Adjusting Mixture and shut down RPM

I have recently spent a day with a LAME covering of the 100 hourly engine maintenance schedule and processes and part of this was the adjusting of the mixture.

The item that arose was that the correct adjusting of the mixture was achieved through setting the engine to idle (i.e. approx 650 RPM for the IO-540) and then upon setting mixture to cutoff the RPM should rise between 30-50 RPM before shutting down.

This was all good, however I was wondering is there any reason that we set the RPM on shut down to approx 1200RPM for normal operations before shutting mixture rather than to idle? I have not found any reason for this other than convention.

Given we have the ability to use engine monitoring like saavyanalysis which would correctly monitor the shut down RPM, we could have this information available and the mixture adjusted into the correct range very easily without needing to subject the engine to ground running and multiple restarts if we just shut the engine down on idle and noted the RPM outputs over time with minor adjustments as we went along.

Any thoughts and information welcome.

The Lycoming SB on use of 100LL fuel comes into play here...

http://www.lycoming.com/Portals/0/techpublications/serviceletters/SL L185B (01-19-1988)/SL185B.pdf

Good info...

Hi Gil,

Thanks for pointing me at that information. Especially about the carbon scavenging chemicals in the fuel. Very interesting read, however, Section B, 4 tells me what to do, but does not give a really good reason for it.

I would imagine that if we went to idle then off quickly the ability to cause carbon deposit problems would be minimal to none.

I can see however, why Lycoming would set the higher RPM process to stop people getting to idle for a long period of time and causing problems.

Do people concur with this or am I missing some other information?