Not had time recently as im also rebuilding my wagon’s tdi block and working on my boys ww2 jeep. Just love spannering.
Just been out to look at her, and once warmed, in neutral, i took her slowly through the revs. And she stumbled at almost exactly 4000 revs upwards. I’ll check the pick up resistance anyway, but i think im with scottie on the carb. The last run i did, i was cruising and opened the choke slightly, and she bogged right down, so my next try was going to be a smaller jet on the left carb, as thats probably the main area for high rev cruising. The right carb is opened up( drilled jet, raised needle and drilled slide holes), but that usually only kicks in for throttle, (was going to bore scope it when running to try to see how far it opened at cruise) although its easier to put back to standard without stripping the bike. I have spare stock slide and 125 jet. Whats your best guess?

