I have a Mercury Mercruiser 5.0 with Turn Key Start that will start but it sounds like the starter will not disengage. I'm not entirely sure but I have a feeling there is an issue with the starting system. I removed the starter and bench tested it and the starter will engage and disengage correctly. Although, when connected to the engine while running it sounds like it is constantly engaged and turning. I know that the starting system will engage the starter until the engine is running but what tells the starter to stop? If anyone has had this issue please chime in, I am unfamiliar how the system works and could use some help.

Thanks

Carbureted right ? From what I know the TKS systems has nothing to do with the starter....you control that.
So check remote solenoid and starter.

The TKS system adds fuel from carb into intake when you do a cold start. This is so the driver doesn't have to do old school throttle 'pumps' to inject the fuel needed for a cold start.

Unhook the small wire that fires the starter at the starter. Hook another wire to the same terminal. Seperate wire not connected to anything. Turn ignition on. Touch your new crank wire to 12 volts. Release as soon as engine starts. Do you get the same sound? Them replace the starter. If no grinding sound, put a voltmeter on the currently unhooked wire that is supposed to fire starter. Does it show any volts?

The starter was already replaced so I know that is not the issue. I'm wondering if the ignition switch or starter solenoid is defective causing the starter to stay engaged. Also, when I disconnect the power/ground from the starter solenoid or disconnect the yellow/red wire for the starting circuit from the ignition switch the engine dies. Not sure why this happens if the engine is already running.

You just answered your own question. That yellow red wire is not supposed to run the ignition. It's supposed to crank the starter only. So the fact that it's making the engine run means it's also putting 12 volts to the starter at the same time.

You just answered your own question. That yellow red wire is not supposed to run the ignition. It's supposed to crank the starter only. So the fact that it's making the engine run means it's also putting 12 volts to the starter at the same time. Chase down the purple wire. And get it seperate from yellow/red.