Limit total advance???

[Excaliber]

I think the vacuum advance and your HG readings at various speeds done when the engine is NOT UNDER LOAD, are a red herring. A snipe hunt. A,,,, well you get the point.

WHY do we even have vacuum advance? What purpose does it serve for PERFORMANCE? It doesn't have anything to do with performance, in that regard, take it out, block it off, throw it away. It's PRIMARY PURPOSE in life is to maximize mpg by enhancing cruising speed/rpm efficiency. That's it, thats all it does, all it was ever intended to do.

You will ever see a vacuum advance unit on a serious race engine, or even a serious street engine. Step 1. Maximize your timing on the base and mechanical side. I have found that most modified engines do indeed run better at low to mid range rpm with 18 to 20 degrees advance. That alone solves SO many problems, like stumbling, hesitation, flat spots, etc. But there are a couple of problems with running this much BASE timing. One is it may be hard for the starter motor to crank the engine, especially when hot. Two is it may be difficult to limit the MAX mechanical advance to maintain the desired total amount. Anywhere from 32 to 38, not generally more than that. Now ADD to that 32-38 the vacuum advance of say, 10 to 12 degrees. The new total may be (38+12) 50 degrees! That is reasonable!!!! Most motors could handle that, like it in fact, maximize your mpg. All though, 50 is pushing the outter limits in some cases. IF, your engine is under light load, like steady state cruise rpm with light throttle, it's amazing how much timing you can throw at it.

The reason is simple: As the HG (vacuum) drops, due to increased throttle opening, climbing a hill, accellerating, heavier load on the engine, the vacuum also drops. The radically advanced timing goes away, drops back to mechanical advance only!

It's unlikely your motor will be pulling anywhere near the HG at 2,500 your reading in the garage as it will at 2,500 rpm under load! Try this, get a really long vacuum hose and mount the vacuum gauge on the window or in the cockpit so you can see what the REAL HG is under various conditions, rpm, load, cruise, WOT, etc.

I run both mechanical and vacuum advance on my side oiler, with an MSD "ready to run" (does not require an MSD box) and comes with a vacuum advance canister. I run so much base timing that I HAD to find a way to limit the mechanical advance. MSD does not sell a stop bushing that offered enough "stop". I had to buy a special, bigger, stop bushing made by a speciallty racing shop just for this purpose. The fact that they are available is indicative of how many people are having the same problem, your problem! To much mechanical advance and no way to limit it enough.

The other thing you need to know is: When does the vacuum advance begin to advance? At what HG? Second is: How MUCH advance does it offer at what HG?

MSD starts at about 15" and offers about 10 to 15 degrees of advance (I forget the exact amounts).

For now, disconnect the vacuum advance, work on the mechanical advance, then go back to the vacuum part. Good luck.

[Rick Parker]

AZFORDMAN:
The overall limit can be changed. The distributor must basically be disassembled. The round part that the rotor slips onto has the mechanical advance mechanism attached to it under the base plate. It has 2 slots, one that may permit 16 crank degrees and another that will allow for instance, 24 crank degrees, the number of available degrees is limited by the length of the chosen slot and the pin that limits its travel. That number plus the intial (18??) is the total advance you will have available. The distance the slots allow can be altered by reducing the length of the slot. (Heliarc works nicely.)
To disassemble it, carefully remove the small wire clip in the center under the rotor, remove the c clips on the vacuume advance mechanism, remove the reluctor without breaking any of the tangs, then remove the baseplate. At that point the mechanical advance will be visable. The distributor bodies are pretty much the same between an electronic as pictured and a point type ignition.

The calibration is best done by someone with access to a Distributor machine.

[azfordman]

Originally Posted by Rick Parker

AZFORDMAN:
The distance the slots allow can be altered by reducing the length of the slot. (Heliarc works nicely.)
To disassemble it, carefully remove the small wire clip in the center under the rotor, remove the c clips on the vacuume advance mechanism, remove the reluctor without breaking any of the tangs, then remove the baseplate. At that point the mechanical advance will be visable. The distributor bodies are pretty much the same between an electronic as pictured and a point type ignition.

