What year 235?

I have never disconnected (and plugged) the vacuum advance line on a 1954 and later 235/261 engine when setting the timing at idle (but, doing that would not harm anything).

http://chevy.oldcarmanualproject.com/shop/1954truck/54ctsm0619.html

http://chevy.oldcarmanualproject.com/chevyowner/54tom31.htm

Here are the 1948-52 instructions:
http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0619.htm

Here are 42-47 instructions:
http://chevy.oldcarmanualproject.com/shop/1942_47/4247csm632.htm

I recall disconnecting/plugging the vacuum advance line for timing adjustment back in the late 60s on a 1948 216 engine. I did what I was told to do. I do not know if that was necessary.

Tim
1954Advance-Design.com
1954 3106 Carryall Suburban
1954 3104 5-window pickup w/Hydra-Matic
The 216 and early 235 engines are not "splash oilers" -- this is a splash oiler.