Sorry if this has already been answered or if I am missing the obvious, but what fuel source do the mutants use? I know the Thundercats and Lunatacs use Thundrillium, but the mutants seem to use something entirely different.
In “The Fireballs of Plun-Darr,” they used fuel from their wrecked spaceship to launch fireballs out of Castle Plun-Darr’s mouth.
That’s an interesting point you raise there, adssse. As Daremonger correctly pointed out that the Mutants did use the fuel from their crashed spaceship to power the Fireball launcher, I’m guessing that they used that same fuel to power their other vehicles.
Though it does raise the question as to how much of the fuel supply did they have stored? They never seem to run out of it! 
Thanks for the response guys!
I do recall them talking about the fuel in The Fireballs of Plun-Darr, but they never seem to go into detail of what it is or as Wilycub pointed out where they ever obtain more. I wasnt sure if I was missing something.