Thats what made me think it might be the governors, one of the Scandinavian countries was the last to operate the cent, & I think a party piece of the drivers was to drive them in a manner to make the governor’s operate which created all kinds of popping & banging, in part due to the unburnt igniting in the exhaust system! although obviously that wouldn’t have applied to you.
Anyway, glad you sorted it.
any Video?
Myford

That makes sence, Plenty of unburnt fuel igniting trust me LOL:diablo:
Video here.

http://spitfirespares.com/SPITFIRESPARES.COM/pages/Spitfire%20MkVIII%20Replica%20page%203.html

right at the bottom of the page is the best one. I havent got one of it runnning properly yet ,still fixing coolant leaks.