I may be over-stating the obvious, but it's
prop/spinner that will cause the vibration,
that is if you don't have a bent crank.

I balance all of my props. Some people have
told me that different brands don't need balancing.
That's simply not true. I rarely find any props that don't
need some balance adjustments.

Next, don't use any plastic spinners. Use only
the solid aluminum spinners, like Dubro.

Now, if you have done what you can to address
the prop & spinner, I have had good luck with
a squirt or two of Armor All per gallon of fuel.
Or if you don't trust this, pour some fuel in a
smaller container & shake to see the amount
of foam that you generate. Add a small amout
of AA & shake it again. No foam.

I add it to every gallon I buy as a precaution.

I have great luck with my 1/2A's using it. They
run at such high RPM's, it is difficult to get the prop
balanced good enough for them...

Good Luck!

Johnny C!