Subject bike is an ’83 Passport C70, a crusty barn find being fixed up and put back on the road for my granddaughter. Almost there. The carb was too roached to rehabilitate, so I installed a new one from Beatrice. The idle was easy to adjust, and the motor idles well. When riding, the issue comes at about ¼ throttle, which I assume is about the point where the idle circuit gives way to the needle. There is a sudden, uncomfortable lurch forward, like the bike wants to jump out from under me. After that, everything revs and runs fine, up to full speed. The same thing happens when throttling down. At about ¼ throttle, it the bike wants to jump back. These transitions should be a lot smoother. So far, I have not fooled with adjusting the needle or jetting. Is this a familiar issue? Is the needle setting too lean? Too rich? Improperly jetted?