Thanks Max
That might also explain the similar problem I have been having with my Pop's old mower - Masport MSAA alloy deck with xm50 bought in late 90s (1997?). It had an old champion plug in (and the original red painted plug in a box was in the shed).
The air filter was awful again so last month I gave it a full on tune up - blades and bolts, 2 oil changes, air filter and prefiler, a clean NGK plug, clean and lube linkages, rpm check (was a bit fast)
And once it was hot it was lightly popping. I was thinking valve or timing (considering someone hit a pair of scissors in the grass - thin short couch grass so no excuse).
I will throw in another plug and see
Its doing 2 lawns now so will probably get a 3rd oil change next time I'm there
