fzs600s unrideable under load

[snapper]

I dont have a 600 and know very little about then but i had this same problem with a mk2 escort ticked over reved but he second it was in gear it just had no power drivable but only just , turned out to be the timing had slipped ! and the advance retard was faulty ,
sorry if it not very helpfull but  you nver know !