ORIGINAL: Al Lewis
That's because it's made of rubber and gasoline eats rubber over a period of time. Others are made of polymers that resist the effects of gasoline. That's not to say that you can't make a black polymer stopper but you would be hard pressed to make a rubber one other then black. The black stopper Toymaker is using is for gas according to the manufacturer.
http://www3.towerhobbies.com/cgi-bin...&I=LXBWL6&P=ML
EXCEPT that the Sullivan "black" stopper is BIG-TIME *N F G*, for sure. It loses TINY parts of itself over time, and that little crap goes exactly where it can do the most interference to a great running engine; the Walbro carb screen.
Has happened to me.
The only stopper for gas I trust now is the DuBro "brown" one. But that may just be my experience only.