The air injection is routed into the exhaust ports of the heads, but could wide open valve be introducing more air than heads can pump out, and the excess ends up in the crankcase?
I vaugely remeber those. One side of the reed valve goes to the exhaust port. The other side goes where? If a reed valve is broken off, it might be possible for exhaust to go backwards through the valve.
What about EGR valve, it is capped from weber conversion.
No, that goes exhaust to intake. If it was way out of wack, the engine wouldn't run good.
Pictures might help. MKaybe someone will spot something.