R;...Just curved an HEI Dist this morning for a friend, they are all way out of what they should be, even the new ones. Did two different aftermarket ones recently both new and way off.
So do this first get this Vacumn can. It is made by Standard Motor Parts SMP VC 302.....Rock auto sells it as a VC 302....Napa part number is VC 1703, ...O'Reilly sells it as a BWD V482.... about 15 bucks.....Standard makes them all just rebranded. Number stamped on the can is 626 10.
If you have the can with the hex nose looks like it takes a big wrench,....the last two of those I Tested on my distributor machine had 22 or more degrees of timing advance...way too much!...It is adjustable for rate only, as to when it starts and stops from Vacumn, not the amount of timing which is what really matters.
So the can I reference above pulls in 10 degrees timing advance at the crank,....that is the number 10 stamped in it.
Get that can put it in 3 screws, just tilt it sideways once loose to remove old one.
You can then check your Centrifigal advance as I described, with a timing light that advances the spark. Once you know that number ensure your base is set for with Centrifigal to total 36 degrees. So if your Centrifigal is 24, set your base at 12, if it is 26 set your base at 10.
If your Centrifigal is way too high or low try one of the aftermarket curve kits, like Mr Gasket. Change the weights that should get you close to 25,26 then set base and recheck.
Now with you vac can pulling 10 you have 46 BTDC for light throttle and super cruising, great response great gas mileage etc. 46 to 48 is what you want.
Now for full manifold, I run that and recommend it highly, car runs smoother at idle and cooler at idle. Your car will run at 20 BTDC with the 10 Base and 10 on the vac can.....if hooked to full manifold Vacumn, it will be smooth as silk. If you use ported you will idle at 10 and have a hotter engine not as good idle.
In about 1965 in order to meet emissions standards just starting, the manufactures wanted to burn off more Nitrios Oxide, the way to burn that off is a hotter exhaust,....the way you get a hotter exhaust is to retard timing,....an inexpensive way to retard timing is to take the vacumn advance away from the distributor at idle...the way to take the vac advance away is to hook the vac to no vac, i.e. A vac port above the throttle plate. ...so it is an emissions improvement.
If you use ported vac everything will be the same off idle, but why do it? Who wants their car to overheat in traffic.....bad timing makes a hot runner.....
So as time permits work on this and you will be pleasantly surprised....use the two light springs in the curve kit, or a light and a middle spring.
I used two black springs today from a Mr.Gasket kit and it started at 900 RPM and all in by 3400 RPM....each Dist is a little different. But you can get real close just with a little work.
Too much timing will cause engine knock and lots of cars have way too much!