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

Hello,

I have a 96 superjet with a mikuna SBN carburetor. It has been running awful. I changed the plugs and fresh gas and made sure the gas is flowing ok. I cannot seem to get a low idle setting that is ok, and which then will be ok giving throttle and getting off throttle. If i try to ride it then it pretty much dies after trying to give some throttle but it is weird. At first I thought maybe it was loading up when the revs come down, but this does not seem to be the issue. The behavior is erratic and does not seem to be a needle setting issue. I notice that after i shut it off (after a few tries to get it running steadily) there are drops of gas dripping off the bottom of the carburetor. The drips seem to build up around the screw holding the plate as indicated (with red arrow) in the picture attached and it is possible that fastener is loose or the gasket is failing right near there, but I am not certain, it could be elsewhere and moving to a low point to drip off.

GSJW8536.JPG

In any case, if some gasket has lost integrity and the gas is leaking like that then I take it this could account for the erratic running issues? I am thinking maybe the thing to do is buy and install a rebuild kit as documented at https://mikuni.com/pdf/sbn_rebuild-kit.pdf

What do you think? Do you figure I am on the right track?
 

smoofers

Rockin' the SQUARE!!!!
Site Supporter
Location
Granbury, TX
I second Mikidymac. The main needle diaphragm is behind that cover and in conjunction with the needle, seat, and spring, controls your pop-off pressure. Pop-off has authority over your off-idle throttle response and when it is out of whack, the ski acts exaclty as you describe. Rebuild your carb with a genuine mikuni kit.
 

E350

Site Supporter
Location
Sacramento Delta
Diaphragm is torn. Had the exact same issue from not running the ski every month over the winter a few years ago. Rebuild with oem Mikuni. No need to touch pop off pressure. Problem solved. Make sure you clean the screen inside the carb. See post # 10 here:


Also see this re ethanol diaphragm destruction:

 
Diaphragm is torn. Had the exact same issue from not running the ski every month over the winter a few years ago. Rebuild with oem Mikuni. No need to touch pop off pressure. Problem solved. Make sure you clean the screen inside the carb. See post # 10 here:


Also see this re ethanol diaphragm destruction:

Yes, thank you E350 for your reply and the links. A torn diaphragm is what I was wondering on too after looking at a video about rebuilding with the carb kit, and seeing the diaphragm is seated right there.

I live in Canada so the jetski is covered on a trailer outside through every winter and only runs 2 1/2 seasons. I guess this is less than ideal.

As regards the ethanol, yes brutal on many older materials for gaskets, diaphragms etc. Same worry for bikes and sleds. I try to only use Shell premium and supposedly (sign on the pumps) this is ethanol free.

I appreciate your input.
 
I second Mikidymac. The main needle diaphragm is behind that cover and in conjunction with the needle, seat, and spring, controls your pop-off pressure. Pop-off has authority over your off-idle throttle response and when it is out of whack, the ski acts exaclty as you describe. Rebuild your carb with a genuine mikuni kit.
Thanks smoofers
 

E350

Site Supporter
Location
Sacramento Delta
WFO you are the man. I don't want to piss you off. Because I am planning on asking you some questions soon. But I think that the only part I needed in the rebuild kit was the diaphragm which was included in the kit. I also gently used a carb cleaner wire index and a can of carb cleaner. But I did buy the needles and seats separately but never used them.
 
The problem here is the rebuild kits do not come with needle and seats and those are your problem here , a rebuild kit will not fix your issues

If fuel is dripping out the diaphragm cover as the op stated it’s not the needle and seat, it’s the diaphragm that is in the kits.

But I would agree and suggest any carb rebuild should include new needles and seats.
 
I misread this but I believe I have still made a valid point, you can buy all the carb pieces separately, half the stuff in the kits is never used, as you just referenced , I buy the diaphragms and all the various gaskets, screws , needle and seats from WESTERN POWER SPORTS , you can get the diaphragms from any Yamaha dealer or Kawasaki dealer as they are also used in the SXR 800, all the Mikuni diaphragms are the same. I have a large divided parts bin I keep all the various carb parts in, that way no matter what the brand I am working on I have the carb parts in stock to fix it with.
 
WFO you are the man. I don't want to piss you off. Because I am planning on asking you some questions soon. But I think that the only part I needed in the rebuild kit was the diaphragm which was included in the kit. I also gently used a carb cleaner wire index and a can of carb cleaner. But I did buy the needles and seats separately but never used them.
Don't worry about pissing me off customers do that every day , I am used to it.
 
Like JC-Superjet said: Do a pop-off test!

But your first pop-off test should be completed with the carburetor completely
assembled.

There are other possibilities for fuel to be in that spot.


Bill M.

Pay attention during disassembly, when you are diagnosing a problem.
 
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A leaking Needle Valve will fizz, hiss, bubble and pop erratically.

A good Needle Valve will consistently pop open around the same pressure with a crispy pop and mist.

Do a search on how to do a Carburetor Pop-Off Test. Get a free Mikuni SBN Manual and start reading.
 

E350

Site Supporter
Location
Sacramento Delta
I am just learning and will also search for the free Mikuni SBN Manual.

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?
 
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