Fuel starvation above 5500 rpm

[Cdn20valve]

Twin 787 1998 speedster.
Feels like im running out of fuel above 5500 rpm. Plugs white on one engine. The other engine is fine. On the problem engine, the rpms go to 7000rpm then drop down to 5500rpm and bounce back and forth. Everything is fine under 5500. Im thinking fuel pump maybe? I rebuilt the carb on the problem engine, as it was doing this on my last ride last fall. But carb rebuild didn't fix it.

 

[Cdn20valve]

When i hold wot on both engines at the same time... They both bounce from 5500/6500rpm, but the problem engine will stall.

Also, my fuel gauge is stuck on empty.. This istarted hapenning around the same time. Im just mentioning it, in case its related somehow

 

[Coastiejoe]

On my 4-tec ski, once on empty my high end RPMs are restricted. This may be the case with 2-strokes as well. Someone who knows for sure hill chime in.


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[Waterluvr]

Make sure your rave valves are clean and all the soft parts look good and the housing bores haven't worn past .400 and swap the water regulators with new or known good used ones on the expansion chambers, by now the regulators will be on their death bed if they haven't been replaced and will cause an exhaust restriction adding back in to much water.

 

[Cdn20valve]

I installed new rave diaphrams last season. I had a look, they are all fine. How do i check the bore of the rave valve housing? Feeler gauge?

Also, i noticed something odd with the check valve above the fuel tank. I can blow through either side. There is what seems like a loose ball in the valve... Is this a problem? Maybe the tank needs to be pressurized to help the fuel pumps draw fuel?

 

[Waterluvr]

If it runs well for the first 75% of throttle range that's where the real issues lie with those Mikunis once you're past that point in the throttle your way past the regulated pop off circuit and air velocity takes over and pulls the diaphragm down and opens up the needle and off you go the diaphragm is down and needle is open and remains that way feeding the high circuit and bombsight until you back off the throttle.

The only issue you might have that's fuel related so long as no one's messed with the jetting is the control arm bent to low and it can't open the needle and seat all the way when the diaphragm pusher bottoms out.

Your tank vent is operating fine, you'd be in a world of hurt if pressurized gasoline was pushing open the needle when the motor is shut off and the crankcase would fill with fuel. That's the primary reason for the shut off selector valve as that goes on to a degree on really hot days after you park the boat.

Try blowing through a straw, then pinch the straw and try again. Little bit easier when it's not pinched yes?

Both your rave valves and water regulator will pinch the exhaust and cause exactly the issues you are having on a 787, it can't breathe right anymore.

You can take a small drill bit and pop a hole right in the center of the red plastic adjuster nuts on the valves and drop a little cocktail straw in there and watch the valves open and close to get a visual on whats going on with them.

For the water regulator you have to swap it with a known good one (or new), I wouldn't fool with a used one anymore old age hasn't been kind to them and I replace them on any ski or boat in for a tune up as of last season that's more than 15 years old.

 

[Cdn20valve]

Some progress... I checked the water regs, and they seemed fine. So? I replaced all fuel lines and pulse lines. I also swapped fuel pumps.

This weekend i took the boat to the water. The problem changed sides! So now im convinced the fuel pump is culprit. I went and got a new one from the dealer. Ill head back onto the lake this weekend to check.

 

[letford]

Hi, did you fix it? I have a Sea-Doo speedster 150 1999 and 1 engine stalls as soon as it hits 6000 rpm?
It's not the carbs and the engines have been reconditioned.
The boat mechanic is stumped.

 

[Dr Honda]

Hi, did you fix it? I have a Sea-Doo speedster 150 1999 and 1 engine stalls as soon as it hits 6000 rpm?
It's not the carbs and the engines have been reconditioned.
The boat mechanic is stumped.

It is the carbs. (most likely)

1) If the pop-off is too high, it will close the needle as the Vac drops. (FYI, after market springs, needles, and seats will do the same thing)

2) You could have a leak in the fuel valve. (Very common)

3) The fuel pump(s) aren't working. Make sure there are no leaks in the pulse lines, and make sure the check valves are sealing. Oh... and if they were "Rebuilt" by someone... make sure... 100%... that the rubber plugs that hold the check valves in place weren't punctured !!! People try to push them into the holes, and is VERY common to poke a hole into them.


When I would have a ski or boat come into my shop with the above problem (and the carbs were "Cleaned" already) I would normally find issues with aftermarket parts. ALL OF THE AFTERMARKET PARTS SUCK !!!!!!!!!!!!!!!! ONLY use OEM Mikuni parts, and the engines will run properly

 

[Dr Honda]

A few more words on the above.

1) If you have an aftermarket airbox on the carbs... you can have the same issue, since the signal changes.

2) They can leak enough to suck air, but not leak fuel. Also, make sure all the hoses are tight, and not lose on the fittings.

3) If you have a crank seal leak that is bad... you may not have enough signal to drive the pump(s). BUT, that will normaly cause the engine to run poorly at low RPMs too.

 

[Cdn20valve]

Hi, did you fix it? I have a Sea-Doo speedster 150 1999 and 1 engine stalls as soon as it hits 6000 rpm?
It's not the carbs and the engines have been reconditioned.
The boat mechanic is stumped.

The pump fixed my problem.

 

[letford]

Thanks for your responses
The reason we think it isn't the carbs is because it only happens on one engine and we swapped the carbs around to check and it still stalls on the same engine.

The mechanic thinks it's something electrical but has no idea what?