May be your harmonic balancer is has spun in it's rubber mount.
Take a screwdriver and follow number one piston up for top dead center
to see if it still matches the timing marks:
http://members.cox.net/wilsond/hb/
God Bless America, Bill O|||||||O
http://www.billhughes.com/

orlin wrote:
>
> Dear Doug,
>
> The vibration is constant. However I can feel it more at the low RPM.
> Before it was worse. After I changed the spark plugs it is not so
> sensible, but it is there.