Hello. Timing related backfiring is typically back through the carb, not the exhaust. The most common culprit for backfiring through the exhaust is having a carburetor that is way to big for the application, but, there are others. That almost means that you have unburnt fuel accumulating in the exhaust, which isn't really a timing issue. Also, a car can have the timing be way off and have it still remain consistent. If it's erratic, it's probably going to be a distributor issue of some sort, as has already been mentioned. The gear on the distributor shaft is wobbling around, or, the cap is wobbling around or something. Maybe the vacuum advance arm is all sticky and notchy for some reason. Could be a lot of things, but, I don't think it's a timing issue, per se.
In most cases when the timing is jumping around, there's too much slack in the timing chain. A worn distributor gear and sloppy bearings in the dist. shaft can also be factors.
If you have a roller cam without a hardened steel distributor driven gear, the gear will wear out in no time. Bronze gears will wear, too.
You can check for timing chain slack by applying a wrench to the crankshaft and rotating the crank forward and back while looking at the rotor. The rotor should move almost immediately after changing the direction of rotation. Even if the timing chain is new, there's still the possibility that it's faulty.
While you're at it, check the position of the rotor when the engine is at TDC on the compression stroke at cyl #1. The rotor should be pointing to where the #1 cyl terminal is positioned on the cap. Place a mark on the dist housing in relation to the #1 terminal before removing the cap. FYI, the distributor rotates in a counterclockwise direction on the V8.
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