How many amps should a 18 coil magneto output? how can i measure with an autorange multimeter?

 

18 coils seems a massive number and I suspect on full power could produce more than the 10amps where a normal multimer's fuse blows. The power produced is controlled by the regulator and the battery takes what it needs, up to the max that the alternator can give. It is extremely difficult to know if any reading is the max that the alternator can provide, as the regulator usually cuts the power as the revs (and alternator output) rise. You don't really need to know. If you get a max of about 14.5v at the battery (assuming a good and fairly well charged battery) your alternator is producing all the power the bike needs. If it can do it with the lights on, it is capable of more than the bike needs.