When I'm setting the max mechanical advance, I want enough RPM to overcome the return springs on the mechanical advance flyweights. That usually happens by 2,000-2,500 RPM. The vacuum advance needs to be disconnected at this time. Taking the RPM back up to cruise speed with the vacuum line connected allows me to measure the difference between maximum centrifugal advance and the additional advance added by the vacuum chamber. Since his vacuum chamber has an adjustment, he can tinker with his total advance number to get the best fuel econony without over-advancing and getting into the detonation range.
Some highly-modified engines need to have the centrifugal advance system restricted to prevent over-advancing when extra initial timing is dialed in to improve throttle response when a very radical camshaft grind is used, but that's a whole different can of worms!
Jerry
