i noticed several threads about timing so here is my contribution in helping others.

first watch this video so u can know what ur doing
http://www.setyourtiming.com/

you will need the following
*basic tools
*timing light
*timing tape or balancer cover


first disconnect ur vacuum advance and plug the fitting so it doesnt cause a vacuum leak. now check ur timing and see what the initial is (8-14 btdc) now plug the vacuum advance to full manifold and notice how much it advances, most canisters are 20+ degrees and will be too much, a 10 degree canister is prefered or an adjustable, but i will show u how to limit yours if u dont have acces to one towards the end of the page

time to add a few parts to make that old hei accurate again. ur gonna need a distributor curve kit like the one below, but make sure the kit u purchase comes with bushings.


instructions and diagrams are provided but here are some pics so u can get a head start on what it is that needs to be done. first remove the cap and rotor and u will see the weights are held with springs, remove these with some needle nose pliers


once the weights are off, u will see some e-clips that are holding the center bracket, take care of these clips as some kits dont come with them. remove the clips and pry up on the center plate evenly on both sides until it comes out


with everything out of the way, you have to grease the tabs im holding, some stock distributors dont have these and have a raised metal section but regardless a light coat of grease to prevent the weights from binding


install the center plate and put the clips back to hold it. from the kit find a set of bushings that resemble ur old ones and put them on the weights first and then on the studs. u can see the other bushing to give u an idea on how they go. once u put the weight back in, go back to the kit and try a set of springs, i went with middle strength first.



INITIAL TIMING
with the vacuum advance still disconnected and the vacuum port on the carb plugged check the intial timing and adjust to where u want by turning the distributor clockwise (advance) or counter clockwise (retard)

MECHANICAL TIMING
once u get the base timing you want, have someone rev the engine little by little and keep the timing light on the balancer and notice how it will start to advance. this is done by the weights swinging out causing the distributor to advance. usually most distributors will only allow 20 degrees of mechanical timing. i chose to put a weak spring with a middle strenght spring and caused my total to be about 34 degrees at approximately 3.5K rpm. ur gonna want to aim for 34-36 degrees of total timing to improve performance. keep in mind that everytime u change springs ur inital timing will change so u will have to go back and adjust. its trial and error but u will find hidden horsepower if ur persistent as this is the hardest step

VACUUM ADVANCE

now going back to the vacuum advance, the vacuum advance is the little canister sticking out on the side of the distributor with a vacuum line to it. the purpose of the vacuum advance is to improve idle, mileage, etc. so it makes sense to hook it up to full manifold vacuum. since there is no vacuum at WOT it wont affect total timing only at idle. if you have one of those 20-25 degree canisters (most common) then u will see your timing has gone way up when you hook ur vacuum advance. the vacuum advance has a rod that moves forward advancing the distributor, we have to limit this movement in order to keep it from advancing. below is a stopper i made from an aluminum header gasket i had laying around. smirk



u will noticed it has a notch on the front for the rod and at the back it has a long slot where u can move it up/down limiting the movement of the rod which will affect how much it advances



here is another one but u get the idea as to how they work, only thing u need to worry is making sure it doesnt hit anything inside, but the HEI distributor has plenty of space



so say ur base timing is 12 degrees, u will want ur vacuum advance to add another 10 degrees so ur engine idles at 22 (12 inital + 10 vacuum) the vacuum will be gone as soon as u hit the gas pedal and will not change ur total timing or anything you've done above it will just simply advance the engine at idle to help improve mileage, idle, etc. this doesnt get added to the total timing its seperate. feel free to join in or correct me wave


here are a few links in case you want to learn more
http://www.gofastforless.com/ignition/advance.htm

http://www.centuryperformance.com/ignition-and-timing-settings-spg-219.html

http://www.enginebuildermag.com/Article/3986/old_iron_tuning_a_carbureted_engine.aspx

http://www.73-87.com/7387garage/drivetrain/hei.htm

http://www.angelfire.com/realm2/84camaro/fix_the_car/coil/coil2.htm

http://www.tuningmadeeasy.com/tuning/tuning-an-ignition-part-1/

http://www.corvette-restoration.com/resources/technical_papers/Timing101.pdf

http://www.chevyhiperformance.com/howto/97438/index.html

http://www.chevyhiperformance.com/howto/46178/index.html