Another alternative is to try an adjustable vacuum advance for performance applications. I have tried to find out adjustable vacuum advance specifications from a number of manufacturers but not one has given me a good reply. From the similar appearance of the various brands' vacuum advance units, it would not surprise me that they all buy their parts from one manufacturer and put their own name on it.
For those who are not familiar with these devices, the adjustment simply limits the maximum advance. There is no way to adjust the vacuum level at which advance starts. However, I was able to measure the Proform adjustable vacuum advance and came up with the following specifications:
|Total Number of Turns||11|
|Vacuum Advance Starts||1" Hg|
|Vacuum Full Advance||9" Hg (with 11 turns)|
|Advance per Turn||3° (measured from 12° @ 4 turns)|
|Maximum Advance||33° (estimated)|
Obviously, with the Proform unit, you would have maximum advance under most normal operating conditions. From the crude measurements that could make from my timing light, full vacuum advance seemed to be closer to 20° than 33° when the advance was turned out completely. The centrifugal advance, which is of course RPM-dependant, may have had an effect on my measurements. Use timing tape, painted-on damper markings, or an adjustable timing light for verification on your own vehicle.
Making changes to the Proform vacuum advance is quite easy since Proform has provided a vacuum can with six sides. Lining up the included Allen key with one of the corners of the can would allow you to make changes in 1/2° increments. Turning the Allen key clockwise increases the maximum advance.
Start off by gently turning the adjustment counter-clockwise until it stops. This is 0° of vacuum advance. Readjust your Allen key so that the tip is pointing up. As a starting point, turn the key 3 full turns. Remember that vacuum advance is a device for maximizing your fuel economy and that your engine will likely never produce audible knock from excessive advance with propane due to its high octane rating. Drive you car through several tanks of fuel and note the fuel consumption. Increase the vacuum advance by 1 turn (clockwise) and drive for several more tanks of fuel. Repeat this procedure until your fuel economy stops improving. Reduce your timing (turn counter-clockwise) in 1/4 turn increments to the position where your fuel economy is best.