The whole system is now clear to me, it's a genuine S25 (25 kmh mofa) setup with carburettor to the left side of the frame. Tomos used a Bing
carburettor instead of a Dellortho or Encarwi one. As long as this works, there is no problem. The system has been on our forum before, like here
. It's in dutch, but it has pictures and maybe you can understand what is been written there.
In your case, you can check for a clogged carburettor and/ or jet (cleaning it out), clogged air filter (just renew or leave it out to test only!), wrong adjustment of the ignition, a combination of things or a case of that air does comes into the system where it's not supposed to. That last thing may be investigated by using the so called "deodorant test". When you spray some of it around the connections where the carburettor is fixed to the intake manifold (when engine is running) and your rpm changes at a specific point when you spray, there you have a leakage of air (in Dutch it's "valse lucht", in German "Falschluft "?)
Like i mentioned before, can you provide information about the color of the spark plug electrodes? Have you already figured out your type of ignition? In some cases the setting of the timing is off. By adjusting it sometimes the complaints will be solved.
In your case you have iron type engine covers, right? To switch to a right side mounted Dellortho is no option because you will have problems with remounting the covers, and/ or the carburettor doesn't fit with a air filter because the mounting point are lower than a frame with plastic covers. Only option is like kinderbrueno mentions to use a straight intake manifold when mounting a Dellortho between frame and engine like his links. This is an Encarwi and egg-shaped cylinder, so not a good example for your cylinder. Yours is more like this: