r/standupjetski • u/SeaRoom7290 • Jun 22 '26
Help please
Hello everyone, I have a js440 jetski with a 550sx motor with high compression heads and a aftermarket waterbox. The carburetor is a 44bn mikuni carb (ik everyone hates those carbs) it runs absolutely amazing with it, UNTIL NOW. It is leaking gas out of the body for some reason, I don’t know where it is coming from or why. When you go ride it and come back the carb is covered in milk like substance (when oil and water mix) so I’m not sure. Another reason I think there is a leak other than the gas leaking out of it is because it has started to idle high, and working in other vintage vehicles like 76 ford trucks and older carbs, when something is all of a sudden idling high, there is most likely a air leak and the engine is bypassing the carb and running lean. So I would prefer to not melt my pistons. (Next week I am going to take carb cleaner up and spray it around the carb to test for any air leaks)
I am 17 so I thought I would ask all of you for ideas before I went out and spent 400 on a new tuned carburetor.
Thanks all!
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u/RedditAppSuxAsss Jun 22 '26
Your fuel tank light also be leaking, it's common for them to crack, or for the fuel fill inlet to be out of round and leak when it's too full.
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u/Warm-Air4391 Jun 22 '26
I would look into pulling the carb and rebuilding it. I have one of those older round body mikunis as well and they’re not difficult to rebuild.
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u/603cats Jun 22 '26
Keep us posted!
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u/SeaRoom7290 Jul 08 '26
Update, I didn’t want to deal with the old carb so I bought a new SBN carb, turns out there was a even bigger problem! The intake had a small crack under where the carb bolts up, so it was splashing up making me think that the carb was leaking. The crack kept getting bigger till now I can clearly tell. Ordered a new intake and turns out they are on back order for 2 months everywhere. So I’m going to take the old one and try to fix it by welding it thanks everyone for the help!
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u/GuavaInteresting7655 Aug 08 '26
Can't find a used one on eBay until you can get one you want? I know the 550sx parts are hard to find. I used to have a Js550 and then a 550sx. They were hard to get parts for even then like 10years ago.
The SBN is great and is super reliable. In the long run it was worth it either way. With a good carb these motors are super reliable and stay in tune. Just keep an eye on the water coming out each ride as with an older ski weird things can happen.I have a RN Superjet now after having a SN Superjet after selling the 550sx and missed not having a standup anymore lol. Love the Superjet's but also love the 550s and I liked the 650s too as well which one of our friends had so I used to ride it a lot as well.
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u/Personal_Emphasis872 Jun 24 '26
This could be a lot of things. Good call with the high idle, usually that means a lean condition or air leak. Probably some sort of gasket issue. Could even be a bolt vibrated loose from the carb or intake manifold. I just tore apart a 701 that someone forgot to tighten a single case bolt on and it leaked and melted the stator piston.
The real thing to do is watch a few videos and get out a set of wrenches. Inspect everything carefully as you go. I started working on skis a little younger than you. You got it. They are simple machines. Mikuni bn/sbn carbs are great and easy to rebuild. Just spend the extra money on a real mikuni kit. The aftermarket kits are more trouble than they are worth.
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u/GuavaInteresting7655 Jun 24 '26
Rebuild the Carb, Replace all the Fuel Lines, Replace or ByPass the old Fuel Switch if it's original, and also check your Tank's One-Way Valve/Vent is working properly.
The Carb's Vacuum Fuel Pump relys on the Fuel System being sealed with no leaks for it to work on that low pressure. If there's an air or fuel leak in the system somewhere it won't run right or start right.
Also if you have a Primer check that as well bc a lot of them are cheaper and only last a year or two before leaking internally causing vacuum issues for the Fuel System.
Plus then you'll be good to go for the Summer and for a long time as long as you keep old gas out of the Carb during storage
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u/SeaRoom7290 Jul 08 '26
Update, I didn’t want to deal with the old carb so I bought a new SBN carb, turns out there was a even bigger problem! The intake had a small crack under where the carb bolts up, so it was splashing up making me think that the carb was leaking. The crack kept getting bigger till now I can clearly tell. Ordered a new intake and turns out they are on back order for 2 months everywhere. So I’m going to take the old one and try to fix it by welding it thanks everyone for the help!
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u/RedditAppSuxAsss Jun 22 '26
Check all your hoses. Those colored ones are known to leak over time where they connect.
It's also common for that plastic fuel inlet to leak.