If you suspect the timing tab is inaccurate, the first item of business is to check it. Make a "positive stop" device from an old spark plug shell and a bolt, install it into the #1 or #6 cylinder, and and slowly rotate the engine until the piston touches the stop bolt. Mark the damper at the zero point on the tab. Rotate the other way until the stop is reached. Mark again. TDC will be exactly halfway between the two marks. Now, with a known accurate TDC mark, you can set the timing properly. Just like the slogan on Sun Electric Co. machines used to say- - - -"We Test- - - -Not Guess!"
Jerry
This is a really great idea! I'm not clear on how to make the positive stop device with a bolt and spark plug as you mention, but it is worth looking into. I hazard a guess here that you move the engine slowly by hand here so as to not damage the engine, correct? I know that pulling all the spark plugs would make it easier to turn the engine by hand. On the other hand, I don't think the timing mark is far off, if it is off at all, so perhaps it's just as good to tune it by feel to get it to it's best performing spot, and easiest starting.
