Can't find THE answer, Rotary Valve 720
- Thread starter Sebastian Bluecrew
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JeremyD615
Well-Known Member
If you do a google search for 1996 Sea Doo Racing Manual, it should come up as a download. There is a section on porting with templates if I remember correctly...
JeremyD615
Well-Known Member
Yeah, I guess I’m looking at it now and they have a porting map for the 787 in some of the racing manuals, but I don’t see one for the 720. That’s too bad, it would have been interesting to see...
JeremyD615
Well-Known Member
The length looks fine. You’re not seeing any fuel flow through the lines? I would check to be sure the fuel pump is sealed and the bolts are tight on the cover. Of course you’re also going to want to check the selector and water separator. Pull the bowl off and see if it is full.
I have emptied it and its not pulling any fuel into it, still empty after cracking the engine
JeremyD615
Well-Known Member
Well you’ve got a massive air leak somewhere then. Trace the line all the way back from the carb inlet and see if you can find something loose or obviously wrong...
Agreed. Either the hoses are swapped somewhere or it's pulling a lot of air in somewhere.
JeremyD615
Well-Known Member
You’ll feel a pulse from the pulse line. It will go back and forth between positive pressure and vacuum... If your pump isn’t even filling the filter bowl, it’s going to be something more than a loose clamp. It would have to be a major leak. Do you have a way to pressure test the fuel system from the carb inlet all the way back to the baffle? If you have a popoff tester, you can use that to test...
I believe a friend of mine has one, I'll do that, with the motor off it should hold pressure? Or hear for a leak?
JeremyD615
Well-Known Member
You are going to pull the line off of the carb inlet and the fuel baffle at the tank. Plug one end of that line, and pressurize the other side to about 4psi and see if it holds pressure. You can do the same thing with the return line off of the carb.
Before you do that though, double check to be absolutely sure all the lines are going to the correct places. It’s far more likely that you’ve got a line crossed than a leak large enough to cause a complete loss of pump pressure. If you look closely at the ring around the fuel baffle, you’ll see that it has “on,” “res,” “ret,” and “vent” engraved in the plastic next to each nipple. Trace those lines and make sure they are all going to the correct places.
Before you do that though, double check to be absolutely sure all the lines are going to the correct places. It’s far more likely that you’ve got a line crossed than a leak large enough to cause a complete loss of pump pressure. If you look closely at the ring around the fuel baffle, you’ll see that it has “on,” “res,” “ret,” and “vent” engraved in the plastic next to each nipple. Trace those lines and make sure they are all going to the correct places.
Glad you got it worked out.
R-u-n2it
Active Member
Did you use aftermarket flame arrestors along with the new carbs and pipe? You know the RV valve cutout could be referred to as port timing and I'd be surprised that the Rotax corp would invest in new tooling/CAD/machine programming on a 2 stroke engine cylinder design and not just detune with flashing timing, single carb, and shorter port duration RV on an engine that is 3 generations down from the state of the art and the 951 in pre production development at the very least, they could read the writing on the wall with emission legislation ramping up... anyways larger throat carbs mean lower manifold vacuum, which is what runs the fuel pumps and pulls fuel through the metering jets, the time you said it tried to take off sounds like when aftermarket arrestors are installed and nailing the throttle causes the engine to nearly stall but feathering the throttle at the position where it hesitates can sometimes get enough vacuum to run the fuel pumps and enough fuel pressure to pop-off and the ski will take off, as soon as you let off it will fall on it's face again when you nail it. Test by restricting air intake at the arrestors or drop the pop-0ff spring to the next softer spring, don't just mash the spring to make it shorter.... wink, yes it's shorter but not softer but have seen it tried and worked enough to tell you you're on the right track. Or if you have chokes in the carbs for some reason. If you have a primer try a prime the same time as you throttle up. You still need the 159 RV. A larger cutout is like a longer duration cam on a 4 stroke. A 720 is like a short stroke 787 and no Counter balancer so the crank will have lighter weight on the throws. Keep pluggin away and reply with results, always something to learn. Lee
R-u-n2it
Active Member
Hey, what's with the forum? Just saw the reply saying good, glad you figured it out, then middle of posts a little box saying there were more replies, and they were in between existing replies....
Oh, psage504, I cleaned the little screen out once, reassembled and was worse, disassembled and black paint flakes in screen after just cleaning, DANG factory painted the carb black and painted part of the brass tube where fuel line goes, so when I pulled off fuel line the layer of paint stayed stuck inside the fuel line, installing fuel line scrapped the paint off the inside of it and it went directly into the sreen filter I just cleaned. Now I remove all paint from fuel line connect points. 1 smart fellow, I smelt far..... wait.... Lee
Oh, psage504, I cleaned the little screen out once, reassembled and was worse, disassembled and black paint flakes in screen after just cleaning, DANG factory painted the carb black and painted part of the brass tube where fuel line goes, so when I pulled off fuel line the layer of paint stayed stuck inside the fuel line, installing fuel line scrapped the paint off the inside of it and it went directly into the sreen filter I just cleaned. Now I remove all paint from fuel line connect points. 1 smart fellow, I smelt far..... wait.... Lee
Sebastian Bluecrew
Member
Problem solved.. 159 degree rotary valve woke it up.. And also, there is no difference in port timing on the D model. It is "Detuned" with the 132 degree valve and a single 38mm carb... After the dual 40mm carbs set to 44PSI popoff and a 159 valve, its nuts... the part numbers on those jugs are the same exact for SBT replacements, Group K also confirmed the part number..
noviceskier
Active Member
717D as in detuned, has different valve since it's single carb, than reg with dual carb setup. Swap it with 159 deg valve and you should be fine
You mentioned you've been going through carbs over and over, did you use mikuni rebuild kit? Or ebay.
(Ditto, just checked page num #2 to find you found the issue, glad to see you have your doo running. Besides popoff did you do anything else with the carbs?)
Sebastian Bluecrew
Member
Not really, I did use SBT kits this time, and the jetting is 142.5/75 at 7/8 turn out on high and 1 1/4 on low
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