Lots of possibilities BD. Could be failed piston rings, blown head gasket, sticking valves amongst other things. Previously you said it was recently rebuilt by a shop. You need to begin investigating these different possible conditions, but may also want to reach out to the shop on what if any warranty they offered.
You've already checked basic compression. Time to pull the valve cover and check for full valve opening/closing and then probably do a "leak down" test as that will help better ID where your loss of compression is coming from. Probably better to post about this in the Engine Shop to get the best visibility and responses back on whats going on.
~ Dan 1951 Chevy 3 window 3100 Follow this story in the DITY Gallery "My Grandpa Carl's Truck and How it Became Mine" 1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver) US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)
