1973 Duster 198 ci. 75k original mileage. So I believe all of the emissions crap is still on it. Canister and lines are for sure still on. It starts pretty easily after a few days of sitting. Immediately idles smooth. If you punch it quick, it's revs smooth. Full throttle, pulls smooth. So idle, transition, and full all feel good. Once you get cruising at say 1/4 throttle or whatever.... It's stumbling and surging. I guess it's more of a surge. It is not misfiring. It's just annoying as heck. What should I start looking for ? Or just buy a rebuild carb for it ?