At idle only I suspect camshaft timing, valve or guide issue or a problem in the carburetor idle circuit.

As stated identify if it’s back firing into the intake, or is it popping in the exhaust.

A quick test for valve timing or valve sealing issues is to perform a check of the intake vacuum.

If it’s jumping around, then compression is getting into the intake. Low vacuum look for intake leaks.

Compression a d leakage tests are a good idea to rule out or confirm mechanical issues.