Large RPM and Vacuum drop when shifting from park to gear

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Charlie_es_cool

Charlie_es_cool

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Hello everyone. I have a 1975 Dart with a 318 in it. The engine has headers, unknown cam, weiand intake, and a 670 holley street avenger carb. Transmission is auto. Have vacuum advance on my distributor, and it's hooked up to full manifold vacuum.

I tuned the carb in park with a vacuum gauge and got it to idle at around 1000 rpm and 18-ish inches of vacuum. When I drop it in gear, RPMS drop to around 500-600 and it dies almost immediately. To combat this, I tried tuning it in gear, and the greatest vacuum I get out of it in gear is around 6-7 inches @1000 RPM. The thing is, after I do this I end up with an Idle Speed at park of 1900 RPM!!!!

In park at 1000RPM and 18-ish inches of vacuum, the dial shakes rapidly but it is fairly constant. On the other hand, when it drops in gear the dial shakes rapidly between 5 and 8 inches. I could get a video of this and post it on this thread tomorrow.

My question is, any idea of what could cause this? What confuses me is the following: If I had an vacuum leak or another issue somewhere, would it only affect vacuum in gear or would it affect both?
 
idle timing when it's running at 500-600 rpm? What is the ignition timing at 1000 as well?

If you are going by the book, throw those timing numbers out. Different cam and not knowing what it is is going to be an issue.

I would isolate the intake tract as well. Disconnect and plug EVERYTHING that is connected, PB, etc...

IMO, remove the advance from full manifold vacuum for now. Tune without it.
 
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At first glance, your torque converter could be too 'low stall'. What converter is it?

Or, you could have vacuum leaks. Or...a bunch of other stuff.

How much timing are you running? I'd set if for around 20 degree initial, with the vac advance in operation. and 32-34 degrees total. Ignition advance idle does wonders for idle quality/strength (and you'll see it in good vacuum-at-idle readings).
 
You likely need more timing. Start by adjusting timing with a vacuum gauge for highest reading, then see if the starter "kicks."

You need to post "if" the dist is recurved and what you have been running for timing, both total (no vacuum) and initial.

And it might be too lean.

And even then sometimes the throttles are open so far "with a big cam" that the blades are out of the idle transfer slots
 
  1. Set timming and idle speed with vacuum advance disconnected and plugged.
  2. Put the vacuum advance on the correct ported vacuum port on the carb
What I suspect is happening is with vacuum advance attached to manifold vacuum you have high vacuum at idle and that is pulling your vacuum advance to full, (let's assume 15 deg.) So you set the static timming to 10 BTDC. You put it in gear the engine rpm decreased and now you loose enough vacuum that the vacuum advance drops out and
now you are at 5 ATDC,
 
Take the VA off, and when you put it back on, use PORTED vacuum.

You don’t nearly enough initial. Your advance curve is probably wrong too.
 
Never liked "full manifold" advance. I don't think Ma generally, employed this, always "ported" vacuum
 
Ported vacuum has no purpose other than emissions
 
gregcon said:
Ported vacuum has no purpose other than emissions
Click to expand...
I would disagree, manifold vacuum decreases as the throttle opens, ported vaccine increases as the throttle opens, increasing vacuum increases timing faster than the engine RPM can advance the dist.

Manifold vacuum would decrease when the throttle is opened reducing the amount of advance. Contrary to what you would want.
 
Put that vacuum advance line back on the sparkport.
Make sure the secondaries are closed up tight,but not sticking.
I'm assuming the float level is according to design.
Increase the idle timing to 10/12.
Idle the engine down to 650 in gear,
and it should be fine.
If it's not;then
Now, let the tuning begin

FORGET tuning the idle to the highest vacuum; it DOES NOT WORK. You just proved it. Leave the gauge in the toolbox.

By tuning to the highest vacuum, with the Vcan on the manifold, you cranked a bunch mechanical idle-timing out of it, to get the rpm down. But you could not get the rpm to go any lower because the butterflies were shutting OFF the transfers and it wanted to stall.
Obviously to me, the cure is less advance. By ditching the full-time Vcan, you will have to restore the mechanical timing, which will allow you to increase the throttle opening, to restore the transfer fuel ratio, back to normal. Now your engine will have enough power to not stall going into gear.

