Hello everyone,
I have measured the compression, which is 4.5 bar/ 65.267 PSI (cold). That seems very low to me.
The engine is otherwise running very well, and the piston rings still seem to be intact judging by the engine noise.
What should the minimum pressure be and would you recommend an overhaul?

Best regards, Lennart

I'm guessing ya talking DT250 but sounds very low no matter what bike it is.
If it's running & idling well & feels bouncy on the kickstart just bouncing against compression by hand or foot on the lever with ignition off of course, then i'd suspect the gauge or sealing of the gauge into the plug hole.
Otherwise--better pull the cylinder to be sure.

Yes, it's about the DT250.
I will take another measurement, maybe I didn't set up the dial gauge correctly. As I understand you, the low pressure does not go with a smooth running engine.

Many gauges won't give you an accurate reading on these engines.   

After I wanted to replace the reed valves yesterday, I noticed heavy wear on the piston.
I then dismantled the cylinder and noticed heavy running marks.
I think this could also be the cause of low compression.
As a result, I ordered a Wössner piston and lots of accessories today.
What advantage do the Wössner pistons offer? According to the description, they offer better running characteristics. How does this work in practice? And should I pay attention to anything special when installing this type of piston?

See here a few photos of piston and cylinder:

Photos

I will also have the cylinder, head and side covers powder-coated at the same time.

Looks to me like it's lightly seized at some stage possibly after being rebored a fraction tight or run too hot & fast early on after bore  if its oversize. All a wild guess, but then carried on after cooling for a lots of miles until what we now see. Rings & cylinder condition affects compression but not piston directly unless the rings are stuck in the grooves. Are you going to bore it or just give it a deglaze light hone with new piston kit?. Slide the rings into the bore one at a time in the position they sit on the ring pegs then hold up to the light & see if you can see daylight past them. Set them about halfway from the top to the exhaust port. All academic Anyway if ya going to replace piston & rings.

Interesting shapes on top of the piston in pic 2. Are they just shadows? Like 4 stroke valve pocket marks in the carbon.

A 250 with truly only 60 pounds of compression is not likely to even start and as I said before, gauges are notoriously inaccurate on these engines.  I found this out myself after rebuilding one of mine showing low compression with my (expensive!) Snap On tools gauge...  it ran okay and didn't make noise.  The new bore and piston/rings didn't change the compression much.  It was the gauge, not the engine. 

I'm probably wasting my time typing this...  the scarring you are seeing on the intake side is 100% from ingesting dirt.  That era model has a service bulletin on the air box seams not sealed from the factory allowing unfiltered air into the engine.  So check that...  or of course it could be someone ran without a properly oiled filter or something too.  (If it had seized, there would be scarring on the opposite side of the piston as well).

Finally, the most important thing with the new piston is finding a shop that knows how to bore these old machines and set the clearance for the new oversize piston correctly.   If you're not boring oversize, be very careful.  Wossner might be sized quite a bit different than OEM....  a difference of just the thickness of a human hair will create issues...  either rattling if new piston is a hair smaller or seizing if a hair larger. 
 

I forgot to add that unless you cleaned the old piston, it's showing remarkably little varnish staining on the sides below the rings.  This generally indicates a pretty darn good ring seal. 

Thanks for the answers, none of them were a waste of time markt ;-)
I had a feeling from the running marks on the intake side that the engine used to run too lean.

The old piston size was the original 70.00 mm, I have now ordered a 70.50 mm piston. I have a professional engine rebuilder in the neighbourhood who knows how to do this and specialises in old engines.
RT325: Can I use it to test the roundness of the bore, or would that be the aim?
It was just a shadow in picture 2 ;-)

Markt, so I mustn't rely on the dial gauge, got it. The compression was probably much higher. Do you know the reason why it doesn't work like that on that type of engines?
To be honest, I'm also unsure whether I installed the air filter correctly. I oiled it too much at first, so the engine ran far too rich and didn't take any throttle. I've cleaned it and now maybe not enough oil. I'll have another look at it. I thought it wouldn't be that important. It was probably a mistake.
I know that you should put grease between the air filter and the air filter housing (i.e. the frontside of the filter) so that no dust can be sucked in between.
Maybe it would make sense to use an non-yamaha tuning air filter, which hasnt to be oiled?

Another question: Is the engine originally powder-coated or painted? I wanted to have the engine powder-coated, but I'm worried that the thick layer of powder on the cooling fins will reduce the cooling capacity and cause thermal problems. In the pictures i saw, the whole engine is black including the cooling fins.