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2001 Seadoo Sportster LE. Just got it!

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Sportster-2001-951C-Stock said:
This one is fuel return to tank, the black 5/16" is fuel supply from water separator. It can rotate, pull and twist it will come off nipple but shouldn't leak, if it leaks, fuel will drip b/c it's under a slight pressure for return to tank.
Click to expand...

Haha. Thought this is a easy one to get more performance lol.


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My minimum

Green is parts to check condition carefully for damage and replace if questionable condition
Yellow is places gum and trash often collect, clean these areas well (don't use carb cleaner on filter, it will melt, use brake cleaner or just compressed air is best IMO)
Red are parts I think should be replaced at least once every 10 years

Use no carburetor cleaner on rubber or most plastic parts, they can be damaged by acetone solvent.
 

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  • Mikuni Drawing.jpg
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Sportster-2001-951C-Stock said:
Mt engine had a flat lean bog in mid throttle, so after cleaning carbs it was still there I increased pilot jet size to #80 two sizes from original #75, I think this motor likes #77.5 better so I purchased them and will install them next time.
Click to expand...

I saw this chart on ebay.
throttle_chart.jpg

But don't quiet understand. Is it showing different valves cutting when piston at different positions?
Or different valves is affecting the power output on different power requirement?

Thanks
 
Sportster-2001-951C-Stock said:
You need Super BN, SBN 46i The "i" is important b/c the fuel pump on MAG carb is upgraded on "i" and uses new style diaphragm/gaskets

I think you don't need any carb parts except for needle/seat (size 2.0)
Click to expand...

http://www.ebay.com/itm/GENUINE-MIKUNI-SBNI-SUPER-BNI-46MM-CARB-CARBURETOR-REBUILD-KIT-SEADOO-XP-98-02-/200776960142?pt=Personal_Watercraft_Parts&hash=item2ebf3d488e&vxp=mtr

Does the carb rebuild kit normally include the needle / seat?
I don't see it in the picture.
 
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I saw my carb is made in Japan.
Does it mean the carb is not the original part?
I also notice there are two screws are in very bad shape in the head with some cuts.


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Bonmotwang said:
I saw this chart on ebay.
View attachment 22542

But don't quiet understand. Is it showing different valves cutting when piston at different positions?
Or different valves is affecting the power output on different power requirement?

Thanks
Click to expand...

That one looks like it's for a slider type butterfly, ours is round rotating butterfly. The slider type has a mixture needle attached to the butterfly that adjusts the jet orifice size. Usually this type of slider carb has just two fuel paths, idle circuit and variable orifice main circuit.

Our carbs have three fuel paths, idle fuel(always on vacuum side of throttle butterfly), pilot holes (transition circuit, is exposed to vacuum as butterfly is partially opened, also serves as an air source for emulsion air of idle circuit) and main circuit (high speed venturi)

Metering needle/seat:
The reason I think it's a good idea to replace the fuel metering needle is because it's tip is rubber and has a lubricant coating impregnated on the surface, this coating wears off over time and the rubber tip becomes grooved as well, then may begin to stick shut or leak, the operation becomes intermittent and can cause issues at low speed operation such as hard starting due to flooding if the valve leaks, or lack of fuel if the tip is stuck in the seat.

So, IMO, a carb isn't properly rebuilt unless the metering needle/seat are replaced (ours is mikuni SBN size 2.0) at a minimum.

The metering diaphragm should not be hard rubber, it should be nearly as soft as a rose petal. Also the two round mylar film fuel pump check valves should seal well around their 360 degree radius, there should be no cracks or wrinkles in these two disk shaped mylar flappers.

In my case, the part throttle throttle response was flat and unresponsive, it was too lean, so I increased the pilot jet size as already mentioned. You should not have to do this if your mid range throttle feels robust, although I wonder if the original #75 jet is just a bit too small.

http://www.mikuni.com/pdf/sbn_manual.pdf
 
Bonmotwang said:
So the original part is Mikuni?


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Click to expand...

Mikuni is a Japanese manufacturer. The needle/seat are not included in most carb kits, perhaps b/c these are the parts that most often need replacement? (LOL, this is only my opinion). I found all the other parts were still in good condition.

Here are a good instruction set for the fuel pump check valves, I think you don't need these if they aren't damaged (see above explanation):
http://www.winderosa.com/files/Download/instructioncheckvalveinstalation.pdf
 
 
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Bonmotwang said:
One is metal and one is rubber??


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Click to expand...

Those two parts are identical (should be identical!) You need one for each carburetor. The metering needle has a rubber tip(it wears out), and the seat has the 2.0 orifice size. The needle itself isn't available separately, the metal seat doesn't wear out IMO, but the o-ring around it should be replaced (n/s comes with o-ring).

So you need two of this item (n/s), IMO, I would change them for good low speed operation. The n/s do not have much effect on high speed operation unless maybe they are completely damaged, they are important for smooth low speed operation.
 
Sportster-2001-951C-Stock said:
Those two parts are identical (should be identical!) You need one for each carburetor. The metering needle has a rubber tip(it wears out), and the seat has the 2.0 orifice size. The needle itself isn't available separately, the metal seat doesn't wear out IMO, but the o-ring around it should be replaced (n/s comes with o-ring).

So you need two of this item (n/s), IMO, I would change them for good low speed operation. The n/s do not have much effect on high speed operation unless maybe they are completely damaged, they are important for smooth low speed operation.
Click to expand...

So the black is just the paint?
two-partrs.jpg
 
Bonmotwang said:
Thanks for the link.

