Are we talking about a generator, or an alternator as a power source? Most voltage spikes are the result of sticking regulator points, or occasionally bad grounds. Be sure there is a ground wire from the generator/alternator housing to the regulator base, and rub a piece of index card stock between the regulator points to be sure you don't have oxidation or roughness there. Don't change any spring tension on the points unless you have a very reliable way of checking the charging voltage. DO NOT trust a dash-mounted voltmeter, as most of them are a WAG at best.
Jerry
