There are four known exhaust leak issue with the 9.5hp Johnson/Evinrude series motor from 1964 to 1973.
Here is a digital parts diagram of your motor and for ordering parts.
https://www.marineengine.com/parts/vintage-evinrude-johnson/380046/catalog.html?https://www.marineengine.com/parts/vintage-evinrude-johnson/380046/38004600001.htm
1. Is the
exhaust housing seal. (Trash or debris could be lodged under between the rubber seal and the wall of the motor exhaust house cover. With a flashlight you can look under the motor or put the motor on its side and visualize the exhaust housing seal.) Worst case you will have to drop the lower unit to inspect the seal.
*** It could be damaged or miss-shaped over the years.
2. Is the
exhaust hose. Could be dried and cracking or torn but not visually seen. With the motor cover off and the motor running, take a bit of soapy water and pour it over the hose to see if bubbling occurs, kind of like checking for a leak on a bike or auto tire.
3. Is the
shift rod boot. (bad news is this part is NLA (no longer available)
** Some have tried to repair it using hardening high tempered gasket sealer. (Make sure it is fuel/oil proof and high temp resistant. I have been told they apply the sealant around the boot to seal up the tear or breakdown. *** I have not tried this so I can not attest to its success, but have heard from experience tinkers that this sometimes works. No guarantees....but if that is the source then you don't have much to loose except running the motor without the cover.
4. Impeller housing and drive shaft
(washer and seal) The washer and seal together help prevent exhaust gases from ascending above the exhausting boot in the motor cover area. These are parts are often missing or worn, because people don't realized there purpose.