Hi, I have a 240z with the L28 conversion with twin Hitachi carbies & Im pretty sure they're series one. Have a Bosch GT40R coil with pertronix electronic ignition, new rotor cap & button, new plugs & leads. Have set timing to 8 degrees btdc, have balanced carbies, have got engine idling at 700rpm & have used colour-tune to set the fuel ratio, which all seems ok. My problem is that the engine brakes down & back-fires out of the carbies at about 2,800rpm. Have tried enrichening & leaning off carbies, have tried adjusting timing, still no difference. The vacuum advance seems to be working, any ideas as to the cause??