Catch 22,
The Arrius engine in the 120 uses basically the same fuel unit as the Ariel.
An inherent problem within that unit is RPM drift versus fuel temperature.
You need an accurate method of measuring the RRPM. I think there is a page in the VEMD that will show you the true RPM. The rotor tach may or may not be so accurate.
It is far easier to correct the RRPM than mess with the hammers. And from memory this is checked at MCP in the cruise. Messing with the hammers should be the last resort.
Track and balance equipment will give you a good cross check of the RRPM.