I have a demon 850 vac secondary carb. I did a quick rebuild on it. New gaskets, needle/seats, etc. I put it back on the boat and its blowing out tons of black smoke (unburnt fuel). I noticed its now dumping a ton of fuel during idle where it wasn't before. Float levels are fine and jet sizes weren't changed. Idle set screws are 1.5 turns out on all 4 corners.

What did I screw up?

Are you sure the floats are set properly sounds like fuel is just running in. Check all other suggestions

Gt might be something they're.

Here's the factory specs for what I believe to be your carb.

On my phone so it's hard to take screenshots and put them together

Double gasket the power valve?

Sent from my SCH-I545 using SoCal Jet Boats mobile app

77 frt 87 rr

Be sure sure to check  the return pressure to see it hasn't changed, good luck

I would make it a return system takes heat and pressure problems away.

Second on the 4.5 try 77 front 87 rear

Also NAPA best filter made by wix is a great filter

I went to power wash down pump to cool down motor and manifolds. 4.0 west marine pump. Works great, no  ore too much pressure,  no more reversion with over transom pipes.

Spray and  even sure your air bleeds are clear

AED makes a good running carb also make sure you give the  engine specs to them. Also get one with screw in air bleeds . Makes  a great different in idling to have the correct idle bleeds.

I run a electric pump and return regulator setup, makes a much easier job for the pump . Performance is much  noticeable with a constant pressure against the needle and seat lowering the chance of flooding . There are some great articles written on the subject especially if you are running juice.

Sent from my GT-P5113 using SoCal Jet Boats mobile app

I don't know you but lots of love and prayer for the little one. Also lots of prayer to you and family keeping strong thru these trying times.

Sent from my GT-P5113 using SoCal Jet Boats mobile app

Go to fuelish tendencies on Century Performance internet all the how to figure fuel needs, great site

Sent from my GT-P5113 using SoCal Jet Boats mobile app

And don't start throwing good parts and wasting good hard money, there are smart people here someone will help. Again don't throw parts away .

Sent from my GT-P5113 using SoCal Jet Boats mobile app