Many 4 and 6 cylinder engines vibrate, some use balancing shafts to help reduce/eliminate it. over head cams helped for additional reciprocating mass, some still have additional balancer shafts. A 4 cycle engine's crankshaft requires 720 degrees of rotation to complete one cycle.. (Crank turns two full spins) a four cylinder engine? a cylinder fires every 180 degrees of rotation. a V8 cylinder fires every 90 degree. sixes are smoother than fours and eights are smoother than sixes.
now lets talk about opposing masses. Horizontally opposed and V type engines with more than one bank run smoother than their inline counterparts.
Did you realize that V8 crank is slightly longer than a four cylinder inline engine? A crankshaft is heavy. the longer any shaft is the more chance of distortion. With weight and the forces being put on it by the rod/pistons ..........
ENGINES RUN SMOOTHER WHEN THE IDLE SPEED FUEL MIXTURE IS A LITTLE RICHER. But a leaner idle emits less pollutant or pollutants they want to pretend to be so interested in. if we all had hydrogen burning cars and the only emission was water would OVER-HUMIDIFYING the atmosphere become a problem? Electric cars emit ozone..
ENGINE DESIGN AND TYPE
FUEL REQIREMENTS
The engine mounts may be starting to go.
Could be a vacuum leak or a bad spark plug / wire
Probably it's a Kia...
I have had all motor mounts replaced twice,new timing belt,tuneup and still vibrates when in drive and st a stop.
