HE V12 with Lucas Ignition
did some timing readings last week at idle with a specific inHG vacuum applied to the Vacuum Advance. These are at idle with timing set at 18BTDC @3000RPM:
0 inHG = 2 BTDC
4 inHG = 4 BTDC
5 inHG = 6 BTDC
6 inHG = 8 BTDC
7 inHG = 10 BTDC
8 inHG = 12 BTDC
9 inHG = 14 BTDC
10 inHG = 16 BTDC
The brown vacuum regulator supplied with my 1988 V12 XJ-S is 10 inHG. But I like my base timing set to 22 BTDC @3000RPM (I have raised required octane from 87 to 93). So this gives me 20 BTDC at idle, which causes a slightly irregular idle sometimes, and seems quite advanced to me for an idle. (I also run 180F thermostats, instead of original 190F, so my idle is probably a wee richer than stock)
There are many GM Vacuum Regulators from the 80s, with a color code. I have no idea what color means what, so I’m buying one at a time and trying it. You can sometimes find these NOS for a decent price of under $20.
White/Yellow GM 14006719
Green GM 14006771
Pink GM 14015597
Black GM 10115851
Grey ???
I am now running the white one, which is also the same one that the XJ6 uses. It supplies 4 inHG to vacuum advance. So my idle timing is now at 8 BTDC instead of 20 BTDC. I do get a better idle, but I’d like it advanced a bit more. (My V12 seems to idle the best at 10-12 BTDC) I have purchased the Green one, and will test it next weekend.
If anybody knows how the color corresponds to inHG, I would love to know! Otherwise, I’ll keep posting my findings.
For those of us who want to lower the advancement at idle a tiny bit without retarding the base timing, there seems to be options. And I don’t like running the vacuum advance off throttle tap, because it’s just too low of an advance at idle for my liking.