It takes either 13 or 30 (someone will correct me : ) #s of pull on the clutch to over-center it, so don't be shy with it in case you were. But if that's not the problem, you might have have worn linkages and pins. It doesn't move much even when new so when you adjust, make sure you adjust it for quite a hard over-center (and perhaps only just making over-center), and then put it on your to-do list to fix the linkage up. Pins are available new, linkages are just plain straps and you can get your local machinist to whip 8 new ones up with a snug fit on the pins. If the holes in the spider are worn badly, you can drill them out to the next pin size. Either way you might want new straps because it could be challenging to drill out the old ones so that the hole center distance is identical on them all, which it kind of has to be