It probably has a pierburg carb which are tricky to sort once they go wrong (unless its been changed for a weber then it won't be in Haynes) but its more likely that you have an air leak in the thick rubber gasket between the carb and the inlet manifold, it gets sucked tight when running at higher rpm, alternatively iirc there is a short vac pipe that comes off the gearbox end of the inlet manifold and links into the servo vac pipe, this tends to get smegged & split on the underside where you can't see it, again when you give the engine some revs it sucks the split shut so OK at higher RPM but will not tickover, usually feels like its only running on 3 cylinders at idle.