Is the fuel chamber where the crank links are? and afterwards will be pushed into combustion chamber?
So it is self regulating the pressure. If there is a leak (the rubber is worn out),I don't think you can get a "pop up" ?
Click to expand...

Yes, that's correct, the spring holds the fuel inlet metering needle valve rubber tip closed against it's seat until the vacuum negative pressure pulls the diaphragm inward into the fuel chamber against the metering spring and the fuel metering valve is opened by lifting the rubber tip from the inlet seat. Fuel flow is regulated at low speed by engine vacuum acting on the rubber diaphragm, at high speed the diaphragm is exposed to full engine vacuum so the metering valve is almost always full open and has lost regulating effect. So the pop-off is most important for regulation at low(er) operating speeds, and the condition of rubber tip is important for fine regulation. measuring pressure required to push rubber tip from the seat (against the spring) is called measuring pop-off pressure and is the gross method to get a rough idea of pressure/spring balance.

The pop-off should be repeatable, otherwise the valve is sticking. Serious racers will polish inside the brass metering needle seat, polish the valve needle top, the lever arm(item 15, both ends) polish the center diaphragm button item 16, and polish the pivot pin item 14, to ensure regulator operation repeatability.

Here's a link to another carb manufacturer's manual, for a similar design carburetor:
http://www.walbro.com/media/21907/WTseries.pdf
 
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Sportster-2001-951C-Stock said:
Yes, that's correct, the spring holds the fuel inlet metering needle valve rubber tip closed against it's seat until the vacuum negative pressure pulls the diaphragm inward into the fuel chamber against the metering spring and the fuel metering valve is opened by lifting the rubber tip from the inlet seat. Fuel flow is regulated at low speed by engine vacuum acting on the rubber diaphragm, at high speed the diaphragm is exposed to full engine vacuum so the metering valve is almost always full open and has lost regulating effect. So the pop-off is most important for regulation at low(er) operating speeds, and the condition of rubber tip is important for fine regulation. measuring pressure required to push rubber tip from the seat (against the spring) is called measuring pop-off pressure and is the gross method to get a rough idea of pressure/spring balance.

The pop-off should be repeatable, otherwise the valve is sticking. Serious racers will polish inside the brass metering needle seat, polish the valve needle top, the lever arm(item 15, both ends) polish the center diaphragm button item 16, and polish the pivot pin item 14, to ensure regulator operation repeatability.

Here's a link to another carb manufacturer's manual, for a similar design carburetor:
http://www.walbro.com/media/21907/WTseries.pdf
Click to expand...

Now I understand that the low speed and high speed has totally different fuel circuit, triggered by manifold pressure and venturi effect.
Very interesting!
 
Good. Now, you can imagine that the low speed circuit has very small passages that will build fuel gum from varnish in the fuel, and the gray Tempo fuel lines also slowly dissolve over time, which clogs the small passages leading fuel into the bore as well as the tiny metering needle inlet pre-filter becomes plugged with gum and debris.

This is why it's important to have properly functioning carbs. Especially in the case there's a running problem such as a lean surging, lack of fuel can cause a lean condition which can damage the engine b/c fuel is used as the carrier for oil lubricant, fuel helps cool the piston, and lean conditions can cause detonation, which will erode and overheat the pistons.

I think it's a bad idea to run this 951 engine at full throttle for long periods of time due to heat building up in the pistons, I avoid doing this for more than a minute or two. These are high performance 2-stroke engines and such they don't have an exceedingly long lifetime. Some racing high performance 2-stroke engines are normally torn down between races, and the expected piston replacement for these racing engines doesn't extend past 50hrs of running time.

It's nothing like a car engine in terms of amount of fuel burned in a short period of time, constant heavy load, no coasting time, marine engines live hard lives, they need oil and fuel!

http://toostroke.blogspot.com/2007/12/piston-speed.html
 
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Bonmotwang said:
Talking about the run time. Is there any low cost way to know how many hours my engine has been running?
Just curious about the history ...

Thanks
Click to expand...

The MPEM computer keeps track of run time, a dealer or someone with candoo s/w can retrieve this variable.

As you can see from Gordon Jennings' article, the reliably safe speed limit for an engine like the 951 would be 6848 RPM to yield a piston speed of 3500 ft/min, but he's talking about a motorcycle engine which has coasting periods, not a marine engine which is under constant load.

Gordon Jennings was an actual expert on the subject of 2-stroke engines, reading his articles (and the BRP shop manual) is time well spent when it comes to understanding the engine in your boat.
 
Sportster-2001-951C-Stock said:
The MPEM computer keeps track of run time, a dealer or someone with candoo s/w can retrieve this variable.

As you can see from Gordon Jennings' article, the reliably safe speed limit for an engine like the 951 would be 6848 RPM to yield a piston speed of 3500 ft/min, but he's talking about a motorcycle engine which has coasting periods, not a marine engine which is under constant load.

Gordon Jennings was an actual expert on the subject of 2-stroke engines, reading his articles (and the BRP shop manual) is time well spent when it comes to understanding the engine in your boat.
Click to expand...

It is a $400 item. Not interested in the hours anymore. LoL
 
The guy who sold me the pump kit and shaft removal tool delayed a whole week to ship out the parts. So I am still sitting tight at this side of the border.
 
Bonmotwang said:
The guy who sold me the pump kit and shaft removal tool delayed a whole week to ship out the parts. So I am still sitting tight at this side of the border.
Click to expand...

I'm still at the border between Mexico and Canada! :)

Forgot to say, if you have a new lanyard key made then it requires connecting the MPEM to the diagnostic tool. That's also your opportunity to get engine hours.

I plan to have a new lanyard made eventually, I have only one.
 
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