There have been ongoing discussions about the Beaver’s fuel consumption and issues with leaning it, specifically, and the issues people are having in Career Mode, as a result.
Here’s a post from the Beaver thread that may help:
There are number of fuel consumption-related posts in that thread. Try using the search function for “consumption”, restricting the results to the thread and take a look at the results.
(Edit: here’s the search result link: Search results for 'Consumption topic:664513 order:latest' - Microsoft Flight Simulator Forums)
I don’t use the Career Mode, but are you able to add more fuel prior to the flight to compensate for any deficiencies in the aircraft’s fuel flow modeling? In other words, fill the tanks more?