I am not able to depress the plunger that locks to the adjuster plate. It moved in slightly, but did not pop back out so I stopped. Can I safely remove the nut and adjuster plate to better access the plunger to work on it? Will the eccentric / sprocket stay in place and stay connected to the chain?

 

The sprocket will stay in place, but you would be in great danger of the very small plunger and its spring falling down into the timing chest. So if you proceed, be sure to stuff rags down below to catch anything. What happens over time and little use, is the adjuster and plunger get stuck due to "varnish" residue from the oil. Try squirting a bit of penetrating oil on the plunger, You won't need much and also catch any surplus with a rag, but there is so little it won't dilute your oil much if or even at all

 

Ok, adjuster and nut removed just fine. I Will completely seal off the interior with a strip of foam I can cut off a sheet of it I have. Started spraying the plunger and adjuster with penetrant (time to change the engine oil anyway).
will remove the radiator for complete access ( and get it cleaned out while at it). Will spray for a few days and then try to get a hold of the plunger and wiggle it out. Heat if necessary.

 

Ok, not pretty bit it’s out. Thanks for letting me know I could remove the adjuster plate; that got me all the access I needed.

Plunger Out

 

After much penetrant time and heat, the shaft has somewhat freed up. I had to use a socket directly on the sprocket nut but without the sprocket.
Modified a socket to fit over the adjusting tools’ cross shaft to rotate everything back counterclockwise to tighten up the chain.
As the shaft never really got loose enough to get a feel for how much of the torque was actually going to tighten the chain, I will have to see if the chain rattle sound goes away. Poked the chain with a screwdriver through the opening it feels tight but still has some give.