Some of them have a plastic intake with O-rings as a seal.
I've heard of them plastic manifolds with hairline cracking but never seen such myself. I've seen manifold O rings that were flatten out but generally the engine won't idle really smooth a very low idle.
Have you tried turning down the idle screw and listen to the engine at a really low idle?
Does it idle really good and smooth at very slow speed after it starts?Are you sure that the after fire solenoid is fully retracting easily and not hanging slightly at first?
I've seen them after fire pintles not retract fully when 12v testing off the engine and I could push in on the pintle gently and the pintle would go in further and I could usually free them up for full retraction by testing with 12v on the workbench.
Keep a heads up when spraying the carb cleaner, most are flammable.
Oil in a squirt can is less hazardous but messy. WD40 is also flammable.
How does the engine act when it does start, does it have oil smoke or act like it might have been flooded or want to start at full throttle with no choke or????
Stay Safe when your are around that dog testing for air leak and people. It can get you from both sides.