Did you ever get your 521 to operate normal?2 quick things,have you checked the fuel for water,it raines alot there ,right?521's leak water thru the fuel sender quite often.pump some fuel thru the filter into a glass bottle.water is gray looking,gas is amber.had this problem on my 521,just quit,wouldn't start.also duel point distrib.are they set correct?sometimes one set stayes closed,won't run!!