NYC of the 70s-90s was considerably different from today: much less safe, bankrupt, gritty, dirty...while you might have seen and smelled some trash, it's not at all like it used to be.

http://all-that-is-interesting.com/1980s-new-york

The areas of Manhattan you'd be likely to visit are very dense and there are generally no back alleys so, even with the system working as intended, there can be a lot of trash that ends up on the sidewalk waiting to be collected. Yes, in the summer especially, Manhattan gets hot and smelly. There's a reason that a "house in the Hamptons" is something of a meme for wealthy New Yorkers.

When I lived in Manhattan one summer, during a time when the city was "rougher around the edges" than today, I was definitely ready to get out of town on a number of weekends.

Probably a trash collection day. Those days during the summer are gonna stink, no real way around it.

The city is remarkably clean and safe for what it is.

That's just garbage day though.