determining RPM's without tach

[p24-1953]

The repair manual says to set the idle speed at 600 rpm, but my car does not have tach, so ho do you determin the RPM. this is on a 53 plymouth

[greg g]

Well 600 is pretty low. start the engine, let it warm up. then back off the idle screw till the engine stumbles or stalls, the give the adjusting screw about 1/2 or 3/4 of a turn. Basically you are looking for the lowest speed which will sustain a nice smooth idle.