the timing tab is not used but O on it tdc is best done when you put the engine to gether but if this was not done it can be a pain in the back .for setting timing if you have a msd i run one blue and a lite sliver spring your timing will start fast and will be all in before 2500. use the red or sliver bushing .i run this at the crank 10 to12 and run it up to 2700 and check it it should be all in from the tab at 2700 it should be 36 this is mark on damper your engine is jump all over ??? it may need some timing and carb work i run a285 at .050 and is 748. lift roller and dose not jump like yours but it dose shake the cement. isyour lifter set rigth ??you would be surprised many people get this wrong and spark plug gap if it msd i run .035 .check 7and 5 if they are mix it will shake but it will run flat