Can someone explain this please?

[1971RR]

My motor is stock so I set the initial timing at the recommened 2 1/2* BTC. I've read some of these posts where people are setting the initial timing at 15*-18*. My car does not seem to like that. Am I understanding properly what you guys are saying? With vacuum advance unplugged, car at idle and setting the timing to 15*+ BTC? Why? To be honest, when I set mine at the 2 1/2* BTC, my vacuum drops to about 11-13. When I jump the timing to around 10*, my vacuum will go to the recommended 15-17. Now mind you, this is based soley on the timing tab and single mark on the balancer. No timing tape or super detailed timing tab. Should I set the timing based on the correct vacuum or correct timing marks?

 

[72RoadrunnerGTX]

The simplified explanation;
There are several factors at play here; first the original 71’ specs are primarily set-up to minimize certain gases present in the tail pipe emissions over any true performance or drivability concerns. The limiting factor is the total mechanical advance set-up in the distributor. On the big blocks around 34-35 degrees is the upper limit before detonation/pinging occurs at WOT. This includes initial and centrifugal crankshaft degrees, vacuum advance disconnected. Without an actuate degree tape you will need a timing light with adjustable timing to verify total advance. The original distributor for that motor will have something like 28-30 crankshaft degrees built into it's curve. Sounds to me like your motor likes somewhere around 10 initial, you will need to modify the limits of your distributor curve to something around 24 crankshaft degrees total to allow the advanced initial without exceeding the total limit. Later Mopar Performance and aftermarket distributors are designed with adjustments to the limit, original distributors can be modified, requires welding and careful filing of distributor parts. Ideally the use of an old-school distributor machine is needed, but can be accomplished without.
Short of messing with or replacing the distributor, find a way to set the timing at total mechanical advance, vacuum advance disconnected, around 34, test drive for no pinging at WOT, make your idle adjustments with whatever initial you are left with.

 

[moperformance]

The simplified explanation;
There are several factors at play here; first the original 71’ specs are primarily set-up to minimize certain gases present in the tail pipe emissions over any true performance or drivability concerns. The limiting factor is the total mechanical advance set-up in the distributor. On the big blocks around 34-35 degrees is the upper limit before detonation/pinging occurs at WOT. This includes initial and centrifugal crankshaft degrees, vacuum advance disconnected. Without an actuate degree tape you will need a timing light with adjustable timing to verify total advance. The original distributor for that motor will something like 28-30 crankshaft degrees built into its cure. Sounds to me like your motor likes somewhere around 10 initial, you will need to modify the limits of your distributor curve to something around 24 crankshaft degrees total to allow the advanced initial without exceeding the total limit. Later Mopar Performance and aftermarket distributors are designed with adjustments to the limit, original distributors can be modified, requires welding and careful filing of distributor parts. Ideally the use of an old-school distributor machine is needed, but can be accomplished without.
Short of messing with or replacing the distributor, find a way to set the timing at total mechanical advance, vacuum advance disconnected, around 34, test drive for no pinging WOT, make your idle adjustments with whatever initial you are left with.

X2

 

[1971RR]

I just installed a Mallory Unilite 4757701. I was curious if I should mess with fine tuning the timing or not. The only mechanical adjustment I could see on that distributor is the vacuum module. You can turn a small 3/32 screw in or out, but I'm not sure what that does. I do know that all-the-way in is definitely a no-no. Its currently set all the way out with two turns in from that point. After I set the timing and re-install the vacuum line to the distributor, there is no change on the timing wheel or idle quality. Stays right between 0* and 5* (I'm assuming that would be close to 2 1/2*). When I raise the RPM the timing mark goes off the timing tab of course since its only tabbed to 10*. The timing mark probably goes to 90* (not timing degrees but in reference to the original location of the mark at idle.) Maybe 3-tabs worth of timing using the length of timing tab on the motor (30ish something degrees maybe?) I dont know at what RPM all the timing is in since I'm under the hood and my tach is on the dash.

 

[72RoadrunnerGTX]

Don’t mess with the vacuum advance just yet. When connected to a ported vacuum port on the carb, as your pictures in the tank venting thread indicates, there isn’t any vacuum advance at idle speeds. Disconnect it until you have the mechanical advance set correctly. I would expect that distributor to have a pretty aggressive mechanical advance curve and low limit right out of the box. If you are not equipped to check/set-up the total advance as described above, check the documentation that came with the distributor. You should find specs for the default curve and limit. Take the total limit, in crankshaft degrees, subtract that from 34, set your initial to the result. Disconnect the vacuum advance, check for pinging at WOT. If it pings, retard a couple of degrees until it doesn’t. If it doesn’t, advance a couple until it does, then back it off. Tighten down the distributor clamp, make note of your new optimal initial timing for that engine, distributor combo. Connect the vacuum advance back-up, check for pinging at cruising or light throttle loads. The adjustment in the vacuum advance hose nipple adjusts amount of vacuum advance in relation to the amount of ported vacuum connected to it.