If you feel that you have located the IFRs, then as a test you can mike a few small wires like from a stranded 16 guage. And use a small wire in the IFRs to limit the flow. The IFRs are small, so the wire should be small. I would try to stay under .010, if you can find one that small.

Also, make sure that the wide band is calibrated correctly. You dropped the mains and the engine didn't like it, but the wide band did reflect the change, didn't it?

If you want to define when the mains come in, here is a simple way to do that; just tape over the openings under the metering block that feed the boosters. When you come off the slot to the mains the engine will just stop due to lack of fuel. A short test drive and you will know exactly where your main circuit is and when you are or aren't on it.

You do need to know if you are cruising on the mains or on the idle/transition circuit. The taped off boosters will tell you that.