The dwell angle is the starting point. Get that right as per book and then timing. make sure that your HT cables are in good nick, perhaps replace old ones. Just as an experiment you could use "super" 98? gas and see if that makes any difference. Make sure that there are absolutly no air leaks in carbs/manifold
Check this thread
http://www.volvoforums.org.uk/showth...=148766&page=2
Also your "new" dizi. is there any slop when they on the dogs engage?