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Hi and thank you for your question.
Two things come to mind. First, have you tried backing off the idle speed adjuster screw? I would suggest setting it manually by removing the flame arrestor and looking down the throats of the carbs. Back it off until there is just the smallest of gaps between the throttle plate and the carb body. We are talking wire size drill bit clearance here. Since it is tending to rev uncontrollably, you are safer to miss it on the low side, and have to use the throttle trigger to keep it running, than having it run wide open with no way to control it. The other thing I want you to check before you try to start it is to make sure you have slack in the throttle cable. If there is no slack in that cable, it can be holding the throttle plates open as well. You can also adjust the throttle cable without the engine running by putting slack in the cable, a lot of slack, then, hold the trigger wide open. It should stop at the grip, Now, grab the pulley on the carb bank and see if the throttle plates are wide open or not. Correct throttle cable adjustment is when the throttle plates hit the wide open stop just as the trigger hits the grip. Water craft throttle cables typically run a lot more free play than motorcycles. As long as it gets the throttle plates wide open, it is not going to affect performance. So you may find your throttle trigger moves quite a bit before it starts picking up the throttle. Just remember that is normal. All you care about is you can get WFO with the trigger on the grip.