I have always been told that impellers flip to the proper direction within 1-2 revolutions of starting the motor.

I actually thought he was kidding about spinning the mains by rotating the motor backwards. If that's the case then I guess I'm screwed on mine.

They can, especially when newer....but.....I wouldn't count on it especially when older. Not all of us change them each year either.

I didn't even think about the impellers. My drives were not on when I did mine. Heck, this time the boat wasn't even on the motors!

I wasn't. I build a lot of things in a local machine shop here that a friend of the family owns and have always been taught never to spin the motor any direction but what it was designed for. At first I thought I was the butt of a joke, but I was amazed to see damage that resulted from it. Could that have caused the issue...yes. Did it in every case? Probably not. The only thing I base this off is one engine builder will rotate a crank either direction and he has a lot of work come back to have the crank turned and new bearings installed. Everyone else rarely, if at all, has work come back. Like I said earlier, to each their own. I'm sticking with what has worked for me even if it may be written off as superstition or wot not.

So without turning the motor back and forth against a stop , how do you find true TDC....Curious minds...

That one engine builder would have had as many come back even if he did not turn it backwards.

But heh, if you never want to find tru TDC, that's your choice.

BTW: When I put my 2 stroke in reverse, it is not done thru a transmission. The motor is let to almost stall, and then the CDI box fires it way before TDC to spin it the other way. Genius ! A 2oz added weight reverse mechanism.

To the OP, I'd be looking at the fuel and/or fuel system. It sounds like water in the fuel. The only time I am aware you need to be conscious of not turning an engine backward is when degreeing a cam. You need to make sure the lash in the timing chain is in the running direction when picking your degrees off the wheel or indicator. If you overshoot and want to backup to land on a mark you may induce inaccuracy. Its best to go all the way around or back way up and come at it again. Maybe that is what they meant by don't turn an engine in reverse by hand?

So, results are in. I bought a correct balancer and put it on. Eyed the timing, It fired on the first revolution of the crank. I set up the timing, took it to the water a couple days later and started running it. I'm doing the after break-in of not staying at the same RPM for extended periods. The motor runs nice and strong. I'm happy.
I also put on a stainless vented Apollo 3 blade 14.75 x 21" prop this year after running a long time on a 23 pitch Michigan wheel aluminum 3 blade. Holeshot is nice and lively even with 800 lbs of people and 200 lbs of fuel. I didn't hammer it, just rolled it on.
I didn't get any flat water or a chance to run wide open, so I can't say how much speed is there, but I'm going to compare both propellers.
I did get a couple of chances to crack the throttle, think I heard a little secondary moan. I miss the old cars...
Does a TB IV have any sort of rev limiter?