On every slant set up I have seen, the vacuum can is hooked up to "venturi" vacuum. Venturi vacuum is MUCH different then manifold vacuum. Manifold vacuum is highest at idle and lowest at WOT, venturi vacuum is lowest at idle AND WOT, and highest somewhere around 25% throttle.

I have gauges t'ed in to both lines and can watch them while driving, it really helps understanding how venturi vacuum works.


TopHat

For vacc. advance you want to make sure it come in just off of idle (check vacc. at the carb)... you want it to be at the max, when you are at cruise... but when 'passing' (i.e. lower vacc. reading, throttle plates tipped open ,but not WOT....) you need to make sure most of it has 'gone', (and if you're running less carb and don't get to '0', then you need to make sure the vacc. adv. is out before you get to that point)....

SO...say for example... you are warmed up at idle, you have no vacc. at the adv. pod, but your steady manifold gauge says you have 20"... you put it into gear (auto tranny, major lugdown, klank...), with the foot on the brake it now says 18" but still no vacc. at the pod.... you then drive off and stay at a constant speed (50 mph...) vacc. gauge says 14-16" depending on hills and such... so vacc. pod should be @ max...

Suddenly a hot looking babe in a 1970 Charger whips by you, and you just gotta catch up.... so first we try to get around that semi in front of us... a little throttle but not WOT.... vacc. gauge says 12"... we should have minimal vacc. an not much advance at the pod... (couple degrees....)

You decide to show her how much the leaning tower of power has stored away and you floor it, the tranny goes into passing gear, the tach goes up and the carb is now pouring in the go juice...) Vacc. gauge says 0-4"... there should be 'no vacc. advance'....

While all this is going on, from the performance stand point......

your mech advance at idle is pretty much 0, just what ever your dialed in the base number (depending on spring choices, you might have a bit more for that initial number).... cruising along.... some advance is ok....

Your mech should run out a few hundred above your cruise line... (aka, if your vehicle is happy at 2800 rpm doing 65... you want max mech advance at 3100 or so... 'stock' distributors wind out about 4500+)

-D.Idiot

p.s. if drag racing.... toss in a set of springs that will give you max mech advance in the mid-1000's, but it's nice to have a set that will keep the weights from bouncing off the ends of the governor at higher rpm....

I believe the vacuum advance should start 1-2" above where the Power Valve (Holley) or power portion of needle (BBS,BBD) ends.

i.e. If the PV activates at 6.5" the vacuum shouldn't start until 7.5-8.5" of vacuum (same idea for power / cruise portion of needle)

So you won't have any vacuum advance if you're in the power circuit of the carb.

I"d also consider a tapered needle ideal, as the lower the vacuum, the richer the mixture. As you get out of the power, the mixture goes leaner; then when it's as lean as it can get and vacuum is still increasing, you start getting vacuum advance.

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Fellows,

I am completely grateful and my level of understanding went up logarithmically. I have never seen this topic even summarized, much less summarized so well as your three inputs have done. Many, many thanks and I bet someday someone else teaching themselves will be just as grateful. I was hoping people I KNOW are tuning bugs like me would respond. I had thought about this topic and drawn pictures, and overthought to the point I could make a good case for each place vac advance should be (hi or lo), and knew I needed advice.

Interesting re. venturi vac, tophat. I run a Holley 390cfm 4bbl and am pretty sure my vac gage is plumbed from carb body into manifold, not venturi. If so I want to plumb another gage into venturi, which will be easy to do since I have 4 venturis each about 4 inches long through risers.
I wish it wasn't 95 degrees and 82 % humidity, I'd do it now!

Thanks so much,
rock
'64d100

Rock,

The venturi vacuum off the carb is hopefully plugged into the distributor pod. If not, it should be.
You could add another vacuum gage to that line to see what the pod is seeing and it tune accordingly for max mileage at cruise.

Like DI pointed out, venturi vacuum is dependent on throttle position, not manifold vacuum.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

I will jump in here and ask another question and post some observations.

I am fine tuning the curve on my brother's 83 Dodge shorty van. 225 (super six), 727, 3.2 rear axle gears.

I am tuning for economy in this bog box, not drag racing, so I figure I need low end torque. The motor stays below 3000 RPM in all but the most aggressive driving, but there is a huge drop off in power after about 2500 RPM. Unfortunately this means that the van gets to about 45 MPH on the highway and they slooooooooowwwwwwwly creeps up to about 55 MPH. On a long straightaway it can get up to 65, but thats about it.

The van has great off the line pickup, but power drops off fast as the RPMs increase. I feel that this has to do with the timing curve.

For refernce, light throttle cruise is about 10-15 inches, aggressive cruise is about 5 inches, and flooring it is 0 inches (duh). Cruise RPM is 2500-3000. Base timing is 12 BTDC (spec for the stock 76 motor inthe van), and the distributor is a Super Six distributor that has (I think) alightly heavier springs and a minorly tweeked vac advance pod. I believe
it has a "9" governor, but I need to pull it apart to double check.

SO: Properly set up, my vacuum advance should not activate until vacuum reaches 5, and should be "all in" at 15. Right?

Mechanical advance for a big heavy rig like a van should come in a bit slower right? But I want it all in by 3000?

