Hi everyone! I have recently bought an r33 gtst 1995 with 100k km on the clock and a few mods, front mount, air filter, full straigjht through system. just recently when i take her above 4500rpm it starts popping and back firing, was wondeing if anyone had any helpful suggestions on the prob? it also idles very lumpy and i thought maybe it hadnt been mapped to run on uk fuel, as it is a fairly recent import? thanks alot!