The lightweight blade of a gmc mower gives the false impression that they have compression, I concurr wit hthe thoughts of maybe the conrod bolts never being torqued enough at the factory, and the very obvious fact that they are very pourous castings.
I would say the problem with the compression would be a HUGE ring gap, I have pulled some down before with a (no bullcrap) 5-10mm ring gap, they will act like they are trying to fire up but they are very weak. ill see if I can digup a photo of at least one bad ring gap case I have seen...
