noob spud problem

This is my first gun. I have a problem. First the chamber is 4" and about 14" long and then is reduced to 2" for the barrel. The bbq sparker is in the rear cap. I am getting plenty of spark. I added about 6 sec worth of aquanet and got a little thump. The potato went about 30 yards. I am real dissapointed. What am I doing wrong? Too much fuel? Or is the sparker need to be in the center of the chamber? Also I have the end of the barrel sharpened to make sure I get a good seal when I push the potato down the barrel.Any help woould be appreciated. ALOHA