787 WOT RPM Forcast: 6900 and falling.

[Krispy]

Well, Over the weekend I shook down my XP and my GSX.


I noticed this problem with my 96 XP. The engine is stock, but I do have a new Solas XO and R&D 85/87 nozzle setup.

I have the carbs set at factory settings, and I am running 89 octane ethanol free fuel, using XPS oil and oil injection.

I went out and putted around the no wake for a while, and then started cruising at about 50-60% throttle until I had nice warm water out of the tell tale at the rear of the boat.

After that, I decided to go for some WOT runs, to check RPM and Speed. My first run was 55.8 mph at 6920 rpm. Pulled strong until I let off.

About 10 mins of cruising later I lined up with my buddies 2011 GTI 130 SE, we got into an informal top end race and I watched my RPM, I was turning about 6900 at full speed, then my RPM dropped. It was as though I had lifted off of the throttle slightly, At that point I was turning 6500 or so RPM. I released the throttle a little, no change, I squezed back to WOT, still 6500 rpm. At that point I turned around and headed back to the dock.

I once on the trailer I checked the plugs, they looked OK, with the PTO looking a good brown, and the MAG ever so slightly light brown.

It seemed as though the engine and exhaust was a bit hotter than normal during my plug change, but I am going off of what it felt like last year so I can't be too sure.

Any idea why I might be loosing RPM after driving a bit?

 

[Matt Braley]

Water regulator or raves leaking

 

[Krispy]

Water regulator or raves leaking

Thanks Matt. I just replaced the diaphragm when I did all the "work" on the ski. How can I verify its function etc?

I have the gaskets for a RAVE cleaning, what failed part would you expect to see in the RAVE? Or just non-smooth operation?

What is leading you to those conclusions (other than experience) how do those things effect that missing 400 RPM?

 

[Matt Braley]

The hotter pipe hints to a faulty reg. When the bellows blow the pipe gets hot enough to burn the coupler between itself and the water box and the pisser line at the top will melt at the fitting. Check with the seat off for leaks.

The Rave valve bellows can leak and let a valve drop at WOT so look for a black exhaust nastiness around the outside of the green or red rubber there. Sometimes the stainless bellow retaining spring falls off or rots in two.

An exhaust leak under the seat will also cut rpm so watch for any smoke under the seat.

 

[Krispy]

Ok, pulled the water control valve apart for inspection. Here are some pics.

Note the new bellows, held on with zip ties.

The bellows has water in it, but I think that is OK. See any problems?




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[Krispy]

Pulled apart the raves, in need of cleaning, only one anomaly.

There is more exhaust muck around the Mag rave than the PTO, check out the green gaskets to see what I mean.

Anything seem out of place here?

Thanks to all for the help!

I am going to be checking exhaust connections for leakage after cleaning and reassembly. Will report back.



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[CReynoldsMIZ]

not seeing it yet. but on the water control valve, you need to trim up the ends of the zip ties. cut them flush with a razor blade so that way they dont poke a hole in the bellows. Mine did this

 

[Matt Braley]

Yeah that stuff looks pretty straight and fresh. On the next water test you can try a few things like:
Open the seat slightly and make a WOT pass to see if it improves(this would hint to exhaust leak)
Try to run with the fuel cap loose(this will cheak tank vents)
Try the RES or ON. Just switch to another to see if it makes a difference. There could be a fuel selector clog or air leak.

It's not uncommon for one cylinder to run a little richer than the other and have a little more exhaust mess.

 

[CReynoldsMIZ]

Hmmm. on inspection of that bellow on the water control valve. Is there a hole in there on the bottom of the picture caused by the zip tie? Kinda looks like it.

 

[LouDoo]

I was just going to comment to trim the zip tie(s) or your gonna get a hole in the bellows, they need to be trimmed flush. If that's not a new bellows you probably need to replace it anyway, they just get weak over time. I believe Nick [MENTION=41828]Minnetonka4me[/MENTION] has them in stock may he can stick one in an envelope.

Lou

 

[kicker]

I used to put a thin dab of RTV on top of the zip tie knuckle.

Instead of zip ties I use an O ring now.

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[Krispy]

Just to update and finish this thread: After reassembly and cleaned raves the issue has not repeated itself.

A couple of weekends ago I took the XP out for a day on the lake. We covered 58.8 miles on about 1 full tank of fuel and not a single problem. Most of the travel was spent cruising at 5200 rpm.

 

[manolocross]

Wait!!! You did not mention if you SMOKED the GTI 130 SE!!!!! Well.......


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[Krispy]

Wait!!! You did not mention if you SMOKED the GTI 130 SE!!!!! Well.......


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We are about neck and neck on the top end, I may have a slight advantage ~1mph. Out of the hole I can kill him, but only if I launch properly. If I just grab a finger full of throttle with the trim up and my weight back it literally exits the water and the pump becomes unhooked. My new method is to trim a little less than 50% up, and put my weight over the bars. Once I am on plane, I go full trim up. Using this method I beat him out of the hole every time.

Still, when I ride his 130se I am amazed at how comfortable and stable it is, for touring it is pretty sweet.