I have a spare slant dist as well as a 340 points dist to use for parts (weights, springs, governors), as well as some spare springs floating about. Any recommendations for good spring combinations to help with power and economy in a van? I know this is pretty much like asking you to tune up my car from acroos the country, but just some ballpark recommendations would be great.

Thanks for the tips.....

_________________
Casually looking for a Clifford hyperpak intake for cheap.

i haven`t done any vac pod tuning but i have played alot with aftermarket and stock springs, i would use the two light spring out of the two stock dist you have, aftermarket spring kit are to light and full adv just of idle. i find with two stock light spring, you can hold adv to 2500/2800rpm range. the slots will have to be shortened if you are running 12/15degs initial timing or you will have to much total adv timing.

Are you sure it's not fuel starvation or the carb in general?

_________________
Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Kinda sounds to me like restricted exhaust. Maybe the catalytic converter melted down? That would kill the economy too.

_________________
Joshua

Reed,

I would get a 74 distributor and recurve it. You have more to work with compared to a 76 unit. Some of the newer units are not very adjustable.

I like to tune not only with the springs and pod, but with the spring post cams. Sometimes I have opened up distributors only to find the primary spring loose and or too light which allows the weight to flop around and give you imediate full mechanical advance right off idle. In this condition it can allow partial advance at idle leaving little left when you need it at higher rpm. So in effect you have lost most of the mechanical advance.

I like to have all the mechanical in by 2500 rpm and tune the rest with the pod. Sounds like your vacuum pod isn't working....and the advance is not there......
What does the timing light show you?

You may have to use a red medium MOPAR spring on the primary and a heavy long looped spring to slow the advance down on the secondary weight. On my 74 distributor that holds it right at 16 mechanical advance at 2500 rpm.

_________________
Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

Good suggestions all. I know the carb has problems, most likely heat soak since it looks like gas is forcing itself out of the carb betwwen the float bowl and the top casting. Also, the catalytic could be bad. I put it on there about a two year ago. It was an aftermarket high flow cat. Now it makes a horrible rattling noise when the motor is running or when I bang on it. I am cutting it off tomorrow to see what's what.

I really don't know what year distributor is on there. I am guessing it is a Super six (77 or 78) distributor, but I am not sure. I can't remember what parts car I pulled it from, or if I grabbed it at the boneyard.

I will get the catalytic and the carb straightened out, but I want to make sure I get the distributor dialed in good too. It will be the last thing I do though. ALthough I am suspiscious since when I got done working on this van last time it had great power and economy.....

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Casually looking for a Clifford hyperpak intake for cheap.

Reed,

I have a load like yours (truck, 3.91, 727) but I have a "built" slant and pinging. If you have no power and no pinging I don't think you should start with your dizzy. you need to have fuel and air right to really appreciate dizzy tweaks, in my experience. Also, fuel and air tweaks are easy....all you need is a good vac gage and a dialback timing light. You can plot Emsvitl curves with them (one to a page) and later come back and fill in your vac advance and dizzy plots on those base graphs.

Once I knew I had my 4bbl adjusted so I knew right where a teeny tweak would go from lean to too lean, then I went back to my dizzy. When you get to that point you will see why I asked this thread question:

If you have a plot of rpm on the x axis and vac in in. of Hg on the Y axis, AND THEN plug your vac can in, and plot one advance line vs rpm for no vac can and another for vac advace with a can, you can readily see the effect of the can, and pretty easily visualize where you need a vacuum advance can to "pull" on the curve set to get vac advance "in". EXCEPT..as you ask and as I asked...where?

Well, Rob (DI) has given us a pretty good range I am testing now...vac advance start about 15 in, Hg and stay in through 20 to 22 in.Hg. I have a 10.5 power valve so Rob's 15 suggestion is only a tad more than the 12.5 recommended in this thread (GREAT sugestion)

And Ted and Rob have both recommended in emails to me about the spring cam adjustments and spring set I am trying this weekend. I think I am pretty close!
rock
'64d100

Thanks Rock! I found out that the catalytic converter on the van had indeed melted and plugged the exhaust. So today I am cutting out the old muffler and catalytic and converting it to just a muffler.

Once that is done, I will test drive it and see how the carb is doing, then play with the distributor as necessary.

The van has 3.2 rear gears so the motor doesn't get too high in the RPM range. I have copied and pasted the recommendations in this thread and will be using them to ballpark the distributor.

_________________
Casually looking for a Clifford hyperpak intake for cheap.

Rather than start I new thread, I thought I'd piggyback onto this one, since I have a similar issue.

My distrubutor's vac advance pod is toat- it does not hold vaccum and timing light test also shows no change from intiial timing when throttle is applied. Yes, there is vac at partial throttle at the carb's (BBD) port.

My distrubutor is really tight (read, hard to pull out of the block- yes, I did remove the hold-down bolt ), so I suspect the shaft may be worn, too.

Here's my question- what would be a good replacement for the vac advance pod and is it worth my while to change the springs for a mostly stock Super Six with a ful 2.25" exhaust system? This is a daily driver, so I'm looking for decent performance with some pep when needed, rather than all out power. I'm guessing I should get a new or rebuilt dist and adjust as necessary, based on suggestions.

TIA,

Casey