The Magnum has a twin needle carb as far as I know (?) and the low speed needle works from idle up to about 50-70% while the main needle sets the top range.
The main needle should be set at full throttle only, if then the low end is too rich that is corrected for by using the low speed needle. So you need to get the engine running slightly rich at full throttle, by adjusting the main needle, only then should you worry about the running below 50% throttle and you fixed that with the low speed needle. The two needles are linked so it is important to tune them in the right order and at their corresponding rpm range.