In addition to the bellows on the shift rod, the large rubber boot between the housings near the bottom can cause them to eat their exhaust when in the barrel or until up on plane. This also happens if the outer housing is slightly deformed so it doesn’t seal around that boot. You can’t see the boot without taking it apart.
Dave

At this link, it’s part number 87:

http://www.marineengine.com/parts/johnson-evinrude-parts.php?year=1969&hp=9.5&model=9R69S&manufacturer=Johnson&section=Lower+Unit+Group