Found one from when they were de-rigging to pull it out after it blew

approach this problem in steps.
1st verify that the TDC on the balancer is truly TDC, unless you have a brand new balancer. I've had a situation on a boat where setting timing according to the balancer, the engine would not idle unless I cranked up timing. I think the balancer had slipped on. the boat I was working on.
2nd. the carb should run on the isle circuit just fine. Adjust the carb idle mixture screws 2 turns out and set the idle screw as close to closed as possible. Like Smitty said, your dieseling problem is because the primary blades are too far open.
3rd, once you can get the boat to idle on the idle circuit, then do a final verification of timing. 34-36 total above 4000 rpm. you may have to turn up the idle speed screw
initally though.
4th once timing is set work on getting the engine to idle with the least amount of primary blades being open. Youll have to play with the idle mixture screws and the idle speed screw some.
SIDE NOTE: spray carb cleaner around the throttle shafts. If the rpms change drastically, your throttle shafts may be worn out, causing the vacuum leak. When they are adjusting timing, are they using a timing light AND a vacuum gauge?

Yesterday it was a struggle to get the idle low enough and consistent enough for them to set the timing but they seemed to get it. It didn't diesel at all yesterday but it still idled high and ran like crap at idle

Do you still have that fuel tank to engine fuel line and see thru filter ? I hope to god not.

Here is the metering rod that's in it. Looks to be marked 6542 which isn't anywhere on the chart. I know the chart isn't the end all be all though. I am going to put a pair of the 7347 in as that is the closest to stock. Looking up spring color codes now

Why?
Fwiw, I do have a regular spin on fuel water separator

Has this carb ever ran/performed correctly since it was rebuilt?

Is this one of those merc 454’s with rectangular port intake manifolds installed on ovsl potrt heads ? If so, what intake gasket was used ?

Those gray marine fuel lines are for outboard engines only. Not rated for enclosed engine compartments. Plus, many of the aftermarket replacements (especially , but not limited to,, Atwood) inner liners dissolve and gum up restricting line and everything past it.

Those glass fuel filters are not coast guard legal, and not even good for cars. Never really a good design from when they where invented.

Yes, it worked fine for 4 hours then motor blew up due to old water damage in one of the cylinders

Yes, rec port intake on peanut port heads. I would have to check with the rebuilder on the intake gasket. Thank you for the knowledge on the glass filter. Super easy to swap out this fuel line anyways so we will do it for sure. I am aware of the issues of the older fuel lines breaking apart from ethanol, I don't think that's our issue here. We don't seem to be having any issues at higher Rpms