Super Jet Gas dripping from mikuna SBN. Do I need a rebuild kit?

[wmazz]

JC-SuperJet - How would I do a pop-off test that would reveal issues in the needle and seat?

Make one, Do you have an air compressor or bicycle pump.

But is it correct that too strong a pop up spring will cause a fuel starvation condition especially at idle/low rpm?

If so, why wouldn't it also cause a fuel starvation condition at high rpm?

Is more pressure created on the other side of the pop up teeter-totter at higher rpm?

Most people will Never Ever, come across a problem that is actually
caused by a high pop-off. Reducing pop-off may cure a problem, but
that does not mean the pop-off was the problem. Too many people
on watercraft forums equate reducing the pop-off as a solution because
they do not understand how a float diaphragm carb operates.

If pop-off was such a problem, why wasn't pop-off a problem while watercraft
was new and under warranty? Don't get me wrong, I worked on new and used
watercraft with hard starting problems, but I never changed the pop-off
to solve a problem.

The only cases where I have seen mid-range fuel starvation is after using
a Chinese Keihin rebuild kit, and on small (less than 50cc) gasoline engines
with Walbro carbs used on weed whackers and RC model airplanes that use
a basic carb with no jets, or replaceable needle seat.

The Mikuni manual is part of the problem, it recommends reducing the
pop-off for a problem and it encourages the idea that pop-off is something
to be changed (as if it the only part of a circuit) .


Bill M.


Segmentlime >> A likely cause of your fuel collecting on the screw is from
the flame arrestor intake. On some carbs, they create a mist of fuel at the
top of the carb at wot. Or if your needle and seat is leaking, fuel can go
all over the place.

 

[WFO Speedracer]

The part that Mazz is leaving out is that high popoff goes with high restriction (read stock) flame arrestors, once you change the restriction you also need to change the popoff to compensate due to the loss of signal at the carb.

 

[E350]

@WFO Speedracer: Is "signal" you are speaking of the pulse of vacuum to pressure to vacuum. . . which is sent from the engine crankcase to the carb fuel pump side?

If so, does having a more restrictive (i.e., oem) flame arrestor actually reduce the strength of that pulse to the carb fuel pump?

If so, I could see that oem or more restrictive flame arrestors would reduce fuel pressure, and then you would need to lessen the popoff pressure to permit the appropriate amount of fuel flow for the more restrictive flame arrestor.

Andu just the opposite with a less restrictive flame arrestor.

Is the above correct?

This is all new to me. I read the Mikuni US Super N Manual and it provided some knowledge about carburetors which I was ignorant of.

But I really don't understand it yet. But want to.

 

[mikidymac]

He is talking about the actual vacuum in the venturi of the carb being sucked into the intake. If you restrict the flow into the bore of the carb from a stock air cleaner it will pull harder and suck more fuel so the pop-off has to be increased. This is just like when you put your hand over your vacuum and the motor speeds up and pulls hard.

 

[E350]

Oh. So @mikidymac there is a stronger vacuum in the venturi of the carb with a more restrictive flame arrestor which would cause more suction at the jet hole in the venturi. So to prevent an overfueling condition caused by the strong suction the popoff needs to be increased to restrict the amount of fuel being pulled into the carb through the jet hole in the venturi?

Like if you took your vacuum cleaner and instead of hooking it up to the 1.5" oem hose, you hooked it up to a 4" hose. Suction would be reduced with the bigger hose (i.e., the less restricted hose).

So less restrictive flame arrestors mean less suction which means popoff pressure must be reduced to prevent an underfueling condition resulting from the weaker suction through the carb venturi?

 

[segmentlime]

So I pulled the carburetor and have opened the regulator side. The diaphragm was intact but quite stiff and hard (that’s what she said) and kind of crinkly and rigid in parts unevenly so across the surface. There is no way this can have been functioning properly as regards a smooth and consistent response to fuel chamber pressure relative to atmospheric pressure so as to actuate the lever arm for appropriate fuel release through the needle assembly.

As well, the bolts holding the cover plate on were not particularly tight, especially the lower one, which may have been allowing a poor seal with diaphragm lip and carb body, and may account for the fuel leaking from there. It seems that it must have been leaking at least for a while because the black paint on the carb body and cover plate has / is peeling off a bit around that bolt location.

The engine has run well with the current setup, and I guess thinking about it the performance and consistent response, idling and low throttle behaviour has deteriorated for a while and recently this season has really worsened. I mention this because this gives me confidence that the overall setup is ok for what it is; that is, the issues are not likely mismatched nozzle or jet sizes, spring strengths, etc. It may be that I could get away with just replacing the diaphragm only, but I figure I will do the whole carb kit and give it a good cleaning while I am at it. I have never done this before so I look forward to learning from this, and given the condition of the diaphragm the other gaskets, check valves etc are probably due.

