Timing questions
So I got my 383 original 2bbl with .484 purple cam 750 holley had heads done with new springs and headers. so a friend of mine said you just rotate the distributer to the spot were it runs best... my question is I know there's more to it and have been reading about it but wonder is ther no specific # to have your timing at? Because it seems past the timing marks on timing cover like maybe runs 20 ish advanced but what should the total timing be and have read about making the mechanical weights open up sooner like changing springs or you can buy plates to put in distributer to limit the total. My dart has nuts don't get me wrong but I like to learn and do things right. Thanks
The factory only needed timing marks for the technician to set the initial timing at the specified idle rpm.
The advance curve was in the distributor, and generally they expected that to be checked on a distributor machine. You'll see in the service manual those advance specs given in distributor degrees and rpm.
Now you've hot rodded the engine. In general it will need a little more spark lead at idle because the combustion conditions in the chamber are worse. Increased valve overlap help flow at higher rpms but at idle cause more exhaust dilution, reduced compression, and colder conditions.
Although your friend has the right idea, it would need to be done with a load on the engine. In fact best with a load and a CO analyzer. Yea right - like you have one in your desk drawer. If your car has an automatic transmission, you can set the timing and then place the transmission in gear. The goal is for the least drop in rpm (and vacuum) when placed in gear. Thats a good indicator the engine is strong in idle.
A few snips from Chrysler on timing in this post.
total timing reading accurate with no load?
My suggestion is to get some timing tape and measure the timing from as slow as the engine will run, then every 200 or 250 rpm up until it stops advancing or you don't feel safe. Then you can plot the advance curve and see what needs to be done, if anything to the advance.
This chart is helpful when you have nothing else to work from.
Distributor starting point for a curve
You can also use the pre-emmissions era 383 and 440 factory timing as a baseline.
See this thread:
Total advance question
Notice the initial was 12.5* BTDC at 550 - 600 rpm. With your cam, its going to need at least a couple degrees more initial, and it may have to idle higher, maybe 700 or 750 rpm.
Be alert to the fact that the distributor may already be advancing at 700 or 750 rpm. IF it is, then when the engine is placed in gear and slows down, timing will decrease. So IF this is happening, it may be neccesary to put more tension on the primary spring inside the distributor.
Direct Connection/Mopar Performance provided a guideline for the higher rpm timing. A copy of that is at the bottom of this article.
How To Limit and Adjust Chrysler Vacuum Advance Cans
That method requires timing tape or a dial back light and of course a tach. In that approach they were most concerned with mid to upper rpm timing and not as concerned with the initial. But I can tell you that using their distributor set as they recommend, initial ended up around 12-15*BTDC.
Finally note. If the distributor you are using was from an early smog era engine (CAP/CAS 1967-74?ish) the advance above idle is very fast and long. That makes it impossible to get the higher initial timing for the pre-CAP era, and what your hot rodded engine will want, without modifications. The best way is to weld up the inside of the slots. However sometimes you can successfully get by using manifold vacuum source for vacuum advance.
