Trouble getting going past idle

[Shortrun]

The engine came in boxes of parts.
We have it back together, 150 psi on both cylinders.

The ski starts easy runs good, i can idle around the lake for hours. But try and give it gas and it begins, launchs, but after about one second it gives up. I can go right back to happily idleing around.

I am thinking carb problem. Of couses they have been apart several times now. Proper jets and clean. Blown air through all the passages. Pop off is a bit above 30 psi.
We did have the idle screws set wrong, we had then at 2 1/4 out and i now think they should be 1 1/4 out

Plus the pop off test (and i have never done this before) was more of a dribble off. It clearly started at one pressure then resealed when the pressure dropped. But there was no pop.

Machine is a 94 with a 580 engine , dual carbs.


I did experiment carefully pulling the choke out while adjusting the throttle and i was able to bring the ski up onto plane and keep it there, maybe 15 to 20 mph. After doing this for a while the spark plugs looked good. Not sure if this means anything.

Any help?

 

[Sportster-2001-951C-Stock]

Lack of fuel, for whatever reason.

Several ideas:

Maybe 1989 had a rubber pick hose/tube inside fuel tank and that'll be rotted off by now if it did. This results in lack of fuel unless tank is ultra full.

Keep in mind, if a 2-stroke is lean hesitating you're at risk of a piston/ring seizure but just idling around there's low risk. at 150psi you're still good.

I would disconnect the fuel line from the fuel pump and blow air not more than 5psi back into the tank, remove fuel cap 1st to avoid pressurizing tank, and listen for air bubbling up through the fuel pickup inside the fuel tank, similar to a fish tank aerator.

30pai popoff sounds about right but it depends on how you got there, specifically the correct spring is muy importante. It's possible to have the wrong spring and mangle the lever arm to get correct pop but that still won't work to flow at high speed.

The mylar film check valve screwed onto the kidney-shaped metering plate is also critical, many aftermarket kits provide the mylar piece that's too thick thus HS(high speed) fuel circuit comes on too late and you wind up with terrible lean hesitation due to no fuel through HS jet that feeds large HS venturi (target shaped venturi) in the carbs bore.

Fuel tank vent check valve might be sticking closed, this results in fuel starvation after several or a few minutes as there's no make-up air entering tank to replace fuel pumped and consumed, eventually fuel pump can no longer pump against the vacuum created inside the tank. Quick check is by loosening the fuel cap to allow atmosphere to enter tank. If engine immediately perks up then suspect the vent check valve is sticking closed.

I could go on, but these are the most common.

 

[Sportster-2001-951C-Stock]

One more thing, the rotary iintake valve may be out of time or have some damage. I kinda think though it wouldn't idle well if it was damaged and would be hard to start but off time or wrong shape (snow-mobile disk?) will have a definite impact on high speed.

 

[Shortrun]

Ski is a 1994 or 5. The engine is a white 580 but I have no idea what ski it initially came out of.

I will go through and check all the items you have mentioned.
Will start with mylar film check valve.

When the carbs were done the old spring in the check valve was reused. No adjustment were made to get the pop off. so should be Ok on that one.

 

[Sportster-2001-951C-Stock]

My XP had a 580 in it I think, traded off many moons ago but a very reliable smooth runner that always impressed for what it was, plenty fast for me on the lake and no complaints.

Agreed, pop-off is unlikely changed appreciably, using original spring. If the metering arm was bent by someone (trying to adjust pop pressure using incorrect spring) there's a possibility the metering needle doesn't lift off the inlet seat enough. likewise, if they used the incorrect diameter metering needle seat orifice.

Note FYI: Pop-off pressure target was mostly based on the airbox flame arrestor design, highly restrictive air boxes used more pop-pressure.

Carb has 3 fuel circuits, idle (adjustable mixture), low speed (no adjustment), high speed (adjustable mixture). It's most difficult to restore the low speed circuit due to the small diameter of the passages and propensity for clogging. Most of the fuel consumed (assuming you aren't always riding full throttle) flows through the low speed circuit.

A partially plugged low speed circuit can be the cause of immediate lean hesitation and engine stop while attempting to accelerate from idle, applying some choke can get you "over the hump" past the LA dead zone and into the HS circuit.

Once the HS circuit begins flowing, you may be able to release the choke and continue running at high throttle position using the HS circuit

The HS circuit has large diameter passages so it's unusual and relatively insensitive to minor clogging, there are no tiny passages where small debris collects and/or corrosion of the aluminum in HS circuit doesn't change the HS calibration enough to make a large difference.

MOST OF THE TIME if it's a carb problem the problem exists in the LS circuit, those three small holes in the carb bore just upstream of the throttle butterfly will be partially plugged. It can be a challenge getting them cleared out, many have given up to discover a brand spanking new shiny carburetor runs like new.

In case you don't have it, Mikuni had their SBN carburetor PDF manual posted last I checked, used to come up on GOOGLE search. This doesn't offer a magic key, just can answer some basic questions you may have and shows the fuel circuits plainly.

Anyway, have a great day

 

[Shortrun]

I do have the Mikuni PDF and have been reading it.
When I was driving the ski I was trying to get past the low speed circuits in the carb which is why I was playing with the choke. But that did not work for me, I could never get past half throttle.

