I recently picked up an 06' STX-12F, I was wondering if there were any aftermarket parts for these to increase horsepower and top end speed. I was also wondering if there was some sort of a governer on these to keep it from going over a set speed.

 

If you want a faster boat, up grade to the 15 F.

If you want a much faster boat, upgrade to the new Ultra 250.

A 12F is a great pwc. Don't get tied into the lake race BS. All of the current 4 strokes are within 8-10 mph of each other. You've got one of the most reliable crafts there is.

Upgrades:

Ride Plate 2-3 mph
Impellor 2-3 mph
Air Intake 1-2 mph

$500-600, plus labor, to increase your craft by 7 mph.

 

STX-12F
There is no governer on this machine. That's all you can get out of it. Upgarding to 1500cc is not easy either. At least $4000 for the parts alone because 1500 uses different shaped piston (but same bore) with longer stroke, different ecu, larger throttle body, different intake manifold, etc. Just keep it well maintained and it will serve you a long time.