Found the AC behind head sink. Took it out this weekend what a blast. Went to pump the head tank and the pump had trouble pulling, heard a hissing and there is a thru hull fitting that appears to be a vent. Put my hand over it and I could feel the suction, is there a valve somewhere to close that off during a pump out?

Depends on the boat, but there might be a overboard dump valve that needs to be flipped. Such may be in the bilge, on the starboard side in the gunwale. But most of them do not have overboard pump out.

It will hiss though through that through valve when it's empty. It's only 6 gallons, so it sucks out pretty fast.

I know it sounds gross, but I used to pump it out with my big shop vac at the house. Since it only had weak girl pee and water in there, it wasn't bad at all. It was a good excuse to clean and disinfect the shop vac afterward. I haven't cleaned my shop vac now since I sold the boat a almost 2 years ago.

I believe mine has the overboard dump, but I always wondered how complete that dump is both for for cleanliness and winterizing. I have thought about the shop vac method as well. I usually scrub out my vac 2x a year anyway, is that odd? Lol

By the way, you said you pumped it out with the vac. Does that mean you you stuck the vac hose down the toilet?

I had my own Pump Out Attachment that screws into deck fill. I would use that, and attach the shop vac hose to that to pump out myself.

Also, the bilge pump under the cabin (for step bottom boats, at the step), may not get every last drop of water out. You can use your shop vac from the outside of the boat, by placing the shop vac hose over the thru-hull fitting, and sucking all the water out through the bilge pump. This works great for when you want to clean any residual water out after a good cabin cleaning.

No overboard dump the valve i thought was dump is the water inlet, didn't realize it uses sea water when toilet flushes. On another topic what drive position vs speed are you guys running? Previous owner had position 3 for 3,000-3,500 RPM which rides nice but I didn't get a chance to open it up last weekend. What position should the limits stop at? Also seems the drives don't end up at the same height with the switch on the throttle, assuming one trim pump runs slightly faster than the other. Is their any adjustment for this? Appreciate the feedback I am learning as I go.

Common issue on all boats with one trim pump being faster than the other. There is no adjustment.
The only thing you could try is to run a lighter weight oil in the slower one.

I always just bumped up the slower one when I stopped trimming. On these boats, about neutral trim is fastest. Always trim for conditions though. There are no trim limiters on the HP transom assemblies. I trimmed out to max by measuring and checked the indicators and made mental note to not go over that.

Use the dash switches to fine tune one of the drives to match the other. I purposely ordered my throttles and had a trim switch on each throttle. Switch on left throttle controlled BOTH drives. Switch on right throttle ONLY controlled the Starboard drive. That way I could use my thumb for trim adjustments when running, but I could use my pinky to fine tune when I got to cruising room; all without leaving the throttles.

Measure and mark neutral on your drives, using a long straight edge. Mark this on your trim indicator window with a sharpie. DRock is right, these boats are set up to run neutral trim. You may only use 2-3 ticks above neutral at cruising RPMs; sometimes.

I've posted about this in the past, but this is the same for every boat.
Set the boat at neutral trim.
Leave the tabs at neutral.
Set the throttles at 3000 or 3500 or maybe 4000 RPM on a long straight, flat, not-busy section of water.
Then "play" with your drive trim switches while watching the decimal places on your GPS speedometer. THAT'S how you find the best/fastest drive trim setting for the boat.

Also, when you get more used to the twin engine boat, you can eventually leave the tabs alone for trimming the boat side to side, and use the drives to change the angle of the bow. There are other posts on this as well.
Short version: If you punch the starboard drive out a bit more than the other, it will lift the port side of the bow.
Trim tabs waste gas.

Good info thanks. Need to school the new guy what position is neutral with the drives and what exactly is being measured?
Drock you said you trimmed up to max and measured to make a mental note not to trim over that limit but what told you the limit? Thanks