So last night I after drivjng home on the interstate from work pinging snd backfiring all the way when i went more than 55, I decided to do additional reseach. Based on my symptoms I chose to retard to timing slightly.... and I mean slightly. I turned it the dizzy counterclockwise maybe a quarter inch and took the truck back on the road. Backfire/ping is gone. Doesnt resolve the motor not wanting to get me to 70, but I’m not up to fixing that yet.

That all brings me to this question. When i had my exhaust done in essenece opening up the airways to 2 1/4 piping, could this have caused a slight timing issue? Either way, slightly retarding it for now I realize isn't scientific but it did fix the immediate issue.

Thiughts?

About the width of a pencil line movement at the distributor equals maybe 2° timing change at the crankshaft. A quarter inch turn of the distributor housing is a whole bunch of timing degrees! What's important to keep in mind here is the mechanical and vacuum advance components inside the distributor are two completely separate independent systems that work together though they are adjusted separately. The mechanical ignition timing advance is always set (and measured) first with the vacuum advance disconnected and plugged.

ASSUMING the ignition timing is the issue, for test purposes we want to determine if A. the vacuum advance is at fault or, B. the mechanical advance is at fault or, C. both/neither are at fault, through a process of fault isolation and elimination.

The first thing to do though when timing these older beasts is to verify that the TDC "0" pointer mark indicated at the damper is truly accurate, best done using a piston stop tool. This is important because original crankshaft vibration absorbers are almost always lunched by now. The heavy steel outer inertia ring tends to slip off axis due to the rubber sandwich dry rotting, making the index timing marks innacurate. If everything proves out OK, proceed with the following:

Disconnect and plug the vacuum advance port. At this time the engine will be running only with the distributor mechanical advance. Keep the idle RPM low around 500 or so to ensure the centrifugal weights aren't starting to pull in any ignition advance in and interfering with setting the base timing accurately. When you're done with this, later you can set the idle RPM wherever you want.

Set the base distributor ignition timing with a timing light, maybe 10° or 12° BTDC, something like that. As a kind of QC check, it's not a bad plan to measure the manifold vacuum at factory idle RPM with a mechanic's vacuum gauge, normally 18" to 20" at sea level and a steady needle on a stock motor. Then rev the engine up (carefully!) in neutral, and again using a timing light, observe the maximum mechanical advance indicated on the crankshaft damper marks. It should top out at around 34° or 36° (Important) BTDC, moving smoothly up and down the scale, no sticking, no "scatter", i.e. the marks on the damper should stay sharp and focused, always returning at idle to the same base timing previously set. Here you're checking to see that the mechanical advance works properly at idle, wide open throttle, and everything in between.

Next take it for a short test drive and put it through the paces. Again to be clear, this is without vacuum advance connected. If it doesn't ping or "knock", might try advancing the timing slightly - say 2° at a time - till it pings again on hard, full throttle acceleration from slow speed and/or stopped. Then back off 2° at a time till it stops. This is your optimum advance point - the initial timing will likely be more than 10° though less than 20°, the idea though is to make sure the timing is not retarded nor excessive at any point and reaches the maximum spec when it's supposed to. Experiment, find out what your particular engine combination likes with the fuel normally used. When it's running OK and you're satisfied lock down the distributor holddown bolt and don't mess with the distributor timing again. "Our work is done here".

Now you're ready to re-connect the vacuum advance canister, these are tunable, usually with a 3/32 (2.5mm) Allen wrench. The takeaway here is to tune or adjust only the vacuum advance at this point. Don't make changes to the distributor timing curve to compensate for an incorrect vacuum advance setting. At least don't retard the distributor timing any. Crane and ofhers suggest starting out with the vacuum canister at maximum vacuum advance (fully clockwise) taking a test drive, and backing off the canister counter-clockwise two (2) turns at a time as required.

Here, you're looking for a a different set of symptoms than when tuning the mechanical timing. Look for a part throttle "rattle", or ping, or a constant knock at steady level ground cruising, or when the engine load just starts to level off, or upon part throttle acceleration. If the ping instantly goes away on acceleration, that means too much vacuum advance. Again it will take some test drives. If using a hot ignition and lots of ignition advance, and/or leaner fuel mixtures it may be necessary to step down a click in the spark plug heat range. Hot spots or excess carbon can cause pre-ignition, this is not the same thing as ignition timing spark knock. The stock heat range plugs are the place to start when tuning, or even a bit on the cold side and work up in heat range. The worst that can happen with too cold of a spark plug is fouling, but a hot plug can cause damage. In most cases the stock OEM plug is excellent. A momentary slight rattle from the vacuum advance when decelerating or accelerating at part throttle every now and then is OK.

The centrifugal weights, springs, stone knives, and bearskins inside the distributor are the main components of the ignition advance and must be working correctly throughout the RPM range before messing with the vacuum advance canister. If the mechanical ignition timing is retarded the "ping" will go away but the engine will run excessively hot and probably overheat, be down on power, and waste lots and lots of gasoline. Hope this helps!