4k headbolts are a joke! your set inside the engine will be eaten up from rust and normal wear and tear, go arp.
i can supply pics of my headbolts i pulled out of my 4k, i wanted to go arp!
mains mean primary jets things in the fuel bowl.
correction jets are on the top near the chokes
http://freespace.virgin.net/tommy.sandham/weber.htm
http://www.vw-resource.com/jets.html
some weber links you should have a good read through to understand how they work
great info even some jetting for 1.5L