The challenge with dual fuel conversions is optimizing ignition timing for each fuel. It is not good enough to just rotate the distributor on older vehicles to get the optimum advance curve because these engines generally could use more initial advance to quicker and at a lower RPM than a gasoline advance curve.
Dual Curve used to make a wide variety of timing advance products and many of these products were integrated with their fuel controllers. Since Autotronic Controls Corp (MSD) has closed down their Dual Curve division, all Dual Curve products are now obsolete. For older vehicles, Dual Curve made a dual advance curve ECU for these applications under PN 40622. While they were still in operation, Dual Curve provided the following response about the operation of their 40622 ECU:
Regarding timing advance, the PN40622 is designed to give an additional 8-10 degrees of advance, after the distributor is recurved.
Referring to Dual Curve's PN40622 instructions pg. 2:
“The Timing Recurve will electronically delay the factory set timing signal 15 degrees. The user must mechanically move the distributor timing ahead or advanced 15 degrees. Add 15 degrees to the original factory gasoline set point. With the 15 degree delay for gasoline, the Recurve can then provide timing advance for the higher octane LPG/CNG fuel. At idle on LPG fuel, only 10 degrees (CNG fuel, only 13 degrees) additional advance will appear on your timing device. The other degrees (5 LPG or 2 CNG) of timing are used to compute a smooth, accurate timing for RPM changes. After the (gray wire loop) fuel timing program is set-up and selected. The fuel selector switch will select the proper timing program via the yellow wire.”