hxseadoo95: I spent all day on mine again today and I have the exact same thing you do. I took my jet pump out. The thing is defiantly not partially seized, it turns no problem. With it off, the engine still does the same thing. However today, I actually got it started! Three times. However it won't turn over every time. I would say every 5 times it will get going. And this is without the additional load of the impeller. It clunks (piston compressing), and then struggles for another second, clunks and then might start cranking.
One thing I learned was to leave your finger on that starter button even after it stalls. It seems to take a second and then will turn over one more time. Before today, I was not holding it down as I thought I was stressing the motor too much.
I am using somewhat cheap jumper cables from my car battery which gives me the best luck of all 4 batteries I've tried. I'm going to get heavier gauge cables (6 AWG) to see if that helps.
Again I have a cheap aftermarket starter bought on the internet but the result is the same as the OEM which I now believe is good. I would be very interested to know your result if you replace your starter motor. I would defiantly spring for the OEM motor to eliminate any doubts. That's where I am.
I'm starting to think its partially a design problem. The starter just is NOT big enough to turn this engine over easily. Once it gets started its ok, but the initial torque is just marginal. Any slight additional friction, load, etc just kills this thing. Even when I pore a tiny bit of gas down the cylinder its not strong enough to turn over any more. That's why i say to spring for the OEM motor as you need every tiny bit of torque you can get. I see these "high torque motors" on the internet that I wonder if they are really any better than the OEM.
Keep us up to date with your problem... Are you thinking of replacing your motor, or what next?