hi Rachel!! nice to hear you have bought a BIS!!!
dont panic!! We are here to help, i see Chris and Richard have helped already,
i own a BIS, and i have had problems with my carburetta in the past, but your on the right track, because they made two types of BIS carbs a proper Weber (which you have) and a not so good polish one.
take off air pan to get to carb, let the engine warm up, then adjust the two screws i have higjlighted in red in the below link, untll the car sounds happy, i.e. doesnt chug, but do it gradually, dont do massive turns. The mixture screw is really hidden, but you can just about get a small thin screw driver in.
see this page i have put on here for you,
www.fiat126.co.uk/webercarb.htmlcan i also add about the over filling of oil, it can cause oil pressure problems, and damage seels??
i really hope this has helped, keep going at it, it will be worth it!! any other probs, leave a post and someone will get back to you!!
regards
Chris