Negative Pressure in Fuel Tank?

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Ski is a '95 XP. Was a non-runner when I bought it.

I've now replaced all TEMPO fuel lines, fuel selector switch, fuel filter/o-ring and rebuilt both carbs with OEM Mikuni kits. Pop-off pressure checks out.

Just took it for a test ride and I'm experiencing a similar issue I still haven't sorted with a '99 XP Ltd.

Under 5k RPM, they run "good enough" (without knowing too much about SeaDoos, I can't say for certain, but I believe a bit of performance is being left on the table). Over 5000 RPM, they are dogs and bog down. Cannot sustain anything over 5k for more than a couple seconds before sputtering.

I pulled the plugs on the '95 and they look good. Definitely not running too lean. Maybe a hint rich if anything but they look chocolaty brown without being soaked. I'm at factory-spec carb settings at 1,600 ft elevation, so a tad rich is probably to be expected.

I just tested the fuel system by unhooking the carb's inlet fuel hose and attaching it to a pump siphon. I wanted to see if I was getting any bubbles or any other weird behavior. Sure enough, the bulb of the pump takes forever to re-expand after a squeeze and I cannot initiate a constant siphon, as if there is no airflow into the tank. I believe the pump is creating negative pressure in the tank.

I unscrewed the gas cap, expecting that to fix the siphon issue, which if it did I would have concluded the air inlet check valve up near the fuel selector was bad. But, opening the cap did nothing and it behaved the same. What else could cause this?
 

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You have a blockage somewhere or you connected a hose wrong.
With the fuel selector to ON or RES you should have free flow of fuel from the tank to the carb.
Seadoos should pull pretty hard above 5,000 rpm, they are not, "Dogs".
 
Turns out the issue was the fuel petcock. I bought it from Jet Ski Plus and apparently it is JUNK. I've bypassed it and now it runs great while I await an OEM one.

Now to replace the VTS motor and get the fuel gauge working, and this ski will be dialed in.
 
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