800rpm is very low for idle, it's supposed to be 1600 rpm (I think, check the manual for the exact number). Have you removed the governor arm and if so, did you follow the instructions in the manual for putting it back on?
Edit: it's supposed to be ~1200 not to exceed 1500. Something is off if your idle speed is 800, and it may be the cause of the vibration. My 1450 smooths out completely around 2600rpm. Check the manual for carburetor,governor and linkage adjustment procedures.
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