A puff back through the carburetor is called a spit. On a 4 stroke it is usually lean mixture or a leaky inlet valve, but it could be caused by wrong valve timing or (maybe) wrong ignition timing. On a crankcase induction, piston-controlled-port 2 stroke like your Victa, it could be caused by lean mixture, wrong ignition timing or leakage past the rings. Lean mixture is not really consistent with the mower starting easily and its speed control working properly. However I'm not happy about your reference to spits coming out from the carburetor to cylinder joint: that joint should not leak, and if it does it will cause lean mixture. I normally regard wrong ignition timing as a bit unlikely on fixed-timing lawnmowers unless the flywheel key is sheared - which happens when the flywheel is on a parallel, but is rare when it is on a taper.
I think I'd start off looking at the seal between the carburetor/inlet pipe and the intake port. Any leakage there would cause problems, which might include spits (though usually loss of governor control happens before that). The second thing I'd check is the compression feel, compared with a good healthy Victa 160. Remember, there should be two sharp compressions for each revolution of the crankshaft: one for crankcase compression, and one for cylinder compression. If the compression is down, the engine won't run well in various respects, perhaps including spits.
