if the power vavle is shot then you should be running rich all the time. choosing the correct power valve is determined by the amout of vacuum you are generating. what is you vacuum at idle in and out of gear? using those values, the rule is either half the value in neutral or 2 points lower when in gear. example vacuum in neutral is 15", then use a 7.5 power valve. but if the power valve is intact then your idling problem is elsewhere.