Vacuum Advance question

So I just picked up a nice 72 Duster with a 440 swap (unknown internals) Seems to have what sounds like a 280-ish advertised duration cam, Holley Street Dominator intake and an Edelbrock carb. Bottom end is said to have flat tops and 452 heads.

First thing first the car needs some tuning as it has a nasty stumble on the bottom end (lean I think) and I noticed the Vacuum advance is not hooked up.

With the vacuum adv. not connected to a vacuum source, does is it run at full advance all the time or is it retarded all the time?

I have only had the car in my possession for less than 36 hours so I have lots to learn about it. Long term, I'll build a 512 for it that will be a bit more hot street orientated but for now I just want to get this thing running better with the parts that are bolted to it.

Thanks in advance

if the vacuum advance isn't hooked up, then you only have the initial advance plus whatever advance is in the mechanical part of the distributor. that could be many different numbers depending on the internals. usually it is around 26-30* at the crank.
add your initial to that and you have your total advance.

the vacuum advance is mostly for light load cruising.

No vac line to the can= no vac adv. check for an immediate/adequate squirt when the throttle is moved (tip in). what is your initial (manifold or ported?). Wouldn't hurt to do a compression test just for a baseline to go in your notes. what is your idle speed? (in gear if auto). that'll get us started.

280ish cam? I wouldn't even try to use it. And I would limit the potential total of the mechanical advance as well.

I'll have to spend some time with it tonight to get some more detailed info... I'm just guessing on the cam based on the lope. My last 440 build had the MP 292 509, this doesn't sound quite as lopey

run a hose to the timed port (passenger side of carb) and see what happens.

Get a advance limit plate from FBO and follow instructions--will make a new ride out of it They are great folks and KNOW hwat to do to help

If you're going to drive on the street (i.e. not WOT all the time) then you need vacuum advance! A "280-ish" cam should have plenty of idle and cruise vacuum. My 383 with a 284/.484 idled at 13" for example.

What we don't know is what kind of mechanical advance you have. Or even what the initial timing is set to. If it's a stock distributor (26+ crank degrees mech advance) then the previous owner probably disconnected the vac advance can to keep it from pinging with the initial advance required for a decent idle.

I recommend you put a timing tape on the damper, and verify TDC is really at the 0 mark too. Then hook up a light and see what you've got as a starting point, before you start making changes blindly.

X2 on this advice, once you know for sure what is going on. In Mopar cast distributors, that plate is the easiest method I know to limit advance to the range your engine needs.

Be aware though, there are two theories on vac advance, timed port v. constant port. FBO is a proponent of constant, which in my case gives me about 30*@ idle. Works very well for me, but all FBO support is going to be under the assumption that you are running advance from a constant vac port. Obviously, both work. More than one way to skin a cat.

Well for the $25 bucks I ordered the FBO part... I do need to buy a timing light. My 30 year old piece of crap don't work any longer. Does anyone have suggestions on a decent budget minded Timing light?

^^^This what you should do first.^^^^
If there is a tag on the distributor, that will be a clue as to how it started life. But that doesn't tell where its at now.

Krautrock's explanation of what vacuum advance does is pretty much it.

If the first stage of the mechanical advance in the distributor is long and fast, don't use the FBO plate. Weld up the inside of the slots (assuming its a distributor with Chrysler guts in it).

It is a chryco dist. I'll check the tag on it tonight. Can you explain your weld up mod in more detail? What are is the result of doing so?

With timing tape you can use any ole timing light that you like. You'll also need a tachometer.

Identify what it has for ignition, Points, dual point, Chrysler electronic or something else.

On the carburetor, go through all of the basics first.
Fuel level,
does the choke open properly
Check that fuel squirts out of the shooters when the throttle is opened.
At hot idle, does turning the idle mixture screws in effect rpm. if so, make 'em a 1/8 to 1/4 turn richer (ccw) than when engine slows while turning them in.

Then see if the stumble occur in N, in gear or both.

before or after going over the basic carb checks
Look at the spark plugs for obvious clues.
At minimum check initial timing.
if a points ignition, check their condition.

Sorry you posted while I was typing. Mopar performance sold a chrysler distributer with Mallory advance mechanism for about 10 years.

Too your question. Yes. Give me a few minutes to wrap up some stuff first.

Here is what I do, and have always done with all my BB Mopars:

I rebuild a regular Chrysler magnetic pickup distributor. I put in Mr. Gasket #925B lightweight springs. I set the total mechanical advance at 34-36° and never hook up the vacuum advance.

My cars idle, run and drive perfect. I get up to 11mpg out of my 505 with 3.54 gears going 70 mph down the highway.

I have installed an analog MSD timing controller on my MSD box and plan on upping the total timing on the one road trip I do with it every year to see if there is a mileage difference.

The advance weights have a pin that rides in slot.
The length of the slot determines the amount of advance possible. The maximumn amount that can be advanced can be reduced by shortening the slots.

Welding is one way to make the slots shorter.
Here's a plate with 30 advance degrees in the slots welded up in the inside for less advance. Then a little filing to make both slots the same.

Notice when its installed that the primary spring has more tension on it. Similarly, the weight with the long looped secondary spring has less distance to travel before it is restrained by that spring. This is generally what we want when running for best efficiency (powerwise) and a vacuum advance.

Note: This is a smallblock distributor

Here's what is going on. These long advance curves came in with the need to reduce emissions, especially at idle. To do this, the idle mixtures were made leaner (reducing CO and HC) and initial timing lead was reduced (increasing cylinder and exhaust temperatures at idle). This hurt efficiency a little, so idle speeds were increased slightly.

Lets look at original 440 Hi performance timing specs to see why it usually makes sense to shorten the inside of slots.

If these advances are stopped short (welding the outside or using FBO's plate), the secondary curve never happens. When using vacuum advance, it means that under part throttle loads between 2000 and 3000 rpm, the total timing is often too much.

Shortening the inside means the existing spring set continues to provide an advance curve close to ideal for the engine. Much easier than finding springs and hours experimenting.

PS. Do all intial testing and checking without the vacuum advance. This is generally true with factory specs too.

Both are right.
Everything is right.
They're both the same.
All fixed!

What about looking at this the other way... at idle the vacuum advance is all-in (to give a cleaner idle with a big cam), which is too much for the engine under load at low rpm. So couldn't you also say that it's reducing the advance to the proper amount when you touch the throttle?

good stuff here.... i know the edelbrock carb needs work too. i know holley stuff not the edelweber carbs. This weekend Im gonna pull the valve covers to see whats under there. Was driving the car a little to get a feel for the motor. Feels real soft compared to a build I did some years back. I think some serious tuning days are in order.