I usually put the tabs down 3/4 of the way while getting on plane then I bring them all the way up when on plane and adjust my out drive trim up to gain optimum ride and speed in calm water. When it's rougher conditions I run the tabs down a touch at a time till the bow comes down some and gives me the best ride.

I built up a 500 efi and fine tuned it on the water in a steppedd hull 27 fountain, boat ran 74 to 76 when it was stock depending on how much fuel and passengers, runs 77 to 80 ish now depending on load. I havent seen many single engine 500 efi boats run upper 70's, not sure if a 288 is a "fast"hull or not but my first question is what do you have for a prop, if you tell me 26 non labbed four blade theres your prob, takes a good 600+ hp to turn a 26 on a v hull and at that usually needs to be labbed. So you either have a fkd up tune, hurt motor or too tall of prop if boat wont rpm (aside from putting extra batterys and passengers in cabin/bow or something blatantly obvious), Smitty

Is that a GPS speedometer you are getting your numbers from?

Have you ruled out a bad batch of gas? Checked the fuel separator for water? I'm just focusing on the performance before the oil purging. Other than blow-by around the rings why would a crankcase pressurize so much?

Fwiw - I'm just a shade tree hack with the wrenches myself just here to learn.

I don't intend to ruin your day however I would recommend doing a compression check. We had the same issue on a 500 efi that was just rebuilt prior to us buying it, motor ran great but would occasionally push a little oil out dipstick. Did a compression check and number 6 cylinder was down just a bit. Pulled motor and had a chipped piston and small portion of ring missing. Id start with a compression check, that will point you in right direction.

Since i started flowing and cleaning injectors this is a re-ocurring story, 15+ year efi motors, never had injectors flowed, tested or cleaned, 1 low flowing injector kills a cylinder, lots of oil blowing out breathers because of excessive blowby on a hurt piston

It does have a 26p prop non labbed.

Do a Compression Test and a Leak Down Test..... You have to find out if its healthy , if its blowing oil out the dipstick , its either to much oil in the pan , or excessive blowby from a bad cylinder.
Also pull the oil filler cap off the valve cover, if its puffing pressure out like a steam train , you've got problems.

Well you read my previous comment on that before you even told me you had that prop I haven't seen a 500efi turn a 26 non labbed in a big single V to where it needs to be like 52 or 5400 you need a labbed 26 prop at a minimum and even then you'll probably only turn 5200 but that's part of why it's only turning 4800, if you're coming too hot boat I see you're in Michigan I can bring a 26 lab for you to try

I can also tell you if your motor has a stock ECU at 4800 it only gets 28 degrees of timing and jumps to 36 degrees by 5200 maybe even at 5000 I don't remember but I can tell you if you don't hit that 5000 RPMs and jump to the 36 degrees at 100 map the boat will never pull