In addition to all of the above:

Vapor lock? After an hour the motor should be getting pretty hot. You might want to see if your fuel hose has moved out of the original routing or if something is making it run hotter than normal.

Or you could just stop every hour for a beverage break and get yourself and the mower cooled off.

__________________
Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.