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do this^^^^^^ ....its a bit better than my dodgy method of 50/50 chance and a screwdriver ,even though my method works for me eventually.significant vac leaks can cause back firing and flames from the carb.
personally i check for number 1 TDC by taking off the rocker cover. the harmonic balancer and piston goes around twice for each cycle (once on power, once on induction) so even sticking a screwdriver in the hole you still have a 50% chance of getting it wrong. not to mention you have just stuck a screw driver in your engine. the thought of that makes me shudder.
with the rocker cover off you can watch the #1 intake valve close which means you are on the correct stroke. and sticking a bit of thin wire in the spark plug hole is less likely to damage anything.
once you are sure the intake valve has just closed and you are on top dead with your piece of wire, the mark on your harmonic balancer should be lined up with the 0 mark on your timing cover. if not your blanacer has spun or your cam timing is out.
so lets say the balancer lines up with your piece of wire and the timing mark, and your intake valve has just closed. you can safely guess your at #1TDC. move to your dizzy. pop off the cap and look where the rotor button is pointed. you can proceed in 2 ways.
1. switch all your leads to suit where the rotor button is pointed
2. undo the retaining bolt, lift the dizzy about 4 inches and rotate the rotor button to suit your cap.
either way works fine, but switching your leads may mean the shorter ones cant reach your sparkplugs.
drop the cap back on, tighten everything back up and fire her up.
dont forget to check that your rotor button is going CLOCK WISE. and your firing order is [insert correct firing order here]