It all depends on your speed also, when you turn you create a lot more drag on the boat and this is the initial reason for it to slow down. If your not running weight and in moderate turn you cant maintain speed with additional throttle then you might look at your fuel filter, look at your carb settings, and ensure that you have full throttle operation at the carb when you move the throttle to WOT.
Depending on your prop, you might be cavitating through turns due to the extra drag and the prop cant maintain speed.
Malo <--- Means--Evil or Mean One. This explains a lot.
2013 Mojo 2.5 Skylon Tower. Bestia < Beast >
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