The calibration is best done by someone with access to a Distributor machine.

Rick, I take it the base plate is in my picture with the 8 and 6 stamped into it, correct? Where is this wire clip that is supposed to be under the rotor? Someone else had mentioned welding the slots.....are these slots on the underside of the base plate? I will get out and try to pull this apart tomorrow. I appreciate the input! Mark

[dcdoug]

Originally Posted by azfordman

Rick, I take it the base plate is in my picture with the 8 and 6 stamped into it, correct? Where is this wire clip that is supposed to be under the rotor? Someone else had mentioned welding the slots.....are these slots on the underside of the base plate? I will get out and try to pull this apart tomorrow. I appreciate the input! Mark

The wire clip is down in the rotor housing hole and is a bit of a pain to get out. I had to mess around with it with needle nose pliers for a while to get it out. Remove it, unscrew everything, and then you will see that the bottom plate that everything is bolted to is removable and beneath that is the advance mechanism.

Here is an article that might be useful. It's for slightly older model Ford distributors, but most of the principles should be the same.

http://www.mustangmonthly.com/techar.../photo_12.html

[azfordman]

Originally Posted by Excaliber

I think the vacuum advance and your HG readings at various speeds done when the engine is NOT UNDER LOAD, are a red herring. A snipe hunt. A,,,, well you get the point.


The reason is simple: As the HG (vacuum) drops, due to increased throttle opening, climbing a hill, accellerating, heavier load on the engine, the vacuum also drops. The radically advanced timing goes away, drops back to mechanical advance only!

It's unlikely your motor will be pulling anywhere near the HG at 2,500 your reading in the garage as it will at 2,500 rpm under load! Try this, get a really long vacuum hose and mount the vacuum gauge on the window or in the cockpit so you can see what the REAL HG is under various conditions, rpm, load, cruise, WOT, etc.


The other thing you need to know is: When does the vacuum advance begin to advance? At what HG? Second is: How MUCH advance does it offer at what HG?

I appreciate the informative post! I had always thought that the intake vacuum decreases considerably when under acceleration. Unfortunately with this car that is just not the case!!! I took the car out after reading your post and here are some results from a drive with vacuum gauge in cockpit:

In 1st gear up to 3000 RPMs the vacuum reading is 20" Hg
In 2nd gear up to 3000 RPMs the vacuum reading is 19" Hg
In 3rd gear up to 3000 RPMs the vacuum reading is 17" Hg
At this point I am already at 55 mph and speding.....

As far as putting a load on the motor, from a rolling start of 25-30 mph

In 3rd gear vacuum drops to 15" Hg
In 4th gear vacuum drops to 10" Hg
In 5th gear vacuum drops to 7" Hg
this is under very light throttle.

Under heavy (WOT) acceleration the vacuum will drop to around 10" Hg initially, but as I am ready to shift, around 5500-6000 Rpms the vacuum is back up around 20" Hg. The vacuum reading rises steadily as the RPMs increase. When shifting, between gears (clutch in and throttle off) the vacuum will jump up to 35" Hg (35" is incorrect...it is actually 25" Hg) then drops down to the 10" Hg as the next gear is engaged and throttle is applied again.

Normal cruising in 4th gear, 2500 RPMs, 60 mph the vacuum is a steady 20" Hg.
My question is, do I have an issue since this motor seems to pull so much vacuum???

As far as the advance goes, with the vacuum line disconnected and plugged, the mechanical advance only goes up to about 28 degrees BTDC, it begins to advance almost initially and is at full advance around 2000 rpm (a guestimate since I do not have a hand held tachometer.) With the vacuum advance hooked up, the timing advances the same way, but goes up to 45 degrees BTDC. So it seems with the vacuum advance disconnected I don't get enough advance, but with it connected I get too much.