NO go without Downshifting

it's got a 2bbl carb, i found the threaded rod and i threaded it in all the way and made no difference. i did see that it slides back and forth on the carb, should there be something there to hold it in one position? will try to post a pic. also not sure on mileage, previous owner said the 273 was rebuilt maybe 40,000 miles ago.