74 Mark Twain Saga (Long Winded)

I purchased a 1974 Mark Twain Runabout it sat for 5 years in a barn un-used. I have done a bunch of work to it before putting it on the water.
Here is a run down of what has been done.
Outdrive (pre Alpha): new bellows, new water pump, new shift shaft seal, new seals, new top cap bearing, new yoke new output bearing and lower seals. Trim cylinder bushings and one trim hose. Bypassed the trim indicator actuator. Used 19 pitch stainless prop.
Fuel System: Drained the fuel tank of old fuel, Installed new cartridge style fuel filter/water separator. Rebuilt the carb complete.
Ignition System: New Pertronix Coil and Electronic Ignition, New plugs, New Wires, New Cap New Rotor.
Cooling System: New Hose and Thermostat.

I'm still working the bugs out of her, and this is the current status after a couple of functional float tests.
1st functional float test. The the engine started to get hot 200 degrees(pre thermostat change). Pulled the throttle back engine flooded would not restart towed back to launch. Tore back into the carb didn't find any smoking gun reset the float level checked needle and seat found okay for service. Changed thermostat.

2nd functional float test. Topped off tank with fuel took the boat out and ran it at various throttle settings from idle to wide open and cruise for about 15 minutes didn't get to far from the cove all in all pretty successful for 15 minute of throttle time. During full throttle runs I noticed the tachometer way above the red line. Pretty sure the 140 couldn't spin a 19 pitch that high I figured the tach was starting to go wonky, and I put it on the to due list. The first time out the tach was solid at 4200 RPM WOT. Notice when I got back to the dock that the tank had vented fuel out of the vent drain on the port side. Not much but at least I know the vent was open. I did fill it up into the fuel fill neck. Won't fill it that full again.

3rd functional float test didn't go so well. The wife and I decided to head up the lake to our friends cove about 5 miles. Got out on the main lake opened the throttle to about 3/4's and headed up the lake. after 10 minutes or so the engine started loosing power I throttled back to about 1/2 throttle and the engine stabilized. After 15 seconds or so I throttled back up to cruise for another couple of minuets and everything seemed fine except the tach was way above the normal range. Entering our friend's cove I slowly throttled back to observe the no wake the engine started to load up and then died. Took about 10 minutes for the engine to clear the excess fuel it finally restarted and we headed back out of the cove a tad bit above the no wake speed as the engine did not want to maintain idle it took some jockeying the throttle and the neutral button to keep her running till I got out on the main lake. I experienced the same loss of power condition as on the way up the lake and stall out in our cove I was able to coast into our dock and restart the boat after about ten minutes and clearing the excess fuel from the carb.

It is the loss of power that has me concerned. Is Ignition break down prop hub slip or something else? Is the tach really telling me the truth? Ordering a new tach today.

pull sparks and have a look,sounds like engine is running very rich. Was fuel tank cleaned thoroughly before adding new fuel ,fuel lines flushed before new fuel was added ?? Needle & seat may have debris in them, causing flooding of engine,plugs and all your issues Your loss of power could be a engine running way too rich!

I would not be worried about the tach so much. You should be able to hear the rpms and if the engine is screaming or not revving up then you should know/hear it!!

But on the back of the tach should be a 4,6,8 selector rotary switch.

With engine running turn switch from 4 to 6 and back and see if that corrects the tach issue.

You run-ability issue sound like carb. You rebuilt it and now it does not function as it should. No blaming you just pointing out the issue.

When you throttled down and it started to "load up" that could be several issues.
First I would remove flame arrestor, try to repeat conditions and watch the choke butterfly. Sounds like it is closed or not opening fully.

If choke is open fully and not causing the loading up issue the the only other thing it could be is the carb and Something is a miss.....

If you had a slipping hub the engine rpms would increase with no increase in speed, Also if the hub was bad you would have very little if any forward motion and most likely overheat.

WHen you say "loosing power" that could mean many things........

did it start to stumble, or did it loose speed but motor kept running at high rpms?

if a loss of power occurred did the engine miss fire? any back fire? skipping? did it feel like running out of fuel? Or did it feel like it was flooded and was there any black smoke or a smell of un-burned gas???

Hard to give a specific answer until we get more specific observations..........