First of all it is not a "deceleration valve".
It is not designed to work only on deceleration.
It is designed to work at part throttle to increase the ignition timing to a
point at which the air fuel mixture burns more evenly.
You should be able to buy one at any big name autoparts retailer.
aka AutoZone, Carquest, Oreilys, Pepboys, western auto....etc.
Very common part, very common repair, not rocket science.
You can test yours with a mityvac and a dialback timing light.
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