Couple of thoughts to amplify what has been said.

First, IF your WOT AFR was 13.0'ish, I would not freak out. For a N/A motor that is not a death blow. Would it be safe to run richer initially? Sure. 12.5 is likely a good target but may not be the ideal final AFR for WOT operations.

Full throttle at the helm equals all throttle blades fully opened at the carb must be checked first.

Lastly, I feel the 1.36 drive and a 23" prop is a less than desirable combination. The blade tip speed at WOT for a 23" Mirage and 1.36 gears will be excessive in my opinion and inefficient. I know it's not cheap but 1.5 gears starting with a clean 21" Mirage + and working up to a 23" might be a better solution.

On a side note, would there be any possibility that either the coupler or prop hub might be slipping a bit? I know it's a stretch but something to contemplate.

I hope you get these issues resolved and keep us posted on what works and doesn't work. Best of luck.

If the coupler or prop hub was slipping wouldnt his rpm much higher ?

Scary part about 13 afr is a carbed single plane manifold setup doesnt distribute fuel very well. One cylinder might be 12.5 while the other at 13.5 etc. Thats why on a engine like buds is wanna error to the fat side

Even my port injected dually gas truck goes to around 12.6 at like anything over 75 percent throttle to help keep it alive pulling a trailer up a hill. And with port efi it has waaaay better fuel distribution

4 sizes up on all four corners and picked up a bunch of speed, never hurts to double check timing on the water either. There's nothing more rewarding than picking up double digit gains with some quality time tuning on the water!

Budman, I am feeling your pain here as it took me a long time to get my issues worked out. Went through the whole fuel system including new carb on mine plus quite a few different props. I am still not ecstatic about my setup but I was coming from a torque monster blower motor...an NA motor just will never have the same torque. Anyway, 1.36 throws me for a loop but it is easy to pitch down to a 21, before you do anything you could do a full throttle shut down and see how the plugs look. Going up a couple jet sizes is easy.

Well iam just finishing a old mirage intruder 217 and found a few suprises that is how much a hull can drag a boat down that has some weight in it. I was playing with 3 blade merc props and............. constantly getting high slip and the fact your slip remain's constant even at high plane or Wot suggest a high lift 4 blade........... and if you can only get 4600 with a 23 3blade on that combo with that TQ is crazy. My slip number's were almost the same as your's when i went to a high rake 4 blade the slip dropped to 8% and i finally got the boat dancing on it's tail

22' 4000lbs 1.47 gearing 4600 wot 23p 4blade ss the boats are similar in size but im running a tired old 330 454. Sounds like blade area and timing...running lean and hard and not a pop?

Yep, pain it is. I paid for the dyno session partially in the hopes of avoiding any major tuning issues. Thought it would be drop the motor in the boat and go. Not so much.

Coupler is new, and Prop is solid hub. Thanks for the suggestion, though.

All valid points and I concur with the hub/coupler issue. Just something to ponder. I'm coming from a port injected marine motor and often overlook the nuances of carb manifold fuel/air distribution.

The problem with pitching down in this case with 1.36 gears is that you are spinning a low pitch blade much faster (rotationally) and the hydrodynamic behavior is not optimum for THAT prop. It becomes less efficient.

Merc only recommends gearing up when you are out of prop selection, confuses me why that ratio would be on that boat. I can see it being stock on a cat...just not that boat.

In regards to the drive ratio / prop issue, I feel like I pretty much covered that one years ago with my 454. I swapped to a 1.5 drive with a 23 M+, and saw no difference in speed. I really don't think that's the problem here.

For comparison, my buddy is running the identical hull with an HP500 clone with Lightning headers, 800 Holley, Dart intake, etc. He is running a 1.5 Bravo 1 with a 25 Mirage Plus, and is seeing around 71 MPH. That is the range that I was expecting this combo to run in.