Strange timing issue - '68 318
The very first thing I recommend on any old car is to use a piston stop and VERIFY that the timing mark is actually accurate. This may come from my days when I used to find a fair number (mainly Ford FE engines) that were not.
Get yourself, or make, a piston stop
Next, all of these cars in the time era of your car use what is called ported vacuum for distributor advance. This means that at idle, there is NO vacuum, therefore you have the distributor hooked to the wrong carburetor port.
Also, you should endeavor to be sure that the advance mechanisms, both mechanical and vacuum, are actually working. Mechanical advance can get rusty, stuck, worn, springs broken, or debri wedged in the mechanism. Vacuum advance can leak, thus not working.
You can easily measure around the outside of your dampener and figure how many "degrees per inch" to mark it so you can measure what the advance is doing, or borrow/ buy a "dial up" timing light.
The very first thing I recommend on any old car is to use a piston stop and VERIFY that the timing mark is actually accurate. This may come from my days when I used to find a fair number (mainly Ford FE engines) that were not.
Get yourself, or make, a piston stop
Piston stop
You can buy or make one. Here's one I made a LONG time ago:
Remove the no1 plug and make sure the piston is down quite a ways. Remove the battery ground for safety. With a wrench, rotate the engine until rotation stops. With a commercial, or adjustable stop, you may need to adjust the length. You are not trying to stop the piston at TDC but rather down in the bore some amount.
With the engine stopped against the device, carefally make an accurate temporary mark under TDC on the timing tab onto the balancer.
Now do the same thing, rotating CCW.
You will have two temporary marks some distance apart. True TDC will be halfway in between, and if the original is correct that is where it will be.
Next, all of these cars in the time era of your car use what is called ported vacuum for distributor advance. This means that at idle, there is NO vacuum, therefore you have the distributor hooked to the wrong carburetor port.
Also, you should endeavor to be sure that the advance mechanisms, both mechanical and vacuum, are actually working. Mechanical advance can get rusty, stuck, worn, springs broken, or debri wedged in the mechanism. Vacuum advance can leak, thus not working.
You can easily measure around the outside of your dampener and figure how many "degrees per inch" to mark it so you can measure what the advance is doing, or borrow/ buy a "dial up" timing light.