A mate of mine has a GPZ 1000RX 1986 model. Bike hadn´t been ridden for some years and he has done a lot of work getting it running and overhauling brakes, new tyres, etc. The most recent work was a new head gasket. Now the engine, which he says was running well after the gasket was fitted, has developed a bad misfire. Removing the HT leads one by one with the engine running shows up cylinders 2 and 4 as possible faults. New plugs were fitted but putting the old plugs back didn´t help, neither did swapping the plugs with the two good cyclinders. There is spark on the two