1990 Honda Civic EX starter will not turn over even though the electronics are fine.

soundfs like the starter brushes are about wore down to not always contact the armature in the starter, hammer on the starter when it does this to shake the brushes down to contact , if that works your starter is wearing out at the brushes

Already Checked the Carbon brush heads. They are in complete contact with the armature. Only half worn out. My hunch is it might be an issue with the ground

Sounds like loose battery connection or loose negative battery connection to frame or motor. Inspect battery terminal connections. Are they the original ones? Is the adjustment screw tightened all the way down with no more left to tighten it? Your battery terminal should have some extra room available on screw to tighten it up some more(when fully tightened) if it does not have any extra room to tighten it the terminal is worn out and needs to be replaced.

is it the starter solenoid? That can be intermittent- how are you testing the starter? have you taken it out? hook it up to a battery or a battery charger then you can see it work- if there is power to the starter, then it should spin- why not just get another starter? nobody just rebuilds these things anymore- 'cept for maybe me

Re-did the battery terminals. They are tightened till the end. The negative connections are both on the frame and engine mounting. The motor does not get a negative charge through any cable but gets one from the engine. @ jamnblues . I actually had the solenoid replaced by a new one, its gear groves were worn out. (a common issue in this model) . Started having this issue after replacing it. And have tested it a couple of times. When I remove the starter from the engine and hook it directly on a battery the solenoid keeps on spinning as smooth as knife through butter. But when mounted back on the engine it starts to play up. That is one reason why I haven't replaced the starter till now.