DDK - Die Deutschen Klassiker

Any idea what can cause this?

Engine running perfectly otherwise, even better than it ever ran I would say! But while idling it does pop and occasionally spit flames! As cool as it looks I can't help but think that some must be wrong in the fuel mixture or maybe plugs?

Video here:
https://www.youtube.com/watch?v=cFEso0JGd18

Any hint appreciated!!

Ignition retarded? Or exhaust valve sticking open (bent valve, very tight tappet clearances?)

Exhaust leak?

Nice "feature" though

What Keith said. Unless you fitted a turbo and anti-lag system?

Ignition was the issue! It has now been fixed and the car is running really smoothly!!

Thanks everyone for the help!