The amount of IDLE-timing you can run is governed by;
where the butterflies like to be up on the transfers, and
The rpm that you target, to idle at.
A stock cam might idle at 500 in gear. Each bigger cam might like 50 rpm more. By me saying to target 650 in gear, I am guessing that your cam is ~ 3 sizes bigger than stock or 212 to 218 @.050, IMO, a safe guess. It is my hope that this guess will put your transfer slot exposure underneath the butterflies to about square to perhaps a lil taller than wide. If you want to, you can just take the carb off and look, and set it there, but if you do, then LEAVE it there. Do not set your idle speed with the speed screw; instead, use timing. Set it to 700rpm with timing, and then put it into gear and fine tune the rpm with timing. If it bangs into gear use less timing. If it wants to stall, first twiddle the mixture screw a bit richer. If still wants to stall, add timing.

After you get really close,
then you can try up to a turn more or less on the speed screw, but keep track of where you are at all times, relative to the starting point, so that, if the need arises, you can get back to it, without taking the carb off again.

You will know when the butterflies are too far down the transfers by the hesitation that shows up during a slow throttle tip-in . Too rich is harder to feel; so I always go leaner until it stumbles, then back up.

Good luck
 
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A ported advance does increase timing as you tip-in the throttle -which is exactly what you don't want.

It works far better to have the timing ALREADY advanced at idle. It gives high vacuum and a strong idle.

If that's true, why use vacuum advance at all? Well, it you don't have it, and you're starting at 20 degrees advance, you'll get pinging once you tip-in the throttle and start accelerating.

So...you advance the ignition at idle, then when you open the throttle, the timing RETARDS a little (like 10 degrees or so). Then, the mechanical advance is tuned to bring the spark advance back in as you accelerate. If you want to be at 14 degrees just off idle, it's better to start above that value and drop to it than to start below it and go up to it.

It works wonderfully. If you haven't tried it, don't say it doesn't work. Vacuum advance was used to meet emissions regs at idle. It's not what the engine wants.

You can see this if you map an EFI engine's spark curve. You pull the timing way up at idle, then let it decay as load increases and vacuum decreases. Pretty much every EFI engine ever born does it. The above method is the poor man's way of copying that.

As for mechanical advance not being able to advance as fast as you'd like....trust me, a centrifugal weight can accelerate far faster than a vacuum pod.
 
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You need to see what stall converter you have i believe that is where I would start. You'll be surprised at how that wakes your car up. And Idle in gear won't be a problem.
 
gregcon said:
A ported advance does increase timing as you tip-in the throttle -which is exactly what you don't want.
Click to expand...

I don't want to start something;
but,I gotta disagree with this; I think it needs to be qualified.
But I also get what you are trying to convey, and
It's pretty obvious you know your way around timing controls.

But it seems to me that you are not speaking to how all streeters are operated. like;
I have a manual trans,
At Part Throttle;
my 367, at 180psi, generates an excessive amount of torque way down low.So I have to be careful to NOT run too much timing down there, because the car then wants to get to bucking. So I purposely have a small amount of idle-timing; just 12 to 14 degrees.
Then it ramps up to 28*@2800, then to 34* at 3400.
But my Vcan , is able to add 22* at any time. So cruising at 2800 it could be pulling 48*. This engine at WOT will blow the 295 tires off thru two and three gears, so I am not interested in max anything.
But one thing I like to do is idle thru the burger joints at 4 mpg, parading, I call it.
To get down to 4mph with 3.55s and 27s takes an in-gear and driving rpm of 550rpm. To do this, without bucking, takes 5* of advance. This CAN NOT be done with 22* of full-time Vcan operation; my t-slot sync would be impossible to drive.

Yes I could achieve some/most of that with full-time manifold vacuum.
But the thing I cannot achieve is a sufficiently low idle speed, whilst having an appropriate transfer slot sync. . Nor could I ever drive it at low speeds without riding the brake or slipping the clutch; AND, it would be impossible to parade with, a thing I like to do.

Automatic guys have an easier time of it; just throw stuff at it until it knocks. That don't make it right.
 
gregcon said:
. If you haven't tried it, don't say it doesn't work
Click to expand...
Not saying it can't work. I have played with it and never liked the results.

I'm just in favor of setting things that are not working back to the way they were designed and starting from there. (And no I don't mean replace the HP add on parts with stock parts)
 
My comments are aimed at the OP's type of car (automatic, fairly mild engine). As cars get more radical, their timing needs become different....though I still like a lot of initial advance.

That's why I like the MSD distributors - their mech advance weights, and vac cans, are way easier to tune and more capable than the stock Mopar units. I know they are Chevy-based, and that kinda pains me, but ....

I don't have an EFI car now, but when I get my R3 project going I will...I'll dig up this thread and post a screenshot of the timing map. The bottom line (for me) is advance does wonders, as long as it's kept in check with load and RPM.
 
gregcon said:
My comments are aimed at the OP's type of car (automatic, fairly mild engine). As cars get more radical, their timing needs become different....though I still like a lot of initial advance.