I will add checking the 3 small holes (low speed circuit) you mention
But am still hoping, right now, that the mylar film check valve is the problem because it would explain why both carbs are not working, equally.

I'm really wishing it was not a 1 hour drive to the nearest water.

 

[Sportster-2001-951C-Stock]

Yep, and using the choke method does take some finesse as you probably discovered, it's easy to flood the engine doing that.

If you disassemble the carbs again, be aggressive with cleaning that low speed circuit, remove the low speed brass jet and idle mixture screw, blow all that out with good air pressure in both directions, and a little white vinegar flushed through as an acid might help loosen the aluminum corrosion. Those three holes can close up imperceptibly and this is the most difficult part of restoring these carbs so you might want to poke a wire through them from inside the bore to reopen and ream out any corrosion there.

I can tell you mine were closed up just enough to cause a huge hesitation that would stall the engine.

But it's important to make triple sure there are no inward air leaks in your fuel lines b/c if the fuel pump is sucking air through a small leak it's a real show stopper.

 

[Shortrun]

I checked the mylar check valve and found the new ones are 0.004 inch. The ones that were removed are 0.002 inch thick. So i put the old ones back in. Fingers crossed.

I presurized the fuel line and i could hear the bubbling in the tank. I then put the fuel line in a container and then pressurized the fuel return hose. To check the fuel flow. It was good, about 1 pint in about 10 seconds. So i think all is good here.

I did verify good flow out the 3 low speed jets. I put WD40 and then used air about 10 psi and watched it mist out all 3 holes. Although i agree with you, just because the passages are open does not mean they are not restricked. I also put a small wire in the little holes and wiggled them around in there, they only went in about 1/8 inch.

I have not checked the rotary valve yet. Did not see any damage when assembling. I did not check to see if it is the proper disk. I hope to try the ski with the changes made so far.

I am going to try and measure fuel pressure but i am not sure i can do this?

 

[Sportster-2001-951C-Stock]

Interesting factoid on the mylar HS check valves, I've never actually measured them so thanks for the detail.

1/8" bottomed in the three LS outlets sounds about right, one hole might be visible while brass LS jet removed. I blow through that passage ans out the idle mixture screw hole with air turned up high, 90psi or more probably.

Good job checking for fuel line leaks.

Fuel Pressure: I suppose you could place a Tee in the fuel line feeding the 2nd carburetor to measure pressure there and I'm gonna guess it won't be more than 3psi on a 10psi fuel pressure gauge. Air moving through fuel lines would be important b/c fuel is definitely displaced in that case. No leaks, no air, and I'm confident you gave the fuel pump a resperatory test as per several sites describe (Seadoosource dot com is one). You can test fuel pump on or off the engine.

Thoughts on fuel pumps: SBNi series (i for improved) carbs are later and have larger pumps (perhaps for larger engines)

Usually a fuel pump problem will be pretty obvious during disassembly, you'll see corrosion pitting around the perimeter of the aluminum seats where the mylar disks seal against. Occasionally the rubber disk grommets holding the disks might be ruptured if a tool was pushed through them during assembly, or the disks themselves might be warped or cracked.

In some cases the film used to manufacture replacement fuel pump disks will be slightly potato-chipped due to the large sheets of mylar are rolled up on rolls still warm during manufacturing of the film. In extreme cases they may be curled enough they don't seal well against the seat so Mikuni places a mark on the convex side to denote which side preferably goes against the seat. It's difficult to tell which side is marked and fuel washes this mark away in a short time. Usually not an issue but should be mentioned as an gottcha.

Otherwise, the rebuildable on-carb pumps are quite robust, rarely cause problems and replacing the disks is typically unnecessary IMO. The divorced pumps used in some applications do wear out and they're a tad small to begin with, I suspect.

Maybe you can find a fuel pressure spec, I don't recall if Seadoo published a spec.

 

[Shortrun]

Good news. We tested the ski today, had the air box off during the ride so could easily get at the adjustments.
It ran pretty good, got to high speed ok. It did have a little bog at low speed. After playing with the low speed, turning it out. Then the high speed. Evetually it was worse, ran poorer the more we adjusted.
So i will remove the carbs, i want to clean the low speed jets more. Get the low and high adjustment the same on both carbs.

Do you think having the air box off will have effect on running?
Anyway, i think the original problem is fixed (the little maylar valves i am guessing) and we have some tuning to do.

 

[Sportster-2001-951C-Stock]

I think you won't know till you put the airbox on. Lots of guys remove the airbox without rejetting then wonder what's causing off idle lean hesitation. Some slight part throttle 4-stroking rich burble is desirable IMO, but lean hesitation is not to be tolerated.

Set your mixture screws to suggested factory spec. If this setup doesn't work, there's something wrong like corrosion in the carb most likely.

The idle mixture adjustment does just that, you should be able to verify it works as intended. Same goes for the HS mixture adjustment. If not, something's wrong.

There should be no part-throttle hesitation. There's no mixture screw adjustment for this, just brass LS jet and those three holes are the team that work together for controlling part-throttle mixture. Some slight 4-stroking rich burble is acceptable while operating part-throttle as long as it clears out quickly as the throttle is opened further. By the time you're on plane, it should be singing a sweet steady tone and should pull hard, feel strong.