Some applications use a slow closing throttle device known as a dashpot. They are used to delay or slow the throttle closing the last few degrees to prevent engine stalling at the lower speeds and to eliminate a sudden peak of hydrocarbon emissions on sudden deceleration.
At idle, the manifold pressure is very low and results in good vaporization of the air-fuel mixture in the intake manifold. When the throttle valve is opened, manifold pressure increases. This increase in pressure increases the boiling point of the liquid and prevents 100% vaporization of the air-fuel mixture. During these periods of high manifold pressures, there are some wet particles of fuel clinging to the inside of the intake manifold which is known as “wet manifold.” During sudden deceleration, the manifold pressure goes back to a low-pressure state, the wet particles clinging to the inside of the intake manifold go back to a vapor state and are taken into the engine as a rich mixture. This is known as “manifold flash” and can cause the engine to die out, especially at low speeds. The dashpot slows the closing of the throttle the last few degrees to give the engine time to clear itself of manifold flash.
Unfortunately at the time of posting, no replacements are available that we know of.