Presumably you don't want the bike to be firing 5 deg. too early so you would like to eliminate this problem. If there is a false trigger to the strobe, the engine might not be firing early, but you would like to identify and eliminate that problem as well if that is the case.
Although there's not enough information to diagnose why you are having the problem, the first thing I would do would be to substitute a completely different timing light, powered by an independent battery, in case the one you are using is being triggered by a false signal from the distributor via the battery. The next place I would look for the problem would be the distributor, using an independent strobe to examine it while a second one is on the degree wheel. I also would be studying the pulses in the HT lines themselves using an oscilloscope with a direct voltage pickup (i.e. not an inductive pickup) at the plug cap. If neither of these approaches identified the problem, my next step probably would be to try to convince the owner to let me install a rotary shaft encoder on the engine itself to get a direct timing reading rather than rely on a strobe flashing on a protractor.
