Check also the needle and seat for the carbs. You said that the carb had some gritty material in them, and that gritty material could have blocked the needle and seat. I would use air to blow out the carbs, and you can use a clean white paper towel to catch any debris from the air to see what was in the carb.

Fuel is what cools the piston down and prevents the pistons from holing, check the fuel lines for any leaks kinks or blockage in the fuel pump outlets carb inlets. It seems like it starved from fuel and its been running on oil.

Check to see that you don't have a clogged check valve in the vent line! That could cause fuel starvation in the carbs.

incorrect spark plug heat range can also contribute to the lean condition that burned those pistons .

@zech007 That is true. I am going to to a proper index test today and video tape it

I’m new to jet skis but an automotive gear head. I’ve owned my own speed shop and teach engine building classes at a tech college. I just purchased 2 Polaris skis that are supposed to have bad stators. How do I check them, is there a good manual to get, where is the best place to get parts? Thanks in advance.

I have Polaris slx 99 the problem is fuel pump is bad so I need mix the oil and gas together before put and the tank sometime i have a problem for get star I luck and online about the fuel pump but I can't get they not make anymore the say