Problem with securing seat
 

I just acquired a forest green Dahon Boardwalk 7 speed with Shimano r derailleur.
The sear tube has a plastic shim and the tube top has a welded/brazed collar with (two) arms with holes at each rear side. It also has a Quick Release cam skewer to clamp the seatpost to secure the seat in position. The seatpost says “34” on the lower part.
I am unable to make this work. Even tightened to the point where both arms begin to touch, the seat slips. I used tools to tighten further, stripped the plastic(?) threads in the skewer knurled knob.
Does thus bike require a different shim? Different clamp? Different something else? The seat post fits snuggly into the loosened seatpost/shim - just can’t tighten it to non slip.
Thanks, in advance for any help. I can’t post pictures yet since I am a newbie.

You can try carbon paste, but you may need to buy a better qualify clamp that has a deep hex fitting so you can leverage it tight. I did. Chromag clamp worked wonders for me.

The most common solution to this problem is to clean the seatpost and tightening collar. Like, really, really clean.

I had a problem with my seatpost slipping down a while back. Different bike, but same problem.

Someone here suggested that I sand the seatpost a bit where it's clamped with rough (not fine) sandpaper.

It worked. It's worth a try, I guess. Obviously, there's a limit on how many times you can do that, but it's held pretty well for a long time for me.

Once it is sanded then it should not need doing again, unless one is in the habit of adjusting the saddle often enough for wear to occur. The folders I fold usually have the saddle really low so that I don't touch it.

Plastic wears out. Dahon USA sells replacements.

I have a seatpost on a different bike, I think it is slightly undersized, but just barely. It is painted black. I sprayed on a couple more layers of back paint, let it harden up for a week before re-installing. That was enough to get the size up to where it should be, no longer slips.

But in your case where you say that two arms touch, that does not sound good. Is it possible that the seatpost has been changed to a different size?

I do not have a Dahon, so consider my comments as generic, not specific to your bike.