Hi LJ,
It could also be a valve spring gone soft !! If the engine was sitting for years with one compressed then it may have
compressed to the point of being very lazy!!!!
Another consideration is that when an engine is idle for years the rings get stuck in the grooves
what can happen when the engine is started is the rings give enough comp to start but wear rapidly or break as the heat builds up leading to consequentual damage
Ask me how I know
My RGS hasnt been started since around 1992, Iiwont risk starting it until I have had it all apart and the sludge at least cleaned out
I hope you do not have the black cloud following you around like me
Good luck
JOhn o R