explosions on my 318..

It can be timing, too rich, or even lean.

Wiki says

"Backfiring can occur in carbureted engines that are running lean where the air-fuel mixture has insufficient fuel and whenever the timing is too advanced.

As the engine runs leaner or if there is less time for the fuel to burn in the combustion chamber, there is a tendency for incomplete combustion.
The result of a misfire or incomplete combustion is that unburned fuel or flammable hydrocarbons are delivered to the exhaust manifold where it may ignite unpredictably

Another backfire situation occurs when the engine is running rich (with excess fuel) and there is incomplete combustion during the Otto cycle, with similar results."

Hope this helps!