RE: Gas engine vibration
An engine that vibrates all through the rpm range in my opinion is junk and should not be used for RC. Provided that the prop and spinner are not the cause.
An engine that is well balanced will only vibrate at or near idle if set up correctly......Causes for excessive vibration.
Flexing firewall.
Sloppy standoffs.
Too low of an idle speed.
Too heavy of a prop causing excessive power pulses.
Airframe mass inadequite to absorb the power pulses.
Harmonic matches (those times when one or more control surfaces shake but only at certain rpm settings.)
Harmonics come into play during some rpm settings in any engine/airframe setup and should be watched and monitored closely. How many times have we read posts about an airframe failing while flying at half throttle straight and level. Most probably caused by a small reduction in throttle setting witch can cause an engine to shudder a bit. If the shudder is in the range of vibration/harmonics, the airframe is being subjected to sever forces.