I am working on s '67 122. Near the end of the summer I rebuilt the SU carbs and tuned them by hand, enough so the engine ran pretty smooth at all rpms, idled well, and started easily. The timing was also set generally but not with a strobe light, and the carbs weren't balanced with any machine, just by observation. I went to college in the fall and came back over fall break, when the weather was about 40 degrees. I tried to start the car and eventually succeeded by fiddling with the mixture. However, I had to manually choke the carbs by holding my hands over the intake (with the air cleaners off), and it wouldn't hold an idle. I had to continually rev the motor to keep it running, or adjust the idle until it was way higher than it was.

Does anyone know why this is the case, and what I could do the fix this? I richened the mixture by degrees, until it was about two flats richer than before, but that only partially fixed the problem.