[drolston]

Getting a tri-power setup to idle correctly depends on getting the linkage and associated return springs set up so that with foot off the pedal, the throttle butterfly valves in the end carbs are both exactly and perfectly closed. The idle jets on the end carbs also need to be completely closed. The only way to do that is to take the body of the carbs off, leaving bases still tightly bolted to the manifold, and the throttle linkage in place. Taking off the carb tops is easy with Stromberg 97s, as all screws are accessible from above. Then adjust the linkage joining the end carbs so that both go fully closed together.

But that alone is not enough, because there is always a little slop or flexibility in the linkage and throttle shafts. For me the key was having stout return springs on the driver side throttle shafts of both end carbs. If you look very closely at the left carb in the photo of the linkage side, you can see the spring that goes around the throttle shaft, between the linkage arm and the carb base. One end of the spring rests on the base of the carb, the other is hooked under the linkage arm, pulling it towards the closed position. The return spring on the accelerator pump tends to close the throttle, but that alone is not enough.

With the end carb butterflies tightly closed, all idle airflow and mixture is controlled by the center carb, making it possible to reliably tune the idle, and avoid stalling at stop lights due to vacuum leak past the secondary carbs.