Chapter 1: Section 7: Page 59: Line 55 (1036)

 Some were inventors with light-engines that could run a bicycle all day but at nightfall stopped abruptly, causing the bike to fall over with you on it, if you weren't careful.

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"Some"
Pynchon can get pretty confusing with referrants at times due to his narrative's loose connection to any one time, place, or perspective. This "some" is simpler than most because it's either referring to the scientific enthusiasts who merely came to the city for the experiment or the ones who were locked up in the insane asylum. This should be an important distinction and maybe subject to your distrust of authority figures and how petty they can be when challenged on any of their beliefs. Did they throw a guy in a mental institution because he invented a self-propelled bike that shut off at night? If the bike didn't also have pedals to continue being used in a regular manner as soon as the engine stopped, I'd have thrown him in an asylum as well.

"light-engines"
I desperately want to read this as some grand science fiction space engine but I'm sure it just means "solar-powered."

"at nightfall stopped abruptly, causing the bike to fall over with you on it, if you weren't careful"
This just sounds like a conservative trying to scaremonger the pubic away from solar-powered engines. "Oh, it works perfectly fine during the day, does it? But what about . . . NIGHT TIME! Have you thought of that? A child not realizing night was super slowly coming on the way night always comes on could be killed when the bike abruptly shuts off! Would you have that on your conscious? No? Then stick with the coal burning steam versions that only occasionally blow up in their inventor's faces, killing them!"

Looking at the sentence less literally, we can read in it one of Pynchon's themes of the book. The book began with the Thelonious Monk quote, "It's always night, or we wouldn't need light," suggesting, perhaps, that without purposeful intervention, things would be dark, bleak, despairing. Our bikes will fall over if we aren't careful. The light sustains us, shows us the way, allows us to proceed in a forward manner towards the future. This can also be more metaphor than literal in the sense of light, or illumination, being seen as knowledge. Without the constant influx of scientific knowledge, we will eventually crash (if we aren't careful!).