Primer seals for internal leakage.
Not just the seals but the primer lines as well, also check the induction elbows are not leaking, another on the induction side it loosen the rubber joint between the elbow and the pipes that come out of the sump and ensure those are fully swaged into the sump, IE they do not move, have known them to be loose and drawing air on Lycomings.
I take it they are the correct plugs for the engine, check.
Other possibility is a sticking valve.
https://www.lycoming.com/sites/defau...20Sticking.pdf