That's why I like the MSD distributors - their mech advance weights, and vac cans, are way easier to tune and more capable than the stock Mopar units. I know they are Chevy-based, and that kinda pains me, but ....

I don't have an EFI car now, but when I get my R3 project going I will...I'll dig up this thread and post a screenshot of the timing map. The bottom line (for me) is advance does wonders, as long as it's kept in check with load and RPM.
Click to expand...


The GM advance is my least favorite. It’s sloppy, doesn’t repeat well, but it is CHEAP to manufacture, which is why GM, MSD and rest use it.
 
That's kinda what I like about it....the Mopar system uses a much better design hinge point - bronze bushings that are bigger and fitted more closely. But that's also its downfall...they are a lot more prone to getting sticky or completely stuck. It's like the old story of the AR-15 versus the AK47...the AR-15 is much better built but the AK47 will keep firing, even full of sand, due to the sloppiness.

Then, the MSD/GM design is much, much faster and easier to access for tuning. Plus, they offer a variety of springs which Mopar really doesn't. Finally, the MSD/GM also uses bushings which allow you to further define the total advance. With the Mopar setup, it's filing and filling time.

I've not had any issue with repeatability...again, I think the sloppiness works well to not allow stickiness.
 
Thank you all for the replies.

The common theme here is looking into what my timing is at idle, and how the manifold vacuum is managing my vacuum advance. I don't own a timing light, so I'll buy one and start looking into what's going on with my timing and pick up from there. Will post an update after I gather this data.
 
Charlie_es_cool said:
I don't own a timing light,
Click to expand...
To cure this problem, that's actually a good thing. just follow my previous post, #11. No timing light needed.
The only thing you need a timing light for, is to set the Power-Timing to an amount less than what will cause detonation at WOT. But since you are just guessing, and then road-testing to listen for it, or to feel the power loss, or to witness the loss of speed on a timed run..... and then making an appropriate change, you can even get away without a timing light for this.
Having a lite just makes dialing it in a wee bit faster. And perhaps a tad safer.
As for setting the idle timing, to an arbitrarily chosen number;
if your engine wants between 10 and 19, by single degrees, there is a 90% chance you will chose a wrong number.
If your engine wants between 12 and 16, there is an 80% chance you will choose wrong. But, by single degrees,if you guess wrong every time;
there are actually 5 choices the first time, 4 the second, 3 the third, 2 the fourth and finally the right one comes up. So that doesn't total just 5, it sums to 15. At least that is what they taught us in math class. Question everything!
The method in post 11, will get it right, the first or second time, because the engine will tell you what she wants.
 
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I don't want to hijack this post but all the info here has been great as I have wondered the same things on my car. My question is how to test if your vacuum advance is working. In theory when I am at idle and set my timing at a certain point with the vacuum advance disconnected at lets say 10* and pulling about 15-16 in of vacuum at idle, when I reconnect the advance to the spark port on the side of the carb then shouldn't it jump forward a few degrees? If it doesn't does that mean my vacuum advance canister is bad? Same as if I accelerate the engine with or without it connected it seems like the timing advances at the same exact rate in any part of the rpm range
 
Projectile Dart said:
I don't want to hijack this post but all the info here has been great as I have wondered the same things on my car. My question is how to test if your vacuum advance is working. In theory when I am at idle and set my timing at a certain point with the vacuum advance disconnected at lets say 10* and pulling about 15-16 in of vacuum at idle, when I reconnect the advance to the spark port on the side of the carb then shouldn't it jump forward a few degrees? If it doesn't does that mean my vacuum advance canister is bad? Same as if I accelerate the engine with or without it connected it seems like the timing advances at the same exact rate in any part of the rpm range
Click to expand...


No, ported vacuum will NOT move the timing at idle.
 
Just suck on the line;if the can is good, the rpm will go up.

Cans are rated from about 5 to11, in distributor degrees, and the arms are stamped to reflect this. You double the number for crank degrees.
Inside the nipple is a preload adjuster that you can delay the onset of timing. But I noticed that if I crank mine to the end, it also does not deliver it's rated amount. I can't say if they all do that.

If you map your mechanical timing as degrees to rpm
and your Vcan as degrees to vacuum,
Then you can determine at any rpm and load setting, what advance your engine is getting.
IDK how useful this is information can be to you..... jus saying.
I used it to help determine my cruise timing. You might hardly believe how much cruise-timing my engine liked.
 
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