Expert: Ernest (Ernie) Kenward - 7/4/2011

Question
I read on a sticker under the hood that the timing is supposed to be set at 8 degrees BTDC. Is this set manually? I have everything lined up to the marks as it instructs in my repair manual, but those marks are set to TDC. Could this be responsible for a stumbling idle and lack of power?

Answer
In answering this, I am assuming that your finding that the timing appears to be set right AT TDC - in other words, at zero degrees - was found by running the engine at specified idle, with any vacuum advance signal to the distributor capped off, and then using a timing light triggered from the appropriate plug lead (almost always cylinder 1) to view the timing mark as the engine idles.  This may seem obvious, even trite, but one is wise to never assume!

Too, my own direct experience is not with "modern" (ie: last 25 years) EFI vehicles, but with non-electronically controlled engines.

Having said that, 8 degrees BTDC is very common as an initial timing set point for gasoline engines.  The ignition is advanced in the first place so as to allow optimal use of the energy released by the combustion during the firing down stroke of the piston.  It takes a little time for the combustion to propagate after ignition so the ignition is advanced so as to better time the combustion so that the greatest amount of expansion takes place during that firing stroke, and to minimise energy being wasted to the exhaust or being applied to cross purpose while the compression stroke is still under way.  Further, as the engine loads, the ignition timing must be further advanced to maintain optimum use of energy from combustion.  To this end, the spark is further advanced as a function of loading; in pre-electronic cars, it was common to use a coordinated combination of "centrifugal" advance, which is simply a function of distributor (and engine) shaft speed and "vacuum" advance, which made use of the venturi vacuum in the main jet venturi(s) through which the combustion air passes at a rate which increases as throttling is reduced and the engine produces more work.  Note that venturi vacuum is NOT the same as manifold vacuum; manifold vacuum is highest at idle, throttle butterflies closed, a point at which venturi vacuum is essentially nonexistent.  Conversely, venturi vacuum is maximum for an engine at heavy load and wide-open throttle (WOT), a point where manifold vacuum is virtually non-existent.

If your engine truly has been set by someone to an initial timing of 0 degrees BTDC, in other words, right AT TDC, it certainly would idle poorly and not load well!

Getting this right ideally means more than simply solving the miss-setting.  In the engines of my experience, setting the timing is first a matter of setting up appropriate instrumentation - at least a tachometer and a vacuum gauge, to monitor MANIFOLD vacuum.  The first is to establish the correct idle speed (usually in D or D2 (wheels securely BLOCKED) if the transmission is automatic, and the second is to see review the changes in vacuum as the setting that follows progresses.  After that, one loosens the distributor holding clamp so that the distributor housing can be GENTLY AND SLOWLY ROTATED WHILE ONE WATCHES THE TIMING MARKS on the damper to SEE THE CHANGE.  IF all is otherwise well, one should hear a significant improvement in idle as the timing approaches 8 BTDC, plus the manifold vacuum should rise to at least 16, but better up to about 20 inches of mercury.  Lower levels of vacuum often suggest leaks in vacuum hoses, a problem which is very common but also very easy to fix.  It could also indicate leaks elsewhere, in manifold gaskets for example, or could be indicative of a worn engine that can no longer develop sufficient compression nor vacuum.

Above, I said "getting this right means more than simply solving the miss-setting" - You have the manual (the OEM one, I hope, as this is best), and this is a great start, and indicates that you are serious about this.  Carry on reading, not just that, but other publications as you find them.  While your car is "new" technology, far beyond this good "starter" book I am going to suggest, Mike Urich's "Holley Carburetor Handbook: Selection, Tuning and Repair" is a great primer in the first principles of air and fuel induction systems used in gasoline engines.  The modern electronic approach largely uses digital models of the mechanisms employed in traditional systems, adding greater finesse through more inputs (humidity, barometric pressure, and the end product of combustion to track oxygen utilisation) and much greater speed of response in adjusting the input fuel to maximise power and minimise waste and emissions.  (In my field, we continue to demonstrate protective-relaying principles using the "ancient" electro-magnetic instruments, because they make accessible the understanding of the core concepts, which are then quite easily applied to understanding - and producing - the programs used in the digital relays which do so much more by virtue of their power and speed.)

Haven't had the time to write an answer for a while, so you may have lucked out ... or not!   Anyway, we all pursue the things we do because they are fun, so consider it in that light.

Short answer: Resetting your timing correctly probably will solve your problem.
Hope this helps!   
Regards ... EGK