What your experiencing is typical for low compression.
It may be the rings are stuck. Getting it warm and then
letting it cool back down helps to free them up as the
metal parts expand and contract. The detergent it todays
oil along with repeated cycles of running and cooling will
clean them up with time. A little sea foam in the gas
might speed up the process. If you cant get any
improvement you'll need to pull the cylinders to see if
they are still stuck or just worn out.