The experiment...
Fixed pitch, caburettor?
Maintaining height:
Reduce throttle (reduces fuel flow) as far as possible.
Lean to max rpm (reduces fuel flow)
Repeat until fuel flow is minimal still maintaining height.
NB this maximises endurance. It may not be good for the engine, or for safety, or anything else. It single-mindedly maximises endurance on the assumption that endurance is limited by fuel running out.