Vacuum advance

[Dennis Hudson]

Finally finished rebuild of my 83 2.3 ranger. Weber carb and msd ignition. Had trouble with the ignition mechanical vacume advance until I discovered that I didnt need the vacuum advance. The msd provides the perfect advance on its own. Just plugged up the vacuum on the carb, and bingo, it runs like a clock.

 

[bobbywalter]

yaaaaay. does it have some essie creations on it or fairly stock?

 

[19Walt93]

If it's street driven you can gain gas mileage by making the vacuum advance work. Attach the hose to the spark port on the carb so it gets vacuum under acceleration not directly to manifold vacuum and it'll work best. When the engine is running at lower revs under minimal load the vacuum advance will add more than strict centrifugal advance.

 

[Dennis Hudson]

If it's street driven you can gain gas mileage by making the vacuum advance work. Attach the hose to the spark port on the carb so it gets vacuum under acceleration not directly to manifold vacuum and it'll work best. When the engine is running at lower revs under minimal load the vacuum advance will add more than strict centrifugal advance.

Sounds like something I could try, but I'm not sure what you mean by "spark port".
This is a 32 36 Weber Thanks

 

[19Walt93]

The spark port will have no vacuum at idle and will get a stronger vacuum signal as you open the throttle. If you look at the carb from the side, the spark port will usually be located just a little above the throttle plate.

 

[Dirtman]

Vacuum sucks.

 

[85_Ranger4x4]

The spark port will have no vacuum at idle and will get a stronger vacuum signal as you open the throttle. If you look at the carb from the side, the spark port will usually be located just a little above the throttle plate.

May also be call ported vacuum.

 

[RonD]

Yes, read about "ported vacuum advance" vs regular vacuum advance

Vacuum advance is needed unless you use some type of ICM(ignition control module), i.e. TFI, EDIS, ect.....

A quick, how spark needs to be timed:
You want spark to occur before TDC because there is a delay between spark and FULL explosive ignition of the air fuel mix
It takes a few milliseconds for the flame front to spread
And you want FULL explosive ignition to occur After TDC, of course, so it pushes the piston down adding power to the crank shaft
The most power is added if FULL explosive ignition happens at 8deg After TDC, before that the piston and rod don't have enough leverage to push on the crank
After 12deg or so the volume of the space between piston and head is getting to large so FULL explosive ignition will expand more so less power is added to crank

The 14.7:1 air:fuel ratio has a fixed time it takes from spark to FULL explosive ignition, a known time
As RPMs increase the time it takes for a piston to travel from say 10deg BTDC to 10deg ATDC changes, I know, DUH
But the time it takes from Spark to FULL explosive ignition stays the same
So very simple to time spark advance, because both times are known, even though one is changing
Centrifugal(RPM) advance is used for this timing
As RPMs increase, spark timing moves from 12deg BTDC to 20deg BTDC to 30deg BTDC, so FULL explosive ignition can occur at 8deg ATDC

But.............there is a problem
14.7:1 ratio has a known flame spread time
But when you press down on the gas pedal to accelerate that ratio changes, it gets a RICHER mix, like 13.0:1
A Richer mix causes flame spread to happen FASTER.
So spark timing must change or your performance will suffer
If RPM advance was at 22deg BTDC when you stepped on gas pedal then it would need to change to say 20deg BTDC to keep FULL explosive ignition at 8deg ATDC

This is where Vacuum Advance comes in
Vacuum in the engine changes with LOAD
When you step on the gas pedal regular vacuum goes down and Ported vacuum goes up

You do need to use one or the other, but depends on your vacuum advance setup in the distributor
You always set base spark timing after engine is warmed up and with Vacuum Advance unhooked

This is the simplified version, an example
There are many, many articles and debates about Vacuum advance, which is better Ported or Regular and best approach to get it right for perfect performance

 

[Dennis Hudson]

The spark port will have no vacuum at idle and will get a stronger vacuum signal as you open the throttle. If you look at the carb from the side, the spark port will usually be located just a little above the throttle plate.

When I first put it all together, I did connect the vacuum advance to the vacuum advance port on the carb. It ran fine at low rpm but would miss and stutter at higher rpm. So I suspected the old distributor, and replaced it. Same problem. I disconnected the vacuum advance from the carburetor, and plugged it. Thats when it seems to be running perfectly at all rpms. I can see that the advance is happening with my old timing light. Mystery!

 

[RonD]

No mystery, distributor has centrifugal(RPM) advance, the weights and springs inside it, so any engine with that type of distributor will have spark advance

Vacuum advance is just for engine LOAD, which varies, and vacuum varies with it, which is why vacuum advance is used
Maybe you need regular vacuum for your vacuum advance, or maybe Ford used a dual vacuum advance setup on your model

Ford, and other car makers, used a temperature unit, usually, on a coolant passage in the intake
It had 3 vacuum ports
One for the distributor
One for Ported vacuum
One for Regular vacuum

Engine temp would decide which vacuum advance would go to distributor

Or maybe your vacuum advance needs to be tuned, on many of these you would remove the hose and insert a hex key in that port to adjust the vacuum advance responsiveness
Or on the vacuum advance "arm" inside the distributor

So nothing mysterious about it, when fuel mix changes the burn rate changes, so vacuum advance is needed for best performance, its just Math or Physics

 

[franklin2]

That is what the problem is, the vacuum advance is adding too much timing. Since you have a aftermarket distributor, it may be adjustable. Some of the Ford's were adjustable also. You stuck a allen wrench up in the vacuum port and turn a screw in there to adjust it.

Using the vacuum advance is not mandatory. Your engine will run fine without it. The extra advance at light engine loads just increases engine efficiency, meaning better fuel mileage.

The problem you are having is very common on a stock Ford distributors when the smog stuff is taken off. Ford ran the static timing on their distributors very low (8-10 BTDC or lower) ran a very lazy mechanical advance, and ran a lot of vacuum advance. If you wanted a big improvement in throttle response at lower rpms, you set the static up to 12-14 degrees, as long as it will crank over when hot. But then when you plug in the stock vacuum advance unit and try to use it, you run into the same problem you are having, surging and miss fire at light throttle. An adjustable unit is the solution in these cases also.

 

[19Walt93]

I recurve my distributors to get the centrifugal advance all in by 3000 or so and run vacuum advance rather than just bumping the base timing ahead. The springs and weights are under the base plate and aren't hard to change but you can usually stick a screw driver through the rectangular holes in the base plate and bend the tabs slightly to change the advance while retaining the same springs. Ford usually has one light spring that is always under tension and a stiffer spring that has free play and only adds tension at higher RPM's. An example: say your centrifugal is all in by 3000(unlikely if you haven't tweaked it) and you're driving along at 2000 rpms, having vacuum advance will improve part throttle driveability and gas mileage- centrifugal only would run smooth with less power and less gas mileage. If a little distributor tuning will add both power and gas mileage, why would you NOT do it?

 

[franklin2]

In my experience, relying on the vacuum advance for power never works well. It's there when the throttle is at a constant position, but as soon as you step on the throttle to accelerate, the advance it was giving goes away. You get a much crisper and responsive engine if most of the timing advance is there all the time with the weights + static.

I just advance the static as much as I can because it's easy. Modifying the springs and weights is the proper more permanent way to do it.

 

[Dirtman]

And on the 8th day god created EDIS...

 

[RonD]

The spark advance should drop when you Richen the air/fuel mix because it burns faster, then centrifugal will take over as RPMs increase and mixture Leans back out