#1 piston comes to TDC twice- - - -once on the compression stroke, when the spark plug is supposed to fire, and again 360 degrees of crankshaft rotation later, at the top of the exhaust stroke when both valves are slightly open. If the plug fires then, there will be a backfire, sometimes through the carb, other times out the exhaust.
Take the valve cover off, and watch the valve rocker arms move on the #6 cylinder, all the way back toward the firewall. The last valve in line is the #6 exhaust. Turn the engine, watch the #6 exhaust open by compressing the spring, and keep turning until it closes and the next valve in line (#6 intake) just begins to move. Now- - - -this is important- - - -install the distributor with the rotor pointing toward the #1 spark plug wire. As the #6 cylinder comes to the top of the exhaust stroke with both valves slightly open, #1 cylinder is also on top, with a cylinder full of compressed gas and air ready to fire. If all you do is find TDC on #1 and install the distributor without watching the valve action, there's a 50% chance you'll be firing the wrong cylinder. I've been there and done that, many times!
Jerry
