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seadoo xp 787 will not rev

Randyrandy

New Member
So:
This has been discussed ad nauseam here but I can not figure mine out. I suspect that you all are tired of this discussion, but I have been working on this for a very long time with no success.

The Problem: Revs fine on the trailer. Will only go maybe 5 MPH in the water. When first started it jumps up on plane, then very quickly (3-4 seconds?) slows to estimated 5 mph. It will idle all day and putt around all day. I guesstimate that the revs are maybe 3000 or less.

When the problem occurred, (reported to me, I have never been able to get it to run properly) it apparently happened suddenly while riding.

What I have checked:

1. swapped carbs with known good - traded with my other ski. The other ski runs like a scared cat with the carbs from the machine that will not run properly.
2. replaced fuel lines and cleaned filters in the carb and in the line - no change
3. bypassed the fuel selector, pulling fuel from the reserve connection to the tank. no change
4. unplugged the voltage regulator - no change, then replaced with known good - no change.
5. rebuilt top end. compression 150 both cylinders
6. Rave valves rebuilt, also swapped with known good
7. wear ring not excessively worn (if this was the problem it would rev too high)
8. Inspected the rotary valve. looks new. bad rotary valve usually makes hard starting as well. This one starts right up. did not check timing because it initially revs when first in the water, then it doesn't. If a bad rotory valve timing it would never rev.
9. inspected pulse line. no apparent cracks or leaks. a leaking pulse line would cause the machine to run lean. Pulling the choke kills the engine immediately which could indicate it is running rich, but certainly not lean.
10. drew a sample of the fuel. No water, no discoloration.
11. plenty of oil

Current known or possible suspect items:
1. speed sensor wheel is missing, shaft is there.
Q is there an over speed limiter?
2. mpem, ignition module, other?

What think you? what do I do next? I am thinking to swap ignition modules, however it is not clear where the rev limiter resides (ignition module or mpem).

One of the things that is really bothersome about these forums is that very rarely does someone who started a thread indicate what fixed the problem. It is just left hanging.
 
Just a stab in the dark here, but since you have really pretty much eliminated most other possibilities. Would it be possible that the trigger coil is dirty or improperly gapped? Not even aure if they work like that where they will provide a good enough spark to ignite fuel mixture when not under load but not strong enough spark when under load. Like I said I’m not even sure if that’s how it works or not but you seem to have good compression and fuel so I guess that leaves spark
 
I would also check the waterbox and see if a baffle has broken loose and also the big rubber hose from the waterbox out the back of the ski. The big hose can deteriorate on the inside and choke the engine out like you are experiencing.
 
I forgot to mention that I took off the hose from the water box to the hull exit and inspected. Did not see anything that looked like hose deteriorating. Also swapped the water control valve. What am I looking for inside the water box?

I will recheck the hose

Also is there a speed limiter associated with the paddle speed sensor?

Sounds like a rev limiter to me.

Thanks for the replies. I have had this thing for two years and am getting ready to swap engines between the machines.
 
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No, nothing to do with the speed sensor and no speed limiter.
You could be electrical now, coils, pickup coil MPEM.
 
Should be some static tests for the stator and pick up coil in the manual, not sure about these seadoo’s but for an Arctic cat 4x4 you can also check the stators output AC volts. something maybe to look into
 
I read somewhere that there could be a crack in the stator housing that could cause this. I will start down this path.
 
you mentioned the "inline" fuel filter, are you talking about the stock one or is there an aftermarket inline one?
 
you mentioned the "inline" fuel filter, are you talking about the stock one or is there an aftermarket inline one?
There should be a fuel filter under the front "hood" just behind the pull out bin. Make sure its clean. There are also fuel filters in the carbs. You said the swapped carbs were from a "known good" machine? How do you know? Did you check the needle valve positions?
 
you just don't want one that is aftermarket and inline in the fuel line. I had a Polaris that I put one in years ago thinking that it would be a good idea but turns out it would not get enough gas at high throttle and would act like it was running out of gas.
 
In line fuel filter is the stock strainer is more like it.

I know the carbs are good because my other machine was running fine, and I took the carbs off that and put them on the broke machine with no change. The other machine ran fine with the carbs taken off the broke machine.

I will be trying the ground, and inspecting the water box next.
 
The only thing I could think of is the MPEM malfunctioning if you got a spare one to swap out you could try that 🤷‍♂️
 
Man I wish you lived close to me I love this shiznit. LOL

Let me ask a silly question as this issue recently kicked my azz. How rigid are your fuel lines? If your fuel lines flex too much the fuel pump doesn't work very well. I had a GSX with new fuel lines installed which I didn't replace when I installed the new engine. I had a high old time testing, testing, testing checking checking and checking. Always something new to learn. Ha ha. Good luck !!
 
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Good thought.
Fuel lines are the black 1/4 inch rubber fuel lines just like I have used on all my skis.

I got this ski real!y cheap. Previous owner gave up on it.
 
Getting out the box, have you checked the magneto and RPM pick up on the mag cover? Have you removed the mag cover and looked inside? I've seen them get funky inside that area. That trigger can go bad. You could have problems with the coil breaking down. You are too the point where you'll have to start chasing down individual wires and plug connections. These skis are sneaky. Good Luck !!
 
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