work.