Backfire 440-6

[Phil1974]

Hi People, I have a small Problem with my RR. I drove it the on staurday about 100miles and on monday i cruised in the City. The car start to backfire and then I could not go over 2000rpm. If I try it start to backfire and don't want to idle. It runs normal, no shaking or noise. Any Ideas?
Btw thanks for the nice welcome here in this Forum!!

 

[D575]

fouled plugs Just a guess