I found this On-Line. Was it only true: "Rumours and legends were spun around the chainsaw in the kitchen that has no gas.
One of the most complete ones goes like this:
1.) a leaf of the rubber tree can be used to pump gas from the car to the chainsaw,
2.) the chainsaw can be used to remove some of the wodden planks blocking the window in the secret room,
3.) the planks can be used to repair the staircase in the library,
4.) Above is a secret room by Ted
All this is pure hoax, there is no gas for the chainsaw in Maniac Mansion. Actually, you can find the gas on Mars in Zak McKracken, but when trying to pick it up, the player is responded "This is for a different game"