id suggest the orginal distributor would have a curve suitable for the grade of gasoline available at the time. i'd suggest todays octane rateing is achived by different means than 50 years ago.. ethanol is added and oxygenation of the fuel is now done. Start with your oem unit and see what it wants at idle work out how much more initial timing above the book nunber it wants to idle, without a smell of unburnt fuel or grubby tail pipe end. Then get that removed from the total timing the advance mechanism wants to put in at 3000 rpm.
your 196?/ 197? factory manual probably specified 5BTDC and a total of 34 at 3000-4000 rpm
a specialist with a Krypton tune distributor machine or similar can do this for you
i live in the UK with different fuel from you, the standards are the same, we all get the same cars these days, but the base stock used means refining and achiveing octane is done differently country to country and climate to climate, to cater for heat cold and avilability of light crude
my 5 BTDC initial timing 34 Total timing in 1971, as described in the factory manual for what I would call the freely avilable 1971 4 star 98RON petrol, is no good today on current British unleaded E5 Super 95RON petrol.
The GAS has changed
When it was a standard motor i ran 10 BTDC initial and 30 total at 3000 for a clean running standard set up on modern fuel.
since it had a compression hike, cam, 1.72:1 rockers and way more carb
I now run 15 BTDC with a total of 28. high compression open chamber semi hemi head
my dizzy is MOPAR electronic above the plate and BOSCH below, it runs Bosch advance set up. (its australian, think of it like a Porsche or BMW 6 distributor with Mopar electronic pickup and reluctor under the cap)
to achive this change the distibutor advance needed to be limited
distributor was put in a vice so input shaft is locked along with the case.
timing disk was bored so it fitted over the rotor arm mounting
a wooden dowel was stripped to a point and wedged into the rotor arm mounting
i gripped and twisted the rotor against the advance so that the advance mechanism advances, and i measure usuing my wooden pointer and timing disk, how many degrees the distributor can advance. you will see somewhere between 12 and 15 in a stanard oem distributor in most cases, which translates to 24* to 30* at the crank
you then remove 1 degree of distributor advance for every 2 at the crank you need to get rid of.
i wanted to start at an initial advance of 15 BTDC and end a 28BTDC (3000 rpm) so i needed to see 13 degrees of advance at the crank which equates to 6.5 in the distrbutor
i did this by welding up an awkward profile cut into the advance armature that uses a spring mounting pin as a stop, and filing it back until my distributor had only 6.5 dgrees this limited how far the weights swung out and therfore how much advance the distributor supplies
13 distributor added degrees at the crank + 15 initial timing gave me 28 total at 3000rpm
motor will take up to 34 but just gets noiser and less happy so 28* is what it is.
To be honest its not so shabby just locked at 15, ELB distributor handy for first start and break in.
if you have a standard mopar dizzy you life is easy, try FBO for an advance limiting disc that can fit a standard mopar (american) distributor. (not my wierd Chrysler oem licencee hybrid)
easy to fit and does this job without welding
once you have clean running idle and smooth quiet running at mid to full throttle
worry about springs to decide how fast or slow that advance should be applied and what rpm it starts at.... mine needs so little it was not much of a worry for me it wants 15 at idle and so it can start, and then all-in more or less from then on...
but your's may still want quite a lot, state of tune plays a part, so the "Shape" of the curve is more imprtant, springs provide the start point and rate of advance in relation to rpm
then work on the vaccum can for clean running on cruise and run down.
I use mopar electronic style (australian mopar OEM licence from bosch) with a 4 pin HEI and a coil off a 1980s electronic igntion jaguar with no ballast to burn out.
basically the cheapest but best bang for my buck in a car where i don't care about limiters and other race focused facilities.
Dave
