Loud Pop on Lawn Mower Start

Hey, I've been struggling to figure out the issue with my lawn mower, so I thought I'd ask here.

I have a Briggs Quantum XTS 60 push mower, and sometimes when I start it, the exhaust produces a loud "pop" - like a gunshot. This happens only when I start it - otherwise, it works great. Recently I cleaned the carb and bought a batch of new fuel, and thought one of those things is probably the culprit.

So, I took the carb apart again, cleaned it all up, bought new gaskets, changed everything, mounted it back up - pop on the very first start. Then I used a different batch of fuel - and it happened again.

I can't really pinpoint it to anything, and I can't get it to pop every time. Sometimes it pops when it's cold, sometimes it pops when it's warm. But always only at the start.

I guess it could be something to do with the choke, vacuum pulling too much fuel, but as I said it pops even when the choke is open and the engine is warm.

I'd appreciate anyone feedback that would help me understand the issue, as I've been getting quite interested in small engines, but this one thing puzzles me.

Edit: Oh yeah - I tried a new spark plug too.