What causes the air intake to shut on acceleration?

On my S-type the air intake flap oulls towards shut when I accelerate. As it’s vacuum opersted I suspect a hose or connection….:scream:
Where should I look?

IIRC, there’s a small vacuum tank on the engine bay firewall just below the flap. I don’t know a lot about it, but it may be the equivalent of the vacuum tank reservoir for the brakes, and if it leaks, or has been bypassed, the varying engine vacuum could cause the flap to oscillate. Just a guess, though.

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Yes, as the guy above says. The problem could be the reservoir tank. If this or the connection hoses are leaky there isn’t enough engine vacuum to hold the flap open during acceleration.
I had a situation where the flap would rise and fall according to how much I had my foot down. It worked like a vacuum gauge would.

If it’s like an XJ6, I think there’s a one way valve in the circuit which stops the loss if vacuum when t hhe manifold vacuum varies.The workshop manual should detail, I expect?

PS. The small circular device on the bulkhead below the flap is the actuating valve. The reservoir tank is about the size of an oil filter and is under the rhs front wing, top left. Not to be confused with the brake vacuum tank which is much bigger. This may have been bypassed completely, as it’s in a perfect position to rot out.
I made a new one from an old oil filter bowl.