Datsun Roadster Forum-311s.org

Advancing the timing doesn't cool down the combustion
It heats it up and leans the mixture the explanation does not make sense
This is a cold start swt. to help get rid of hydro carbon emissions . All late sixties to mid 70s vehicles had heat riser valves , a double vac advance ( ferd and Chrysler )
To heat the exhaust manifold quick to burn these emissions asap.
it's not going to over heat the block starting cold.
, vac advance is a emissions device too it Robs power from the engine using hard full power
I think the manual is saying if it over heats the fan clutch will cool it like if the swt. Sticks

redroadster:

Dann's (Rallye) explanation corroborates David's regarding advancing the timing:

"... 2000's came with a 'safety valve' which was located on the thermostat housing. Normally the vacuum advance unit does not function at idle, as there is no vacuum in front of the carb throttle plate, where it draws its energy. At idle there is a huge amount of vacuum in the manifold, way more than the vacuum advance unit would ever see during its normal operation. What this safety valve does is allow manifold vacuum to reach the vacuum advance line if the engine temp. rises at idle to 220+ degrees F from being timed at 0 degrees. This huge vacuum throws the vacuum advance full-open, which would advance the timing about 12-14 degrees which in turn would raise the idle; both would bring the temp. down quickly." Rallye, 1977 (italics mine)

The exhaust manifold heat risers you mention brings back memories of penetrating spray and hammer to try to get the rusty things free!

Jon