Starter engagement

[The Veg]

My starter will spin eagerly but does not engage, so I removed it for a look. It appears to have been freshly rebuilt not long before I purchased the car and looks nice and fresh and has a rebuilder's label on it. What I found was that I can easily move the pinion by hand into the engaged position, but that it tends to stay there unless gently nudged and then it retracts. I presume that this may be related to it not moving into the engaged position under power, however it takes very little effort to either move it or unstick it. Does this sound more like a mechanical issue or a lack of lubrication, or some other problem?

(as an aside, from what little I could see of the flywheel through the starter's hole, the flywheel looks new- must've been done when a PO rebuilt the engine )

[oldelanman]

It's probably just dry.
I would try up-ending the starter on a jar of paraffin or diesel so the Bendix is submerged and let it soak for a while. When it's moving freely drain it off and wipe off the excess but don't dry the spiral. The very thin oil is enough to lubricate without attracting clutch dust.
It's an "old school" method but it has always worked for me.

[Craven]

There is a very light pinion return spring, is it fitted, if you lubricate use a small amount of very light oil.
Exploded view of starter can be found on internet.