I was not, and that was the problem.
Thank you!
Dumb rookie mistake.

I am having trouble finding/modifying fuel values.
I am trying to bring up the value of Rocket Fuel up from 225MJ to 250MJ by adding the following line to my prototypes/item.lua file:

data.raw.item["rocket-fuel"].fuel_value = "250MJ"

Nothing I have tried is having any effect on the game.

Any ...