Originally Posted by
Vintage Schwinn
It is a GIANT bicycle re-badged for Schwinn. It is a decent basic bicycle with SUNTOUR derailleurs front and rear. Beyond the closure of Chicago factory, Schwinn increasingly turned to importing much more. Schwinn's contracts with GIANT propelled GIANT into a major player in the bicycle industry such that they soon overtook Schwinn in both status and importance.....................................
I was going to speculate that the bike was a Giant frame. I have a Nishiki Sport from that era also Giant and it is a solid frame. Light enough and stiff, lighter than Schwinn normally used. The Sugino cranks are an improvement over what Schwinn would have historically used at the price point.
One thing that you might want to consider as an upgrade and I would put this off for consideration until after you get it running right would be quick release for the wheels. This will involve new axles and skewers,