These dashpots were in wide use in the '70's. We had just experienced the Arab Oil Embargo and the manufacturers were forced to drop compression ratios dramatically to cope with the cat-pi** gas that was available at the time. They also began to run the motors hotter to try to help with emissions and placate the "green" people. If there were any deposits in the chambers or burrs on the spark plug or even a head gasket that overhung a little, they could glow red from the heat of combustion. Then when you turned the key off, the throttle blades were still open at the idle position and the motor would pull in enough fuel/air mixture to diesel, with the hot spots in the chamber igniting the mix.

The fix was to install a dashpot that held the throttle blades open to the idle position when the key was on. With the key off, the dashpot relaxed and closed the throttle blades fully so that no mixture was introduced into the motor. No mixture to burn, no dieseling.