Backfire under loads

[liftlawssuck]

Originally Posted by geezer#99

That kind of timing tells you that your balancer/timing tab is really inaccurate or you’re reading your timing in the wrong spot on your 305.
A 305 should have a timing tube at the 12 o’clock position that you sight down to the balancer. Right behind the water pump.
Are you timing it with this or is there a timing tab at the 2 o’clock position?

Some info here.
https://www.crankshaftcoalition.com/..._TDC_Lines_SBC

The only tab on the cover is in the 2 o'clock position. This is what I am timing it to. There are 2 marks on the balancer and I am using the 1st line that comes up to the tab. However if I use the 2nd line it doesn't run much different really. So it having the tab on 12 o'clock would make a world of difference. I'm trying to time to incorrect marks. This would make a lot of sense.

So which timing mark on the balancer would be the correct one to use?