Thats because youre running out of power below 1500rpm. :^) If it were in a motorcycle it'd probably climb hills fine.
Thats what i'm trying to figure out, 1500rpm really seems to be about the floor. (though I think the Dodge viper can cruise at 1200rpm as the single sole exception) Yet the drop from 1500 to 750 is potentially half the fuel usage... IF the power is enough i'm wondering what else stands in the way? Were I to do this in a car i'd be doing it on a hot rod with a 10 liter engine. For my more practical and useful tractor projects the engine will be pretty oversized for the load. A heavy flywheel would be added to help maintain smoothness as you engage the load. Whether additional mods to restrict flow or optimize torque peaks for even lower in the curve would be done is to be researched.
For reference I had a 1962 chevy 235 inline six that would idle at under 200rpm. It was so quiet you couldn't tell it was running standing by the hood if there was the slightest wind noise.