Thermoquad race tune

There are a couple other things besides the air door spring that affect the transition to the secondaries.

One is the very small bleed hole in the front wall of the RH secondary barrel. It connects with the vacuum diaphragm that's linked to the air valve. The front edge of the secondary throttle valve blocks this hole off when the valves are closed. As soon as they start to open, air at atmospheric pressure enters the bleed circuit, and over a couple seconds this spoils the vacuum that's holding the air valve closed.

The other is the initial opening of the air valve--.040" or so. This is a little like the initial kicker opening on an automatic choke. When the secondary throttle plates first start to open, this opening allows some air to flow in past the secondary nozzles, but only a little. When the secondary plates are just starting to open, for a moment that creates a very high vacuum at the nozzles and a rich mixture.

Both these things are another way of doing what a separate secondary enrichment nozzle or a separate accelerator pump for the secondary circuit would do. They work with the air valve spring to keep the FAR from being too lean (or too rich) during the short time the valve is opening.

At least on some TQ's, the accelerator pump linkage is also designed to provide an adjustable shot of fuel when the secondaries begin to open.

So there are really three or four things that interact to control FAR during the transition to the secondaries, and not just the air valve spring. I would think that slight adjustments to any or all of them might make for more power during those few seconds of transition.