My 1/16" aluminum TIG rod goes in the same depth in the front passage as it does in the back.

Edit: finally was able to get my head in there to actually see the passage in the back and it is NOT drilled.

Had it idling at about 1000 rpms from a cold start and had 10-12 degrees of timing. Backed off the throttle just slightly to see if I could adjust the idle mixture screws and was able to make some change on the vacuum gauge with the throttle at that position , until rpms started to go back up. I backed off the throttle more and then it stumbled and quit.

It needed a good amount of throttle to start after that and I even had a backfire through the carb. Once running again it needed enough throttle to hold 2000 rpms, and would still run like this keeping the throttle there. As soon as you back off the throttle towards nuetral it stumbles and quits.

what ignition , distributor, coil are you running

With no vacuum leak, its a carb problem

You’re chasing your tail. Rebuild the carb. It’s cheap and doesn’t take that long. Only takes the smallest piece of crap to mess that up. Bring #1 to TDC and redrop the dizzy verifying firing order. Make your timing marks for 0°. Start from scratch and stop trusting other peoples work

If it idles down better cold than warm that would make me think it’s way rich. You should have a hose coming from each valve cover going up to breather. With engine running however fast it has to to stay running, put your thumbs over the end of those breather hoses. They should have a slight pressure build up. If they have vacuum on the hoses your intake gasket is leaking vacuum into the crank case.
Sounds like carb needs to come apart though.

Also disconnect choke and make sure it stays open while you’re doing all this

what is the list number on the carb?it is stamped on the front of the choke horn.

Thunderbolt 4 (the style that mounts to the distributor, not the manifold) distributor and coil appear to be stock mercruiser.

Only numbers I can find are on the side of the metering block.