I'm having an issue that perhaps some of you have had - the front derailleur on my Univega Gran Turismo (1980), a Suntour Vx, can't shift into the smallest chainring (triple crank) under load. It's a low-normal derailleur, and either the spring just isn't strong enough or it's worn out , but I think a top normal model would allow me to yank it into gear with some pressure on the pedals.

If I'm not mistaken, I thought that was one of Suntour's major innovations, the top normal front derailleur. Why isn't mine like that, and which models should I look out for?