Hi Frodo, and welcome to Outdoorking. It's surprising that cognomen was not grabbed years ago, but now you've got it, keep it.
Ultimately you may have to go through the proper procedure for tuning and diagnosing, but I can make an off the top of the head guess. Your mower has gravity fuel feed. It appears that when you turn left the mixture goes lean. The obvious thing to suspect is that there is an obstruction in the fuel tank that floats around a bit but blocks the outlet when you turn left. You might try removing and cleaning out the tank and fuel line. If that doesn't work it will be time to take a look at the carburetor, where the fuel enters upstream of the needle and seat, and then in the float bowl. I'm guessing you have a large flo-jet carburetor, and there could be issues in the float mechanism if it has been messed up by a previous tenant.
Please report back after you clean out the tank and line, and tell us whether that was the problem. If not, we can lead you through a process for getting the carburetor up to scratch.
