2002 SSV Supra - High Idle - WOT Bogging down

[INSANEREINDEER]

I've been helping my brother with his boat he bought last year (750 hrs). We've replaced everything the previous owner failed to do, got it cleaned up, and have almost everything working. The boat has a 350 Indmar Assualt TBI.

We are still having two major issues that I can not figure out.

First is a high idle. It cranks up an runs at 800, then after putting it in gear and driving for a bit, it stays at 1000-1100. I have yet to verify the throttle cable isn't sticking by pulling it off, but it is always resting on the stop so I don't think this is the issue. The engine needed a new IACV when he purchased it, so we replaced the IACV and TPS with one from an LT1 corvette from a parts store rather than Indmar. I also suspect this could be the issue, but doubtful with my experience in GM parts. I am going to get the parts we pulled from it, and verify the IACV is the same length. Yesterday, we also tried to swap it out with a MAP sensor I had laying around with matching P/N. No change. The only gasket/hose we have not replaced is the intake manifold to the head. Spraying carb cleaner around it doesn't change the RPMs. TBI gasket, TBI adapter plate gasket, PCV hose, CTS,& MAP hose all replaced. Flame arrestor was cleaned as well. We have hooked a laptop scanner to it my buddy had and all parameters seemed to be within reason. Where should I look next? I'm running out of ideas and I'm am very familiar with GM engines and EFI tuning.

The other issue is if you're idling and slam WOT, it bogs down and will die. I need to check the base timing. I also need to verify fuel pressure which I read was supposed to be over 40psi. Is there a screen in the tank on these boats that could possibly be clogged? It runs fine other than slamming WOT. Also starts with no issues.

Other than that, my last place to check is a compression test but that wouldn't cause the problems from my experience. I know it doesn't take much to make the TBIs run correctly, but this one is being stubborn.