How do you use the weed whacker on steamworks island?

Answer 1

You need the dirty beaker from inside the museum om Main Street (upper right).

Notice that there are 9 lines on the beaker. In the Research Lab, fill the Dirty Beaker up with 3 lines blue, 2 lines green, and 4 lines red.

Answer 2

The Weed Whacker is an attachment to the Mech that Zack is building in the garage next to Sully's place. When you have recovered the two parts needed to finish the Mech (the Steam Motor and the Crank), you can complete its construction and add the Weed Whacker onto it. This will let you clear the mass of plants from the annex (storeroom) entrance at the far left of the island.

Once the Mech is complete, click on the Weed Whacker to install it. When you reach the overgrown entrance, hit your "spacebar" to make the Whacker work. You can aim it up and down with your mouse, and move the Mech back and forth to reach all of the vines. Three or four properly aimed thrusts should clear the doorway.

Inside the Annex, you will use the Weed Whacker to "splat" attacking plant creatures, and to knock open a stuck Giant Fan at the final entrance.

(see related question)

Answer 3

Use the Weed Whacker on the weeds by pressing Spacebar.