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Can you fill me in with exactly what the engine does or doesn't do and it's history?
Is this a recent problem? Does it make a particular sound? Does it accelerate? Good top end?
Has the engine been sitting or did this just happen?
It does sound as the linkage is not correct.
Your engine is a little different from the engines here in the US so I don't have a service manual or parts book that is a direct match for your engine.
That being said have you tried to adjust the idle mixture screw on the top side of the carbs?
Remove the air silencer from the front of the carbs and make sure all the throttle plates close equally and open the same all the way.
the 600 to 700 rpm is that in neutral? In the water? If so your engine is idling too low. Raise the idle speed to about 950 in neutral in the water, when you put it in gear the rpm will drop a couple hundred rpm and you should have a good even idle. This too could be from mis adjusted linkage.
Post back what you have. We can work from there.
I would also replace the water pump impeller and thermostats.
I think once the linkage is adjusted correctly and raising the idle you will be fine.
Let me know how it does.
That isn't sand in the float chamber but is a byproduct of old fuel. As the old fuel evaporates it sometimes reacts with the aluminum and other chemicals in the fuel causing this. Try not to use any fuel over a few months old, particularly in a fuel tank outside. I'd also recommend manually draining the carbs after each use by removing the drain screw in the float bowl. Drain into some paper towels.