Is it a stock pipe, pics?
I have seen them leak right at the cylinder where it slips on the exhaust manifold I have also seen the head pipes crack now is the time to pull the seat and start it up look to see where its leaking then pull the whole pipe for cleaning and inspection and so you can reseal the system.
http://www.pilotodyssey.com/Siliconesealer.htmUsually what causes them to leak and or crack is bad isolators and or missing springs.
Wow I see you can still get the springs and rubber mounts
http://www.partzilla.com/parts/search/Honda/ATV/1979/FL250+A/MUFFLER/parts.htmlIf your
Engine mounts are broke and allow the Engine to move more than its suppose to this can also make the exhaust leak and break the head pipe.