1970 challenger 340-6 has an original vacuum advance diaphragm. Tested with a vacuum pump and it goes down real slowly is this normal or how long should it hold vacuum?

If I have to replace it I'll need to replace it with an original. Anyone know what part number I need to look for? Do they make accurate reproductions or will I be looking at NOS?

If it goes down,it has a bad diaphragm,it can be fixed.

How can I fix it?

if it wont hold vacuum then it is shot. any points SB one will fit tho the amt of adv (stamped on the arm will vary) and you can shim the notch in the arm to reduce it and file the notch so it will go in farther (difficult but can be done) for more advance or just plug an play with the one your parts house gives you (you can adj the tip in point with a 3/32" allen wrench). if you need NOS or a repro I ain't up on that.

Manual Trans 3514265

Auto Trans 3620704

How can the bad diaphragm be fixed? I can't seem to find a 3620704 are there other part numbers that would work? It doesn't have to be date code correct or anything just needs to look like the original.

have your parts house pull one up for you & take yours in & do a side by side visual then make your decision

Depends on who you know,had this one done a week ago.Also,you won't find one for any Prestolite dual point at a parts house.

Max, who does em? EDIT "Joesixpack" on here might have an NOS one for sale. If he does it'd sure be cheaper than having one redone

Trade secret,I can get them done but was requested by the vendor not to give out their information.

Robert,I guarantee you if you find a "correct" Prestolite N.O.S 70 dual point vacuum advance,be it big or small block it will be four times what I can get them done.Here is a sample.

http://www.ebay.com/itm/1970-1971-Cuda-Challenger-Road-Runner-GTX-Charger-Hemi-VACUUM-ADVANCE-NOS-MoPar-/361035250057?hash=item540f60b989:g:1R4AAOSwxH1T~-z8&vxp=mtr

Max I believe you!

I don't know anyone. What should I do to solve my issue?

Check your PM's

Do the vacuum diaphragms have any markings, part numbers etc., on them? What does dual point mean and how can I tell if mine is dual point or not?

Here's the parts house one and an original dual point. Not sure it looks much different than an original except it's much brighter and the hexagon section is longer.

For fitment the critical part is the shape of the arm. Are you good there (both the same & will be if they pulled the correct one for a points SB) and is the finish/longer hex acceptable to you? Also what number is showing on the arms?

The distributor in question if what OP posted is correct is from a 340-6 which would be a Prestolite dual point.The Chrysler single point and the electronic use a different breaker plate and the advance is not interchangeable with the dual point plate.

Do the vacuum diaphragms have any markings, part numbers etc., on them? What does dual point mean and how can I tell if mine is dual point or not?

Alright, got it. OP it'll have a # on the arm (& likely a letter such as 16L) & that # times 2 is the amt of max vac adv that can will give you on the crank. I ain't sure what the letter is for.