This is the front suspension mechanism of my newly acquired Dahon Jetstream:



And this is the front suspension mechanism of my newly acquired Dahon Jetstream upside down:



And this is the big-ass nut in the middle that looks like it might be a tension and/or rebound adjuster of some sort:



Now, when I tried to negotiate this thing with a wrench, I found that it was locked as far as it would go clockwise. I was able to easily turn it a few turns out (counterclockwise,) but I was afraid that if I turned it too far, the whole thing would come apart, so I turned it back in to its clockwise locked position. With the bike on the floor, I'm able to compress the suspension about a quarter of the way through its travel with my full body weight on the handlebars.

So, what am I supposed to do with this thing? Is it actually adjustable, the same way the rear shock is adjustable?

If the suspension piston is within the headtube, there might be adjustment available from above the folding stem.
Fold the stem and check if there is any kind of adjustment from above, likely via a long allen key.

That big nut looks like it secures the suspension fork to the head tube, not an adjuster.

I folded the handlepost down and, as expected, there's the standard Dahon headset tension bolt:



I shone a torch down the 10mm hex hole and thought I saw something beneath that, so I undid the bolt and took a closer look. What I found is a pressed-in stopper with a hole in the middle:



The light-colored ring is the pressed-in stopper; a little farther down the hole in the center is what's apparently a piece of elastomer.

I dropped a couple of long allen wrenches of different sizes in there to see if they'd grab onto something abd they didn't, but they did bounce off of a piece of elastomer of some sort, so now at least we know what's doing the cushioning work. Now I'm thinking that maybe the big nut at the bottom is just there to keep the elastomer spring contained, and maybe it can be swapped with harder or softer compounds as desired. I don't know.

The plastic block on your tension bolt looks cracked

The jetstream ex had a outer shock that is serviceable. Unfortunately, the jetstream p8 has sealed cartridge in the stear tube. I had one broken (sagging shock) back in 2008 and I needed a new fork unit. These days, there are not easy to find. I saw some copies on aliexpress and I know of a shop who had a stock of hammerhead fork which they sold to customers to scavenge and adapt to their broken jetstream fork.

The P8 OE rear shock is sealed. There was a crane creek upgrade that was serviceable. If yours has the OE rear shock, you can swap it for a A5 shock with valve to adjust the pressure. They are on ebay.

As I rode it with the kids


Rebuilt for sale last year

I saw that as soon as I took delivery of the bike, but it still functions as it should, so I'm not too worried because I'm likely going to swap out the the OEM handlepost for a better engineered "Chinese" one (right) that does away with the little plastic thingy and is more reliable for it anyway:

This is the one that came on the bike; is it the same one you're talking about?



I pumped it up to 100psi before I took the bike on its first shakedown ride around the neighborhood and it was rock hard with zero compliance. I've since dropped it down to 50psi and could gain back some give on my man-cave floor, but I haven't ridden it again as of yet.

The 2nd generation of jetstream frame uses a std 150mm length rear shock. these are adjustable so you can setup the sag and with the knob, you can control it.

Yup. This is a 150mm air shock with rebound adjust. I'm still a long way from trying it out on some terrain, but 50psi seemed to work well the second time out on neighborhood streets. It's certainly more compliant than the fork!