951 - high speed shut down problem.

[kaspermm]

Hi guys.


cant find any good info on this problem.


99 gsx ltd. 951 - rebuild carbs, new fuel lines, new fuel selector, new filters + inline filter.


the first 1,5 hours of this rebuild the ski ran good and strong - and could keep the power close to WOT as long as I would - (I am not running WOT for very long since the engine i new)


the problem is now: cant keep it running above 3/4 throttle for very long without it dies. if I keep throttle position it shuts off.

the ski can be started immediately and run without problem.

if I let go of the throttle I can keep it alive at reduced speed.

It is in my opinion, a clear fuel delivery problem...

I have checked filters, and the stock water sep. filter was a mess.. have cleaned it, but the problem remains.



what kind of fuel delivery problem like this, can appear after 1.5 hours of perfect running?


any input is very much appreciated.

thank you.

 

[SabrToothSqrl]

check your carb settings. i'm thinking it's too rich? too lean? how do the plugs look? compression test?

 

[Sportster-2001-951C-Stock]

Well, the symptoms sound like maybe air is being sucked by the fuel pump b/c there's a leak somewhere. Usually the water separator cup seal or the cup has a small crack at the threads, from my experience. Does it also experience this issue on reserve fuel setting? If yes, I would pressure check the fuel lines from the engine to the reserve valve, remove fuel supply hose from fuel pump and turn reserve selector to off and put some pressure in the line to confirm the water separator is leak-tight.

"the stock water sep. filter was a mess.."

If there's enough trash floating around in the fuel some may have made it's way through separator into the carbs and there's only one way to fix that. This filter isn't designed to filter out tons of trash, just an occasional piece of trash. The large marine type spin-on water separator is your best bet if you cannot find a way to clean all the trash out of your fuel tank.

Don't rely on that small water separator filter element to filter a fuel tank full of trash, it doesn't have enough surface area to strain more than incidental amounts of trash.

 

[kaspermm]

the carbs are totaly stock.

low - 1,75 out

high - 0- out.


plugs looks black/dark brown - because oil in fuel + injection oil.

did compression before summarization 150- 150 psi.

 

[kaspermm]

thank you for your replys.


I also have the inline fuel filter and this looks clean, so I dont think the carb filter is clogged.

but maybe a large piece of trash can float around and block the suction from tank?

and wouldent a air leak show it self in different areas, and not just at the same spot at 3/4 throttle.

 

[Sportster-2001-951C-Stock]

"a large piece of trash can float around and block the suction from tank?"

Yes it can, the fuel baffle pickup screen may have trash stuck to it and blocking flow into both reserve and standard suction draw tubes. Try to clean your tank out, a cheap electric fuel pump sucking fuel through a draw tube and into a disposable filter can be used to vacuum up trash from the tank bottom and polish the fuel much like a swimming pool vacuum system removes undesirables from a swimming pool.

"wouldent a air leak show it self in different areas, and not just at the same spot at 3/4 throttle."

Yes it could, but I'm not going to attempt to diagnose a carb issue until after I'm satisfied the fuel delivery system is leak-free.

Okay, moving on....... Under the assumption we are troubleshooting a lean bog at about 3/4 throttle position:

Remember that this type of carb has three overlapping regions of flow delivery, first of which is the idle mixture adjustable orifice known as the idle mixture screw, which flows fuel at throttle idle position. The second circuit that begins flowing fuel as the throttle plate opens from the idle position is the pilot transition circuit, through the "bypass holes" in the throttle bore. This second circuit is limited by the orifice size of the pilot jet. I've found this particular circuit is the source of most of the fuel delivery, except for throttle positions approaching WOT (ie:greater than ~70%) where the third circuit begins to come in, this third circuit is the high speed circuit, which is limited by the high speed jet orifice. There is also an adjustable orifice in this circuit which is supplementary to the fixed orifice.

