I concur with the blend door and vacuum ideas. I'm not sure exactly how your system is supposed to work, but I would think there would be a vacuum reservoir and maybe a check valve here and there to maintain vacuum in the system when manifold vacuum goes to near nothing, as when WOT occurs.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.