this is the problem with the k jets its older technology
i had a 84 gti and i ran that on 95 unleaded and never had pinking it was great just one problem i had was the old filter seal blew out on start up once and all the oil drained out of it
2000 miles later the bigends were rattling.... so i got rid of it....
the digi's are much more advanced they automaticly retard the timming if pinking occurs by the knock sensor...
sound like his ignition timming is way out!
