When I ran a Supa5 it was always a bit rough at low revs – it only runs happily over 3000. There is negligible oomph below that. When I complained to the dealer about surging on overrun he opened up the idle jet to 0.5 mm, so presumably a 50 jet. I didn’t notice any difference. Other jets should be corrected. The proper manual for the 250, in the supplement for the Supa5 says main jet 135, needle jet 70 (without cross bore). For the older four speed the main jet is given as 140, needle jet as 70 (no mention of cross drilling), starting jet 110, slow running jet 35, float needle valve 20, slow running screw 1½ revolutions open. I assume the difference in the main and needle jets is significant – the rest remain the same.

For the needle position:

“Needle position from top 4 to 5 (5 for running in). Besides the driving habit, the sparking plug appearance is decisive for the adjustment.”

Have you checked the cold start plunger? If the rubber stop in the end of it doesn’t seal on the jet it can cause odd problems with the running. When mine was a bit worn I prised it out and turned it over. That improved things a bit.

If I had one now I would look at one of the Bing carbs offered by some of the German spares dealers. It’s a better carb.

Also bin the Haynes manual and get a proper one. I bought a genuine MZ one and found it so much better. It left with one of my Supa5s, but I since downloaded one from somewhere – can’t remember where but they are out there.