Clark only lifts when reving

We recently picked up an older Clark that works well except for one thing, it only lifts if you rev the engine, which is a pain because you have to put it neutral every time you want to lift something. is this an adjustment or worn component issue?

most likely its a weak hydraulic pump it getting a lot of case bypass

If it has had a hard life, and been overloaded, it could be that a overpressure valve is failing or the spring has collapsed and letting pressure bypass the ram or motor and go back to the reservoir, but like Idacal says, the pump could be worn out!

usually when a vane pump or a gear pump wear out (not catastrophic failure) they start bypassing oil through the case drain the tolerances are shot so it takes so many gallons per minute just to build pressure thats why you need more rpm to over power the leakage. but I haven't dealt much with piston pumps so they may wear differently. the way to check is get the case drain flow specs from the manufacture and do a flow test on the case drain that will answer most of the questions. It could be a overpressure valve but for my luck its always the most expensive component it can be.
side note one time I had a hydraulic spool crack in the valve bank that was the weirdest thing to figure out almost rolled a drill rig diagnosing that one, it was on the outrigger jack. and that one gave all kinds of headache to several mechanics I wasn't the one that finally figured out what was going on, just the rig operator.

If it ends up being the pump, I used Flint Hydraulics , had my CAT split the pump , 1/4 cost of what caterpillar wanted . keep us posted

As everyone else has stated, this points to a worn hyd pump. There could be other causes but this is by far the most common.

A hyd pump in good shape should provide max pressure at idle. I've seen lots of old lifts like this, most people just deal with it rather than fixing.