The "vibration" produced by big displacement 4 stroke engines is not from unbalance -but from having the torque input to the prop---at each 720 degrees---- so the prop slows down for most of 720 degrees then takes a big shot --this causes a hell of a kick -and the rotational soft mounts do help soften that kick.
The larger gas engines 2.4 cu in and up -also put out a big kick - but at 360 degree inputs--be it a twin or a single - --if the power is the same - the kick is the same -
In the sizes of the 1.4 cu in four stroke and the 2.4 in two stroke -- a strange thing happens - the kick required to keep the prop moving at say an average 3000 rpm- is really not that different between these two examples
the why --is that as mentioned earlier - the one engine has to hit the prop harder to maintain the same avg speed of the prop at a given rpm.
Now then - adding a soft mount to the two stroke is a great idea--if you can do it and still hold the engine perfectly on line with the crank axis.
That is the trick-
adding soft points at each mount point is usually a bad setup - the engine wobbles easily.
If tho-- you mounted the engine to a wide flat plate - --then put the rubber bits under the extreme corners - the wobble would be more controlled.
From a practical standpoint -this can get to be a pain -
the best bet is to attach the engine solidly to the airframe .
Here is whre the sh-t can hit the fan.
Simply having a big ol chunk of wood hooked to the engine does not insure that the twisting force exerted on the fuselage - does not simply wind up the fuselage and let it spring back each time the engine fires.
If possible - make sure the enge box is solid - does not twist -
make sure the fuselage is as solid as possible - -all the way to the tail.
Some models are simply not good in this test.
I have tried -and it works - cross bracing very light fuselages ( my 2.4 powered models have fuselages weighing under two lbs)- these don't shake .
Is this a bit more work than just glueing together a ARF - by the book?
sure it is - but it also isn't rocket science -- you can -with help from a friend - carefully apply some twist loads to your model -- see if it has an obvious point that is easy to move.
See if a diagonal piece of very small section spruce can be added to stop this movement.
It may well surprise you how smooth the model can be - if the fuselage is simply stiffened and the engine is solidly atached.
My 11 lb models are proving to be very solid and they are all solid mounted engine setups- so being light is not the problem with having vibration -- the construction plays a big part.