20 is pretty normal .
Not to sure timing would be the issue here. Did the symptom occur after a timing adjustment? Did anyone adjust the timing?
Sounds like air getting into the exhaust stream. Maybe the EGR valve is sticking?
Could be also the carb is too rich. Also faulty AIR check valves can let air into the exhaust when not wanted and cause a pop or a burble.
Fact is stranger than Fiction