Most marine nitro carbs don't have a low speed needle. You put the 3rd channel needle between the tank and the engine. I try to keep it close to the engine. If your engine has a needle you can either take it out and block the needle hole, or you can just open it way rich and leave it that way. You then hook the servo up to the third channel needle. This will allow you to change the needle setting some. The needle will also have an adjustment on the top.

The way I use mine is I start with the 3rd channel toward the most rich setting and the top needle also set rich. Once the boat is on the water I lean it out until it runs right. If it gets to the most lean setting but the engine is still too rich, I bring it in, set the 3rd channel all the way rich and then lean the top needle a bit more.