I am in the process of trying to fashion a diy pop-off tester from a little pump/gauge I have for filling the pneumatic wheels of rollerskis.

 

[E350]

I had never done it before either. Just go slow. Be gentle. Every 25 years or so its good to clean that filter, item no. 9.

https://www.x-h2o.com/attachments/mikuni_bn44i-yam_rebuild_kit_diagram-1-pdf.412013/

https://www.eccarburetors.com/assets/images/Mikuni-SBN-Tuning-Manual.pdf

 

[mikidymac]

Ignore the site. LOL but follow this thread.
The Mikuni Carb Rebuild Thread.

 

[E350]

Now I know you are the Man!

I will ignore the site, but I respect the effort you put into that thread with explanation and photos and all. Good Job! And thank you for sharing.

 

[WFO Speedracer]

Oh. So @mikidymac there is a stronger vacuum in the venturi of the carb with a more restrictive flame arrestor which would cause more suction at the jet hole in the venturi. So to prevent an overfueling condition caused by the strong suction the popoff needs to be increased to restrict the amount of fuel being pulled into the carb through the jet hole in the venturi?

Like if you took your vacuum cleaner and instead of hooking it up to the 1.5" oem hose, you hooked it up to a 4" hose. Suction would be reduced with the bigger hose (i.e., the less restricted hose).

So less restrictive flame arrestors mean less suction which means popoff pressure must be reduced to prevent an underfueling condition resulting from the weaker suction through the carb venturi?

Yes you are pretty much spot on, the performance Mikuni's and the OEM Mikunis are completely different animals in terms of fuel flow etc, you can get away with a lot more with the performance Mikuni versions than you can a stock or OEM Mikuni carb, they are much more forgiving in restect to tuning.

 

[wmazz]

He is talking about the actual vacuum in the venturi of the carb being sucked into the intake. If you restrict the flow into the bore of the carb from a stock air cleaner it will pull harder and suck more fuel so the pop-off has to be increased. This is just like when you put your hand over your vacuum and the motor speeds up and pulls hard.

1) Most water craft did not have a restrictive flame arrestors. That includes Yamaha, SeaDoo,
and the majority the Kawasaki's. Only the Kaw JS300, 400, 440, 550, and early 650 have restrictive
flame arrestors.

2) If you replaced the stock flame arrestor with an aftermarket, and readjusted the fuel
screws, or bump up the low jet a few sizes, or the mains, and there was no loss of acceleration
or transition from low to high, WHY would you need to change the pop-off?

3) By far the majority of people that spent money on replacing flame arrestors, removing
the choke, and rejetting their carbs wasted their money!

*** Most ski's would have benefited more by increasing the low jet 2 sizes.


Early 951's benefited by replacing the the flame arrestor because the oem
FA sucked water off the bottom of the hull and into the engine.


Bill M.

SegmentLime >> Pay attention to the o-ring on the needle & seat. Better yet
replace the needle & seat with a genuine mikuni seat.

Pay attention to the lever height.

Perform the pop-off test twice. Once with the carb disassembled, and another
time with the carb completely assembled. The result should be the same. If
not, remove the diaphragm and recheck the lever height.

 

[WFO Speedracer]

1) Most water craft did not have a restrictive flame arrestors. That includes Yamaha, SeaDoo,
and the majority the Kawasaki's. Only the Kaw JS300, 400, 440, 550, and early 650 have restrictive
flame arrestors.

2) If you replaced the stock flame arrestor with an aftermarket, and readjusted the fuel
screws, or bump up the low jet a few sizes, or the mains, and there was no loss of acceleration
or transition from low to high, WHY would you need to change the pop-off?

3) By far the majority of people that spent money on replacing flame arrestors, removing
the choke, and rejetting their carbs wasted their money!

*** Most ski's would have benefited more by increasing the low jet 2 sizes.


Early 951's benefited by replacing the the flame arrestor because the oem
FA sucked water off the bottom of the hull and into the engine.


Bill M.

SegmentLime >> Pay attention to the o-ring on the needle & seat. Better yet
replace the needle & seat with a genuine mikuni seat.

Pay attention to the lever height.

Perform the pop-off test twice. Once with the carb disassembled, and another
time with the carb completely assembled. The result should be the same. If
not, remove the diaphragm and recheck the lever height.

