Using two stock light springs in distributor advance, good idea?
I've been reading about having full mechanical advance in by about 2,500 RPM
Halifaxhops is giving you the real info.
Start here for ballparking an initial timing for your engine combo.
Distributor starting point for a curve
How emissions changed timing curves and why 318-2 bbl timing was different than 273 4 bbl in the middle of this post:
Rich Idle AFR reading
'All in by 2500' is misleading.
Even on a hot rodded engine, the two stage setup is very useful,
especially with electronic ignitions. The switching in electronic amplifiers takes time, and that time shows up as retarding advance that becomes noticible between 3000 and 6000 rpm. There are faster electronic boxes and slower ones and even units that are designed to offset that internally - I think pertronics III may do that and FBOs box might too. The revenator patent suggests it sortof does that as well - but I haven't studied and its probably over my head.
For drag racing using a distributor with no vacuum advance, the heavy spring can be used to hold the WOT steady or even better, a very slow increase with rpm. That's what the Direct Connection 'tach drive' distributors do. Detailed discusion here:
P4120701 Small Block Mech Tach drive Its drivable on the street - I've done it. Its just not ideal.
Both those distributors and the regular Mopar Performance distributors with vacuum advance were best setup at an rpm near 'full advance'. By 2600 - 2800 they are usually at or close to full advance and pretty stable. Instructions for those vac advance equiped MP distributors here:
How to determine the RPM for all in?
Trying to set them at idle can be difficult because its advancing quickly at low rpm. However that can be adjusted if you know what to do. It's all about the initial spring tension.