if that compression truely is at only 80 psi, you've got problems.
a fresh rebuild should be between 120-130+ compression.
who did the rebuild ?
was the cylinder measured/bored/honed to fit the specific piston used ?
has it been leakdown tested ?
is the piston in the correct direction ?
time to start tearing into it and see whats wrong, starting with a leakdown test to check for gasket and seal leaks, a check of the reeds, piston direction (arrow facing exhaust), ect. ect.
click the very bottom link under my sig pic for links to trouble shooting