Hi Grassman,

You don't have 13 volts at your battery.
Fully charged = 12.6VDC, less with weak or dead cells.
You need to calibrate or replace your meter.

Have you inspected the plugs?
Check for a robust spark at EACH spark plug.
Is it a bright white spark or a weak yellow?
You might check compression while you're at it.

Why don't you replace those (likely fouled?) plugs with new?
Gap to 0.028"
Use anti seize on the threads.
Use dielectric silicone on the top of the plugs so the plug wires can be removed easily for service.
You can damage the plug wires by yanking on the wire instead of the molded plug cover. Plug wires are not so forgiving for rough handling, and there are tools
to safely remove them.

Confirm robust spark at each plug.

Bill