Info on the 720 engines

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riffe723

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Here’s what I have ran into. In my previous post I posted about a speedster SK I had bought. The SK has the twin 717/720 motors in it. Later in the thread I posted that my parents were going through some issues and were splitting up and I was given two 1997 SeaDoo SPX and a double trailer. I only wanted one of them originally but they gave me the whole package.

Looking into the boat I had to do some carb adjustment and I noticed that the 720 in the boat have a different manifold and only a single carb per motor. However I’ve worked on the spx for then before and both of those skis had the dual carb set ups. Just interested as to why they did that, and has anyone put the dual setups on the boats? Do they make more power with duals? Obviously I’m thinking single carbs in the boats are related to fuel efficiency.

Like I said. I’m just wondering, does this stuff switch over, and is there a benefit? Likely leave them as is, just curious.
 
This is an older picture but it shows the condition of the SPX’s. Such nice skis for 97 and I know the 720’s were desirable.
 

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Well from my search the spx can’t be a 720 it should be a 782 or something. They are quite different so that must be it.
 
Now my question is why am I seeing some spx with powervalve engines and my spx does not have powervalves.
 
The early SPX had the 720 engine. In '97... the SPX got the 800.

As far as single vs dual carbs... I couldn't tell you why they did that... other than to keep price down. (few $$$ in production = a lot on the sticker) The engine is still rated to the same HP on both, and since the engine has a rotary valve, and only one piston draws at a time... it's not over taxing the carb.
 
From Group K (Klemm Research): Sea Doo Single

Carburation - There are a wide array of high performance carburetors available for the 718cc and 782cc powered pwc's. The main thrust of these carbs is to dramatically increase acceleration and throttle response on pwc's. Along with those increases can come considerable increases in peak rpm ability. While these are all wonderful qualities for a pwc application, they are less attractive in a single motor 14.5 application. Since a perfectly set up 14.5 pump is already cavitating when the throttle is applied quickly, an increase in throttle response will only lead to increased cavitation...not increased initial acceleration. Since the stock carbs and arrestor are easily capable of delivering enough fuel and air for the desired 6800 - 6950 rpm peak, the improved rpm ability of bigger carbs would be "the answer to a question that is not being asked". We do not intend to say that "high performance carbs are no good". However we will say that they are "very" questionably worthwhile in a 14.5 application. What is unquestionable, is the significant increase in fuel consumption that most high performance carbs would net.

All this said, There could be merits to a slight increase in the throat sizing of the stock carburetors in a "sport" application. The 40mm carbs on the 782cc models are actually 37.5mm. Group K offers a modification to these carbs to make them "true" 40mm carbs. This slight increase in bore size adds significantly to acceleration off the turns, without causing significant reductions in "signal" or fuel range. The 38mm carbs of the 718cc models can be bored to 40mm as well, however the signal characteristics and throttle control of this setup are not as good the true bored 40's from the 782cc motors. For 718 owners wanting bigger carbs, we recommend using a set of true-bored 782 cc carbs...they bolt right on to the stock 718 manifold and flame arrestor.
 
1999 speedster sk is not a 14.5 application, it is a 16.8. However some good info thereveithout a doubt.
 
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