D,
Here is the info you asked about as published in the factory specs for the '61 Chevy 261 Jobmaster, US-spec truck engine (extracted from the
Vehicle Information Kit, courtesy of the GM Heritage Center). Not sure it is terribly helpful, since the intended use of the Jobmaster was heavy duty use.
The '62 VIK doesn't include this info, but the distributor PN 1112403 was the same from '55-'62, so I don't think the factory likely changed the advance curves during that time frame. The last image is the similar data from the '61 Chevy passenger car VIK for the Hi-Thrift 235 - same advance curves as for the truck 235 and 261, which is no surprise since it used the same distributor PN.
Initial timing setting for the 261 Jobmaster was spec'ed in '61 as TDC and total advance was only 24-28 deg BTDC at 3500 RPM. Vacuum advance was a heady 22 deg at full vacuum (> 15 in. vacuum), according to this data, but practically, limits to less than about 8-10 degrees for moderate throttle opening (6-8 in. vacuum).
So, by "normal standards," the factory timing shows as significantly retarded, likely due to the intended use of the HD truck engine, which would have seen lots of low RPM, full-throttle use, and the expected use of lower grade fuel. I would expect the Pontiac passenger car 261 may have had a more normal timing curve, initial setting and total advance, probably low to mid-30's total advance and something like a 5 deg BTDC initial setting (similar to the passenger car 235), as Jerry suggests. The timing marks on the flywheel/flexplate may be stamped with the numeric initial setting. Today's fuels will support a bit more total advance, as well.
FWIW, the stock advance curve in the '403 distributor seems to work pretty well, in my experience, both for both (US-spec) 235 and 261, not to say it couldn't be tweaked.
Doug
