dropped from the region above 850 rpm the flag nflg is reset at the step 114 and then the program proceeds to the step 106 to determine whether or not the flag flg value is 1 or not when the engine rotational speed ne drops from the region ne gt 1250 rpm or the region 1250 rpm gt ne gt 1000 rpm the flag flg has already been set to 1 otherwise the flag flg maintains a value of 0 therefore if the answer to the question at the step 106 is affirmative or yes the program proceeds to the step 108 wherein the deenergization period u03b4t of the solenoid 24 is set in accordance with the engine rotational speed ne and the rate of decrease u03b4ne in the engine rotational speed followed by execution of the step 109 and termination of the program for example according to the fig4 table if the rate of decrease u03b4ne is in a region between 25 rpm and 28 rpm the one shot time u03b4t is set to 30 msec when the engine rotational speed ne is in a region below 850 rpm on the other hand if the answer to the question of the step 106 is negative or no the program proceeds to the step 107 wherein the solenoid 54 is energized for 5 seconds that is the opening of the throttle valve.