I thought about that, but that would drop HP a bunch.
I been reading this all along and thought you were having a carburetor related issue (black smoke , black plugs just after an hour of use). but did not want to mislead you. So I did some researching as I would my own tractor and found this guy that had the same problem as you with it stalling under load.
The problem was in the carb. The carb is a Nikki and the "jet" is located in a strange spot. This twobarrel carb has only one jet and it is located at the bottom of the fuel bowl where fuel from the bowl first get pulled from the bowl. The issue I had is that there is an o-ring seal down stream of the jet that sits in fuel 100% of the time. This oring was deteriorated and the seal had failed. Under heavy fuel draw, fuel from the bowl was bypassing the jet and the carb was running rich. Briggs changed this design on later model years.
Are you still getting black smoke and black plugs?