Move on to setting your mechanical advance timing. Insure that the emergency brake is set so no one gets killed. Set the dial of the timing gun to the 35 degree mark, shoot the balancer and slowly bring the engine up to 2500rpm. This will clearly be easier with the extra help of someone sitting in the car to bring the revs up. Turn the distributor until the light falls back down to the 0 degree mark on the balancer. Secure the lockdown on the distributor, tell your buddy to get off the gas, and consider your mechanical advance set.

Total timing equals initial plus mechanical, not including vacuum advance (that's why the vacuum is unplugged). So in this example the total advance is "all in" by 2500rpm and for most small block MoPar engines, this is a good place to have it.

If initial timing is 17 degrees and mechanical advance (total) timing was set at 35 degrees @ 2500rpm, then: Total timing (35º) = initial (17º) + mechanical (18º) "all in" at 2500rpm.

Plug the vacuum hose back into the distributor. You will notice the timing mark shift to well over 50 degrees. It's cool, vacuum advance has just been introduced into the system and its function is to provide extra advance when a vacuum signal is present to improve fuel economy (or so I've been told). Because this is in addition to your initial and mechanical advance (aka your total advance) you notice the degree spike when it's plugged in. The amount of advance can be adjusted by swapping out different vacuum canisters or by putting on an aftermarket adjustable canister. Unless you are a hard core racer, most of the time this is not worth the trouble.

The auto manufacturers have been struggling with the ignition advance curve for 120 years now and are just now getting a solid handle on it.  It’s tough for a single advance curve to cover all the different drivers and driving conditions even when the engines remain the same.

Full throttle timing attributes differ from cruise timing attributes.  This has to to do with the air/fuel mixture being in the neighborhood of 12.5:1 at full throttle and 14.5-14.7:1 under cruise conditions.  There is no standard ignition advance curve that can be applied to all situations as carburetion, intake manifolds, compression ratio, and camming all play into this.  The vacuum advance chamber does allow for the increase in timing under cruise situations and still allow the timing to backup under full throttle by lieu of the vacuum signal to the distributor dropping off when the manifold vacuum drops off.  But here are some basic guide lines.  As fuel mixtures get leaner, then additional ignition advance is required in the lower rpm bands.  The lower the compression ratio, then more total ignition timing is also required than what would be necessary on a higher compression engine.

As the camshaft lobe centerline is decreased, then additional initial ignition timing at low rpms helps with the idle characteristics.  This is simply due to inefficiencies taking place in the combustion chamber and the additional initial ignition advance simply helps to start the combustion burn earlier to help combat those ineffeciencies.  This in turn requires a shorter advance curve so that there is not too much ignition advance at full throttle and in the higher rpm ranges.  Because a street engine is operated under a variety of conditions, the combination of mechanical and vacuum advance helps to cover these.  Moderate acceleration has the vacuum advance dropping off which in turn results in less ignition advance seen by the engine.  Once the engine rpms are held in steady state (cruise), the air/fuel mixture goes leaner which is where the vacuum advance helps by adding additional degrees of ignition advance on top of what is already being supplied by the mechanical advance portion ofthe distributor.  Leaner mixtures are more difficult to start the burn cycle which is why the ignition must be advanced in which to compensate.

On the race engines, total timing is the single attribute I aim for and I simply want an initial timing that allows the engine to startup without ‘ignition locking’.  Cruising ignition timing attributes are not required and are therefore not taken into account.  On my roadster, total timing is set at 36° BTDC with the MSD electronics automatically retarding the timing 20° when the starter is engaged.  Once the starter button is released, timing is at 36° both at idle and at speed.  I do have the capability with the MSD to put some additional retard into the system with a switch if desired.  This would allow a 40-42° ignition advance off the line and if the switch is put on the shifter, then a 4 or 6° ignition retard could be made when the car goes to high gear. While I did that with my FE powered cars, I’m currently not doing that with the Y powered roadster but the capability is there if desired for a harder launch with the additional advance (42°) and still maintain a strong mph with the original timing (36°).

With all that being said, 10-12° intial and 36-38° total at 3500 rpms works well for many street Y’s.  As the camshafts get wilder, then 18-20° initial at idle may work better but this requires that the curve within the distributor be shortened so that the total advance without the vacuum advance is not excessive.  As a general rule, don’t exceed 10° of additional timing with the vacuum advance activated.

Are you using a dial back timing light?  I have found some of these to have the wrong scaling on the knob which in turn gives erroneous readings.  Beyond that, what kind of total timing numbers are you getting with the vacuum advance disconnected?  If the distributor has a very short ignition curve, then the engine would like a higher initial timing setting for overall driving.  Poor quality of fuel would also be another consideration.