Regarding the posters issue.

Just as a general rule whenever I hear of a backfiring issue I reckon it's normally ignition related like norm and max said.

Norm and max are spot on with their advice on these Victa mowers.

It's probably either the points or the ignition coils are bad. With ignition coils they talk about resistance tests on coils but I reckon a lot of the time they are useless tests. Ignition systems need testing under load to stimulate the cylinder pressure on the ignition system which is at its greatest when engine is under load.

If it was me, I've got one of these cheap spark testers were you disconnect the HT lead from spark plug and clip to engine ground and set a decent gap and see what the spark looks like. It should be a nice blue spark that jumps a decent distance on the tester. If you have a working mower I would look at how that spark looks visually versus the mower you posted about. Would give some clues

https://www.repco.com.au/tools-equi...ark-plug-firing-tester-302167/p/a9485434

Then I would buy the ignition module normk is talking about and replace points. Points are a stuff around. Old timers like them because it sounds romantic but they always go bad. Here is a link or even better someone on this page might give you one or sell you one cheap.

Then if that ignition module doesn't fix your issue of misfiring under heavy load I would buy a new ignition coil.

Either the module or coil will fix your issues is bet.

https://pushmowerrepair.com.au/product/universal-ignition-module/