I had a similar problem with my boat. This is what I posted, Ok, found the problem. The steering cable is routed under the expansion chamber in the engine compartment. The steering cable housing had melted from being in contact with the expansion chamber, causing the steering cable itself to be exposed. Which in turn allowed it to "bend" out when turning the steering wheel left. Turning right pulls on the cable which pulls the venturi to the right causing the boat to turn right while turning left pushes the cable which pushes the venturi to the left, but with the cable housing being melted the cable was being pushed out instead of turning the venturi.
Check the cable housing, perhaps it has broken or has melted causing the cable to be exposed
My Webpage
So how did you fix this?