Help ! Newbie, why won’t my boat get up and go.

I’m new to airboats and forums so excuse me if I’m not doing something right. I’ve spent several years building an airboat for my son. Got parts and advice from Chuck at Air boat Pros. He’s been very helpful, but I still can’t get the boat to go over 5 mph. 12’ aluminum hull w/ poly weighs 800 lbs, 4A084 engine, reduction drive, Mikuni carb, headers, IVO 64” adjustable prop. I can get it to run 3,600, presently put more pitch in and runs 3,300. It just won’t get up and go! We’ve lost all interest in Air boats, just want to get it operating and sell it. Can’t figure out what’s wrong. Any help would be greatly appreciated. I have a walk around video, but can’t get it uploaded.