So,,, to avoid making a short story longer.... 3/4 throttle is very near where the large venturi begins to flow, the pilot jet is reaching it's flow limit around there as the high speed circuit should begin flowing fuel. There is a mylar check valve on the inside of the metering plate that ensures air isn't being pulled into the fuel chamber backwards through the primary(large) venturi during low speed operation. Confirm this valve is operating correctly, or the operation of the high-speed circuit may be delayed(lean bog at high-speed transition). More likely, you have some small amount of gum or AlO2 growth in the throttle bore transition ports (also called "bypass holes" in Mikuni carburetor literature) which is limiting fuel delivery near the upper flow limit of transition port fuel delivery. If purging these ports with carburetor solvent(acetone under pressure) and compressed air doesn't resolve the issue it may be necessary to increase the pilot jet orifice size but only after confirming all else, such as no fuel line leaks and the fuel chamber metering needle and lever pop-off pressure/position.

 

[kaspermm]

Thank you very much for your very detailed feedback.

I will try what you have suggested tomorrow in the order you have dictated.

I will be back with updates!

 

[SabrToothSqrl]

the carbs are totaly stock.

low - 1,75 out

high - 0- out.


plugs looks black/dark brown - because oil in fuel + injection oil.

did compression before summarization 150- 150 psi.

1.5 turns out, not 1.75 is spec on 951 carbs. (low speed).

Why is there oil in the fuel? that's just for the first tank.
I hope you aren't even close to WOT for the first tank...

 

[kaspermm]

sorry my mistake... I believe that is said 1.75 in my shop manual.


It is the first tank of gas since rebuild..

before you teach me about breaking in: there are as many opinions about breaking in as there are riders.

This is the way I break my engines in, and I am sure this has no relation to my fuel delivery problem.


My only rule past the 1. hour is not to run WOT more than a few seconds at a time.

durring the first hour I go trough the following procedure:

first idle only 15 min.

complete cool down

idle 5 min - up to 1/2 throttle 10 min varying rpm.

complete cool down

idle 5 min - up to 3/4 throttle 10 min.

complete cool down

idle 5 min - up to 3/4 throttle 10 min.

1. hour done... ride as you like, except extended WOT.



Please I do not wish to make this a discussion about break-in procedures.


I will be back with updates regarding the problem.

 

[Sportster-2001-951C-Stock]

"I believe that is said 1.75 in my shop manual. "

Using a factory air box, 1.75 is certainly not the cause of a lean mixture at 75% throttle. Bumping up the pilot size to #77.5 or #80 should take care of it if the fuel system is in good condition (I'm currently running #80's with a factory air box). Once that's done, you can crack the high speed needle open if you find WOT is still too thin.

Better to start off fat with a 2-stroke, then dial it back from there b/c lean-outs are the road to ruin.

 

[- BiRkS_]

Yep, as said above, you start of rich, and slowly tweak it in. I always set mine just a tad rich. as soon as the engine starts running great, stop. don't try to go further. And then after you're done tweaking, do a plug chop (WOT run for 3 mins, pull safety lanyard, and pull plugs and look at them. You want a tan/ light brown color on the insulator.

Back in the sled racing days, keeping leaning out the mixture until you pop a piston, then turn the high's out 1/16 of a turn!

 

[SabrToothSqrl]

No schooling, just official SEA-DOO manual information.

Break-in is arguably the most important part.

If there are other engine issues, it will not be repaired by adjusting the carbs.

Was this a complete rebuild or was only the top end replaced?

What are the compression readings now?

And while the 99 FSM manual states 1.75 turns, the 2002 version states 1.5 turns + or - .25 turns. (low speed)

The 99 manual was based on the 98, the first (official) year of this engine. The 2002 was based on 4 years of this engine.
(Let's not get into the white 951 with upside down carbs).

the 98-02 carbs are identical.

Did you get plug readings / photos?

How do the Reed valves look?

I doubt it would cause a shut off effect; but the water regulator comes into play at higher RPMs as well.

I chased an engine problem for almost 3 years where I couldn't break 6,000 RPMs. It's fixed now, but I've been inside/out on the 951 for over 5 years now. at the moment there are 5 951 skis and 6 engines at my house.

 

[kaspermm]

hi again.

I finely found the time to work on the ski.

pulled fuel baffle and screen and float droped in tank.. great. but got it out with a long wire.

I cleaned the tank.. not much junk in there, and pick up screen was ok.

removed the carbs. all seemed fine..

removed the inline filter and replaced the hose, so no filter in use.


to it out for a ride.. and ran perfect..

I found no smoking gun. but my issues i apparently solved.


thank you for all your help.