The Polaris machines do not have a switch or setting to limit speed like the newer machines. Polaris stopped producing watercraft in 2005 I believe.
It looks like the 780 has an actual water separater bowl which should be checked, as well as a small white inline filter.
The following listing is from a 96, I bet the 97 has a similar setup. I posted it to give you an idea of what to look for under the seat.
http://www.ebay.com/itm/96-1996-POLA..._Parts&vxp=mtr
Beyond that, look to the main jets in the carbs. Its cheape/easier than getting into diagnosing electrical problems.