It sounds like fuel foaming due to vibration.

You can sometimes reduce the effect of the bubbles by putting a fuel filter in the line as close to the carb or needle as possible, to kind of filter out the bubbles. A few drops of Armorall, as mentioned above, will work too, though too much will cause your glow plugs to stop working too soon. (the silicon will bake on the element, preventing the reaction between the element and the fuel). The best solution is to reduce the foaming by using a smaller tank with padding, or balancing out the engine/prop/spinner a bit.

Many engines can be made to run smoother by mounting an out of balance prop with the heavy blade 180degrees from the piston, so that it basically adds to the crankshaft counterbalance. Most engines are a bit light on the crank counterbalance and this helps balance the whole system.

I didn't see what brand of engine you have. Some engines idle well out of the box, but most will need some break-in and will idle a lot better after they have been run for a bit. Set up the carb as the guys above mentioned, a stable idle at full down stick and full up trim, and an engine kill with full down trim.