1963 Corvette runs rough at normal half turn valve adjustment but smooths out at very loose lash (1 and half turns loose). This was a previous owner rebuild with less than 100 miles but no details or parts list came with the car. Car was parked for over a year, are the hydraulic lifters the most likely problem? Thanks for any help, this one really has me puzzled...jim

 

Yes that might be the problem. If the lifters are seriously damaged, they will need replacement. In the meantime there are additives that _might_ free a stuck lifter. One trick that's been around forever is to put a quart of automatic transmission fluid down the engine oil fill hole and run that as hot as you can. Lucas and Marvel Mystery Oil are said to help as well. I've never seen it work for a stuck lifter but one that was making noise I have seen that work.

 

You should have about .010" clearance from rocker arm to valve with valve full closed. With a collapsed lifter it would be the other way around, having to compensate for the lifter at the rocker arm. As stated since this only has 100 miles, either replace lifters, at least pull them they can usually be cleaned up, and re-used. Is this happening on all cylinders?

Also on the bottom of the lifter, check that the cam lobe isn't "banging" the lifter, this could be a cam shaft issue.

 

hydraulic lifters need clearance? Isn't that only for solid lifters. I've never worked on something that old so educate me.

 

Perhaps .002", this is to make sure the valve is fully closed when it should be