I have a Honda Nova Dash 125cc two-stroke. The problem I've got is that my bike only does about 12km a litter. I've replaced piston rings, base and head gaskets. The piston and chamber were in good condition – so I don't think I'm losing compression. The exhaust doesn't seem to be blocked. Although the exhaust fumes do come out red hot.

I've stripped the carburetor and there doesn't seem to be any wear or gouges in any of the parts.

Anybody has any ideas?

what colour is the spark plug when u remove it?
spark plug colour is a good indication of if the bike is running too rich or lean.
If its black and sooty then its too rich ( too much fuel little air)
if its white or powdery colour its too lean ( too much air not enough fuel)
It should be a light brown or tan colour.
make sure your air filter is cleaned and oiled or replaced if its a paper type.
You may need the carby to be correctly jetted if its running to rich or too lean.
hope this helps a little.

The problem with a two-stroke is that you not see much color difference with the spark-plug. If the spark-plug looks a bit greasy – your fueling is way to rich. If your two-stroke is on the lean side you probably were not complaining on the low fuel economics.

If the choke unit is faulty the spark-plug should show very black and a bit greasy at all times. The choke mechanism carburetor easily checked by removing it from the carburetor body and turning on the ignition; it should, after a few minutes, extend fully to shut off the choke passage. If this is correct then the next area to aim for would be the exhaust. It's a dirty job but you need to look into the pipe and within the silencer and unblock them.

I agree Richard.
my only 2 stroke experience is with 250cc motorcross bikes.
If they are not jetted properly or too much oil is added ( 32:1 works best) then oily residue seeps from the exhaust and blocks the muffler hindering performance.
the exhaust should be cleaned out and the muffler re-packed with new insulation
muffler packing is cheap and its generally an easy job to re pack it. Make sure the perforations in the baffle are clear before wrapping with the packing.
matty....

It's very likely the exhaust, as Matty say clean and repack your exhaust is easy and costs not so much. You can find more information about cleaning and repacking your exhaust system here at article Learn how to repack your exhaust.