Yes, for a short period (~30sec) the engine will probably run with fuel shut off and expect it will run much better during this period if the metering needle is stuck open. The reason is, if needle is stuck open (or leaking badly) for whatever reason, the cylinder will flood with fuel. This probably explains your smoke rings and I'll bet your eyes still burn from the excess gasoline wafting.
Preface: Let me just say I'm a bit of a tightwad, optimistic "repair it" fanatic so keep this in mind...
New fuel lines, definitely, always a good investment. Confirm there are no fuel system leaks anywhere b/c if your fuel pump is sucking air it's not pumping fuel. 5psi is enough pressure to confirm, using pressure decay method. I'd avoid more than 10psi. My quick way - Normally I just remove the fuel line from the fuel pump and blow in there back toward the tank while listening for bubbles from the pickup inside the tank but remember to loosen the fuel cap first, else you might be in for a surprise gush of fuel once you stop blowing.
It's extremely even more so important for 2-stroke marine engines the fuel system is working correctly, else engine damage is waiting just around the corner.
If your carbs haven't been rebuilt in 5 years or MAYBE NEVER, it's time. Do your research, BE THOROUGH and think it all through. Go slowly and ask questions. All of your questions are actually answered somewhere in these hallowed halls and corridors, dig around if you want.
Re-use the original metering springs, muy importante assuming they're original and undamaged by someone. Often repair kits include new springs that are wrong or lousy so hopefully the originals are still there and are undamaged. If they've been bent, cut or modified, they're not gonna work reliably so make sure not to step on them accifentally.
There are four different strength springs depending on the year and engine type so you need the correct springs and during reassembly confirm the pop-off is reasonable. Balance carb A with pop of carb B for smooth low speed throttle performance, within one psi is close enuff.
With the original springs, new needles and undamaged lever arms, it should be correct already. Confirm pop pressure to gain confidence thy pop correctly before reinstalling carbs with new fuel lines b/c all those polymer rubber oil & fuel lines typically rot and begin swelling/corroding/delaminating in 5 years.
You can buy or make your own pop-off gauge, just need to measure the pressure and confirm it's reasonable. A pop-off gauge can help diagnose the defective carb by confirming the metering valve is stuck open. You'll need to plug the fuel return line and pressurize the fuel inlet line of both carbs independently. The inlet line should hold pressure as you pump up pressure eventually reaching enough pressure to pop the valve needle off it's seat, that's the pop pressure.
Go check out seadoosource dot com and google search for Mikuni carburetor SBN factory PDF manual and download the PDF file. It's the basic file which explains the SBN line of Japan Mikuni carbs used on many brands of jetskis like Seadoo and Yamazuki
You might get by without confirming pop-off pressure (I have skipped measuring pop many times but I have bad habits, lol) but if you can swing a pressure gauge setup it'll be one variable eliminated.
Make sure your part numbers match your application and buy quality Mikuni kits unless you are an old hand at these carbs, b/c some of the aftermarket parts should not be substituted and my rule is replace only parts that are worn/damaged. Do no harm.
So on these old carbs assuming they've never been done you may need gaskets, definitely metering needle and seat set, of correct seat orifice diameter and possibly new diaphragms if not still rose-petal soft. Soft metering diaphragms give smooth low speed operation and easier engine restarts.
ie: Don't jump on shotgun replacing all the parts, some of the original stuff will still be fine and aside from replacement metering needles, you might not really need any parts at all if they're in good shape. Could be those carbs are just full of the crud that used to be floating in the fuel tank, (imagine that) and just need cleaning.
Run the mixture needles inward til lightly seated while counting the number of turns to confirm and record previous settings then remove them from the carb body.
Screws will be difficult to remove due to steel/aluminum galvanic bonding that occurs, I like to use anti-sieze compound on those tiny ones during reassembly, or at least some machine oil so they don't gall.
Use a good new screwdriver tip that's hardened, or even an impact driver to help break those screws lose. Not some old junky worn-out ill-fitting bit from the dime store Press hard while removing screws so as not strip out the heads. BTW, These are JIS japanese industrial standard screw heads, which is close but not exactly Philips head so feel free to obtain JIS phillips head screwdriver bits.
Clean, clean, clean, all tiny passages must be clean and blown out with compressed air. Remove the brass jets from the fuel chamber (under the round rubber metering diaphragm and kidney shaped metering plate) and blow out all those tiny passages both directions, alternate using carb cleaner aerosol (pressurized can of acetone). Make sure they're not clogged by squirting the acetone through and watching where it comes out. You'll be surprised the engine could run at all, given the amount of junk in some carbs. Corrosion of the aluminum, junk from the tank and rotting fuel lines.
If you're a total newby to carbs it might be a challenge but anymore with the fuel we have nowadays you may as well learn how.
The other option if you want, is buy new carbs. Some guys go round and round trying to get 'em clean then finally break out the wallet for new carbs (from OSB I believe) to finally get their ski running correctly.
ressure differential) = fuel delivery. If not working correctly this can result in not enough or too much fuel, depending on failure mode (stuck open or stuck closed)
