@Matt Braley would be able to tell you why it was used in 1995 and not after.
I can only tell you the 1997 SPX is not a 720, it is the 787 X4. The 720 SPX stopped being made in 1996.
Not sure if your talking about the SPX, or the 720, more commonly known as the 717cc, turning out 85 horsepower. As far as I know, for sure, that engine was still being used in the 2000 year model skis. The 2005 GTI, GTS, HX and other models, were the last models to run that motor. That was also the last year for it to be produced.
The original thread, pump extensions? This will likely depend on who you ask. In 1988-89, when the first Seadoo came out, it was equipped with the 587 engine, and was really under-powered, so it was pretty much a get on and have fun kinda ski.
In the years that followed, the 717cc (or 720), was the first engine that really had any gusto to it, but without the RAVE valves, was still a bit under powered, IMO. When Rotax patented the RAVE valve, they pretty much took the 717, added a balance shaft, and with the addition of the RAVE valves, increased horsepower from 85 to 110hp. When you think about power in an engine, it does not matter how big or how much fuel you put in an engine, if you cant allow that engine to breath out, your doing nothing but adding back pressure.
Now, to get back to the pump extensions. These really made a big impact before we had VTS, because with the added power of these engines, the front ends wanted to come up, porpoise and cavitate. Before the extensions, we took washers and added them to the top bolts, to angle the nozzle down a bit, to help trim out the ski. When the VTS came out, that changed the ball game.
Where the pump extensions really make a big difference is on the V-TEC, 255 and greater engines, since manufacturers were mandated to not design an engine to operate over 70 mph. That was so easily gone around by impellers, pump extensions, wedges, changes to the resonance, and a butt load of other high performance parts. The actual extensions are almost, a thing of the past.
If your still running an other model ski, you don't really have to invest in a pump extension, because your motor doesn't make the horsepower to make it beneficial. If you can to try a few things, take off your pump, and put a couple washers behind the two upper pump bolts. This will drive the nose down just a little, giving you a better plane. If you want to see if you can bring the hull up a bit, ride more on the pump intake and nozzle, put a couple washers on the bottom two bolts.
Here, someone might say, "that'll take your shaft out of alignment". No, it actually won't. If you've ever taken that shaft out, you'll notice it's kinda rounded at the ends. This is designed that way, to take up any small variables on pump alignment.
