Your current carb has a spark control valve (SCV) and will not send a correct vacuum signal to the duraspark. The best way to solve your problem is to get a high vacuum carb (non-SCV) and use the ported signal from the carb. This will be close to the vacuum signal that your Duraspark was designed to receive. There is some fine tuning that can be done with springs and adjustments inside the duraspark.
If you cannot swap the carb, use manifold vacuum. However, idle/just off idle will have poor performance. Most guys running manifold (eg high revving/race applications) could care less about idle/just off idle performance, but in a daily driver app you will notice a difference.
There is a sticky on the SCV function and the Load-O. Good reading.
John/Fresno
