Can't find THE answer, Rotary Valve 720

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Sebastian Bluecrew

Sebastian Bluecrew

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Hey guys. New to the forum, but I have read the forum for years..

Long story short..
I have a 1997 Seadoo SP with a 720... Completely rebuilt, new everything, overbore 1.0mm, Factory Performance Pipe spec 1 with ECWI and rev limiter, Solas 16/24 Concord, AND, dual 40mm carbs...
So the stock machine was single 38mm carb. But with what I have read on the forums, the FPP will never run on a single 38.. Also read about the rear piston failure and detonation with this pipe so thats why I picked up dual 40mm carbs from an XP800 and rebuilt.. Jetting on these carbs 142.5/70, low screw out 1-1.5, high I tried 0, 1/8, 1/4, 1 turn out and it made no difference.

Here is my issue, before I keep going through these carbs over and over. It is not running right, no WOT or top end fuel circuit at all, falls on its face and once in a while will jump for a second off idle when i throttle it only half way(And man when it does, what a hit that is) then it falls out again.. On the trailer, this thing sounds nastier than my banshee!

This is the answer I cannot find.... The Seadoo manual says this 1997 SP is a "717D" and lists a completely different rotary valve duration and timing.. I am understanding that this SP was limited on engine power by using a single carb, different port timing and a shorter duration valve. Nobody has ever heard of a "717D" and I called SBT, I called a couple local shops... They all tell me there is no such thing. I had one shop pull up the fiche, and he says" Wow, that s funny, Seadoo shows a "?" next to the valve timing on the actual fiche for that year.." He said he never in all his years heard of anything other than a 159 degree valve for a 717/720...

I am using the stock 132 degree valve that came out of it, timed it 4 times, but when I changed to dual carbs does that mean I have to change the rotary valve to a different duration? OR, can I put in a 159 degree like every other 717? I think the jugs are specific to this one very model and probably has different port timing to accommodate this valve duration.

Thanks for the help..
 
The difference is in the rotary valve. I don't think they altered the cylinders on those. Run the dual carb rv with the dual carbs and it should come alive.
 
Matt Braley said:
The difference is in the rotary valve. I don't think they altered the cylinders on those. Run the dual carb rv with the dual carbs and it should come alive.
Click to expand...
What is the proper "Dual carb" RV? From my understanding the RV has nothing to do with single vs dual carb, but rather port timing....
 
My guess is the larger carb throat doesn't resonate well with the smaller effective port?

I'm curious to know the ATC angle the port is closed, my guess is around 65 degrees?
 
And I'm debating to switch back to 38 dual carbs, but I have read nothing but disasters with the factory performance pipe with anything less than 40mm carbs... that is why I bought them... So I either change the RV and keep these carbs to try it, or, go to 38mm and keep this RV.. Not sure..
 
I believe the 717D has different port timing, retarded ignition advance, the 132 degree RV, and the factory rev limiter set at somewhere around 6000rpm. I hate to say it, but you may need both the 159 degree valve and some 717 (non-D) cylinders. That still won’t correct for the ignition timing either... I wonder if an MPEM for a 96 SPX could be made to work with the correct ignition advance... I’d have to compare wiring schematics to see if that would even be an option.
 
Sebastian Bluecrew said:
What is the proper "Dual carb" RV? From my understanding the RV has nothing to do with single vs dual carb, but rather port timing....
Click to expand...

IMO, the RV design due to it's wide power band affords a wider variety of port timing options, more than say, reed and especially piston port would. So I disagree the RV depends on cyl port timing, but more to do with carburetor bore and where the power band is.

http://www.amrca.com/tech/tuners.pdf
 
JeremyD615 said:
I believe the 717D has different port timing, retarded ignition advance, the 132 degree RV, and the factory rev limiter set at somewhere around 6000rpm. I hate to say it, but you may need both the 159 degree valve and some 717 (non-D) cylinders. That still won’t correct for the ignition timing either... I wonder if an MPEM for a 96 SPX could be made to work with the correct ignition advance... I’d have to compare wiring schematics to see if that would even be an option.
Click to expand...
Could I adjust the timing on the stator by a few degrees? Also, I have the rev limiter installed for the Factory Performance Pipe, I think that raises it to 7200 rpm??
 
You’ve got the rev limiter covered. I’m not sure about the ignition advance. The timing curve is programmed into the MPEM, and I don’t know enough about the “D” motors to even begin to tell you what that curve looks like. I’m not even sure if it has timing values loaded for that far beyond the factory rev limit. Matt knows way more about modifying these things than I do, so take his advice before mine. You may be able to pop a 159 valve in and be good to go. I’m just trying to go through what issues you might run into if that doesn’t get it where you want it to be.
 
All good info, thank you.... With what we know about this thread so far, could the incorrect RV cause it to run flawless on trailer but be nutless on the water? I'm going to order a 159 RV to give it a shot anyways..

Matt, have you ran a FPP? Did you stick with stock carbs or did you go more fuel? Should I keep trying to get it working with the 40mm duals that I added or go back to stock 38s?
 
Running on the trailer will tell you absolutely nothing other than it runs. You have to have a load in it to tell you anything.
 
Yep, if it runs on the trailer that only means it's time to splash the hull. It could even seem to run crappy on the trailer and that still wouldn't tell you anything other than it runs, which is what you need to know before splashing it.
 
One more thought, I wonder if the rev limiter is somehow creating an issue but I'd expect you'd find that occurring on the trailer as well.

Running out of fuel is another possibility, if say for instance (for whatever reason) the fuel pump pulse is too weak.
 
Until the rotary valve is swapped, there’s no telling what the issue is. The ski has some parts configured for top end power and some configured for bottom end right now. Get that 159 RV installed, and then we can see where everything sits.
 
You also have to understand that you are in uncharted territory since that engine has different port timing than all the parts that are bolted onto it were made for. It probably isn't ever going to run to full potential with those cylinders.
 
Hey, I am in the same boat as you are, I recently acquired a 1992 xp with a 1997 sp engine in it, the computer and wire harness was also from the sp, the original owner had butchered it up so bad I decided to find a donor ski. Found a 1996 gti, swapped out the wire harness and computer, new key programmed fired right up. Took it out for a test and sucked lol no power nothing, figure it was the carbs. At the time I never knew about the 717D, the more I looked into it the more I realized everything needed to be swapped over, so I have put the exhaust pipe on from the gti and now I have the dual carbs rebuilt from the gti, will swap out the valve over while I have it all apart hopefully by next weekend I can get it out and running and let you know how it goes
 
I’m curious, so please come back and update us...

Another thought, just to see how different the port timing is, would be to compare the ports to the template in one of the old Sea Doo Racing Manuals. I believe the port templates show what the stock ports should look like on a 720 compared to a “race port.” It would be interesting to see how far off the 717D is from the standard 717.
 
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