I have a 1995 formula s and I just replaced both pistons because it was backfiring and was bogging out. Upon investigation, I found that there was a hole in the piston head, and the piston seals were rotted away. I replaced the piston, piston rings, seals, and all that jazz. After that, I pulled about 20 times but it wouldn't start. The key was on, kill switch was off, full of gas and oil, everything was in place, and it backfired. The carbs haven't been cleaned in a while and the spark plugs are over 2 years old, but would that cause it to backfire? As I was putting the sled back together, I accidentally broke one of the seals for the bottom of the piston block. I tried to superglue it back together, but I'm not sure if it worked. I am hesitant to conclude that this is the problem, as I will have to take apart the whole engine apart again just to change one seal. Also, is there any difference between the 95 formula s and the 95 formula ss??