I know there are a bunch of threads out there similar to this, but a lot of them have found solutions and I'm still not sure what to do next.

I recently purchased a used Go Devil 23HP Vanguard, which seemed to be taken care of very well. Roughly 160-165 hours.

I finally got it on my boat on Monday and started it up. Fired up on the second crank and got it to 3850 RPMs. Then I take it to the river and hop in with a few friends (450ish lbs) and I can't get it over 2100 RPMs in the water. It just totally bogs down when dipping the tail in.

So following instructions from what I have found on this site, I did the following:
1) Dump the gas, previous owner said it was from last September. 4 gallons of 93 octane with half a can or slightly more of sea foam. Ran it in the driveway for 10 minutes. Take air filter off, try to dump a little of the sea foam in the intake, run for 2 min. Put air filter back on, run for another 10 minutes. Got 2 more gallons of 91 octane to make the mixture a little less, "rich".

Went to the river got it up to 4150 RPMs, seemed to be running better, but same outcome with the same load. Roughly 2100 RPMs again.

2) New spark plugs and top off the oil. 2 new NKG spark plugs and added more Royal Purple oil. Same result. 2100ish RPMs, maybe a little more by myself. MAX 24-2500 but still only going like 5 mph. Spark plug wires look fine, engine idles and sounds much better than when I started, but it was already pretty good, just bogs down to 2100 in the water with a load. If I'm by myself, maybe 2500 for a little but the previous owner said he could run it 2800-3000 RPM depending on the load.

Previous owner said he replaced the fuel filter a week before selling it to me, Spark plugs in January (did it anyhow), carb I believe was cleaned last August.

So what's next? The duck boat shop up here is closed until Monday so I have the weekend to try and fix it. I don't know much about the engine yet but with help and YouTube I'm willing to learn.

My ideas: air filter (old but not too dirty, tried to replace but AutoZone didn't carry) but this shouldn't make a huge difference. Prop measured 9" exactly (he also said this was a year old) so that wasn't surprising.

Appreciate the help. BTW it is on a 1654 ModV Nova UltraJon. He had it on the same size GatorTail boat.