I guess Bill if you still live in the eighties and have never worked on modern watercraft,

 

[wmazz]

It depends what you consider a "Modern Watercraft." I consider modern watercraft
as super charged 4 strokes! and I did work on early 4-stroke Yamaha, SeaDoo's,
and direct injected 2 strokes.

What we are talking about are 2 stroke dinosaurs that are 15 to 30+ years old,
and many are officially obsolete.

I worked in the largest watercraft and motorcycle market in the United States
between 1979 to 2005. When I started dealers were selling 100 motorcycles
a month + jet skis (back then dealers considered jet ski's as a necessary pita),
9 months a year. In the 90's to 2000's dealers were selling 1000 motorcycles
& 100 watercraft per month plus boats, except for a few months during the
winter.

Sales volume paid off in the service departments, and so did the factory tech
classes that were available during the winter.


Bill M.

By the 90's the oem flame arrestors were better than the aftermarket flame
arrestors sold in the 80's.

 

[E350]

Now I am happy I followed my intuition and replaced the AM with or kept the oem arrestors on our 25to30 year old skis. One AM even had some kind of gauze which choked off air when it got moist.

 

[segmentlime]

Hi

So I got the rebuild kit and some related stuff from @JetManiac - excellent service and knowledgeable support. Thanks Chris!

I am in the process of the rebuild - now fully disassembled, cleaned up in ultrasonic and blown out, and I am just about to start reassembly. I hope someone can help me with some questions about this because I am a bit confused.

This is the first such carb rebuild that I have tried to do.

The parts list and diagram sheet that came with the kit indicates the use of some different, mutually exclusive, parts for the 38 carb versus the 44 carb. I thought my carb is a 44 because the diameter of the throat near the butterfly measures 44mm.; however, some of the parts in question that were on my carb when I disassembled it match those as indicated for a 38.

For example, my carb had part 1a O-Ring, and part 6a O-Ring. As well, the carb did not have the 2nd of the two parts 1 Pump Gasket, (in this case the one missing was the one indicated as that going closest the Pump Body Assembly, the same one indicated as not used on the 38.

Most notably on closer inspection, since starting this note, I now notice it appears that my Pump Body Assembly is what I think is actually part 19a which is indicated as belonging to the 38, and that a part 5a gasket was previously used. I say this because when I look at the new gasket part 5 as is indicated to be used on the 44, I notice that the holes on this gasket do not match the opening of my pump body assembly and this gasket appears to be cut for a pump body assembly with slightly larger openings than those on mine. In the parts diagram this surface of the parts 19 and 19a pump body assemblies is not shown but from the diagram of the gaskets 5 and 5a it would appear the the 38 pump body assembly has smaller openings.

I guess in short it looks like my carb parts with respect to those related to the pump body assembly match those as indicated for a 38 carb and not a 44 carb.

So, am I in error to have identified my carb as a 44 type based on the diameter of the air intake throat and butterfly?

If I am not out to lunch on this aspect then are there some kinds of hybrid carbs with a 44mm throat but employing a 38 type pump body assembly?

I am for sure inclined to put the carb back together using the rebuild parts that seem to match those that came out of it, that is, as indicated for a 38. And this is how I think I will proceed.

Sorry for such a long note, but I would really appreciate if anyone can shed some light on this for me.

 

[segmentlime]

This as reference regarding my previous post

 

[JetManiac]

Use 5a (smaller hole) and 6a.

Do not use 5 and 6.

 

[segmentlime]

Use 5a (smaller hole) and 6a.

Do not use 5 and 6.

Yes thank you. That is what I did. As well I used the part 1a O-Ring on top of the pump assembly. And, I only used one of the 2 provided parts 1 Pump Gasket; this between part 2 Pump Diaphragm and the part 18 Pump plate. The part 1a O-Ring seems to replace the 2nd of the two Pump Gaskets.

I still don’t know though, with the 44mm throat, is my carb an SBN38 or an SBN44?

 

[JetManiac]

Yes thank you. That is what I did. As well I used the part 1a O-Ring on top of the pump assembly. And, I only used one of the 2 provided parts 1 Pump Gasket; this between part 2 Pump Diaphragm and the part 18 Pump plate. The part 1a O-Ring seems to replace the 2nd of the two Pump Gaskets.

I still don’t know though, with the 44mm throat, is my carb an SBN38 or an SBN44?

3 or 4 bolts to f/a on carb top?

Just looked at your pic in the first post, you have a 44mm 61x stock carb.

 

[E350]

Just for giggles, you may want to check your HIN to verify your ski's year of manufacture. The HIN's last two digits represent the year of production:

How to Tell What Year Your WaveRunner Is – AquaSportsPlanet

aquasportsplanet.com