Why is my MJ1200 with SureFlow mod doing this?

[Delphinus]

The tab is there to ensure the prop spins in the right direction, since powerheads can spin both ways. If it starts up in the "reverse" direction the prop should suck itself outwards, and then hit that tab and bounce off and start spinning in the forward direction.

If it is constantly hitting it though, then something's up. One thing I have noticed with these SureFlow kits is that the shaft itself will corrode and fall apart. It looks solid until you pull it out of the magnet and then it's obvious that it's done for. I've found with mine I can't really get more than a year out of these Sureflow kits, but who knows, YMMV, I don't really get a lot of good luck with stuff in this hobby myself. But this is what I would check for on yours.