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  #1  
Old 01-07-2014, 08:10 PM
davidmij davidmij is offline
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Default Re-curved Distibutor acting strange

Greetings, I recently took my distributor to the guy that built my motor to have him re-curve it. He said the curve wasn't far off but the vacuum advance module was. I was surprised at how much nicer it started from a dead stop once reinstalled - very smooth with no hesitation - at least for a week or so.

My problem is that it seems to have changed. The motor starts up and idles once warm at about 680 as it should. (If you remember I have a bit of a cam) Then I drive it, when I come to a stop the idle stays around 1100, it won't drop back down to 680. I checked the manual choke and high idle screw on the Edelbrock carb and they are not the problem. Sometimes when I hit the brakes coming to a stop, it will drop back down to 680, but very rarely. I'm guessing that part has something to do with the power brakes pulling vacuum and letting whatever is in the distributor module go back down. I checked all my vacuum lines. The one to the brakes - the one from the dist to the carb, and the plug in my Edlebrock carb. They are all sealed. Those are the only vacuum lines I have. (I have an electric wiper motor)

If I pull the line from the distributor it drops back down to 680. If I plug the line with my finger it stays at 680. I can suck on the line to the dist vacuum module and it goes back up to 1100. Can anyone tell me what might be wrong or what is going on?

thx, Dave J
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Old 01-08-2014, 12:20 AM
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Quote:
Originally Posted by davidmij View Post

My problem is that it seems to have changed. The motor starts up and idles once warm at about 680 as it should. (If you remember I have a bit of a cam) Then I drive it, when I come to a stop the idle stays around 1100, it won't drop back down to 680. I checked the manual choke and high idle screw on the Edelbrock carb and they are not the problem. Sometimes when I hit the brakes coming to a stop, it will drop back down to 680, but very rarely. I'm guessing that part has something to do with the power brakes pulling vacuum and letting whatever is in the distributor module go back down. I checked all my vacuum lines. The one to the brakes - the one from the dist to the carb, and the plug in my Edlebrock carb. They are all sealed. Those are the only vacuum lines I have. (I have an electric wiper motor)

If I pull the line from the distributor it drops back down to 680. If I plug the line with my finger it stays at 680. I can suck on the line to the dist vacuum module and it goes back up to 1100. Can anyone tell me what might be wrong or what is going on?

thx, Dave J
Does the vacuum hose from the dissy hook up to the carb above the throttle plates or below them??

And was the hose hooked up or not when the timing was set?
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Old 01-08-2014, 03:04 AM
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David, I think there are some misconceptions about curving a distributor. Each one is unique to the application. Assembly plants know the same engines are going in the same cars, so setup is all the same.

A same distributor will be curved differently for a Mustang than for a Crown Victoria, each having the same engine. Customizing the engines for different service (loads) alters specific distributor settings.

You have a new cam with a peak torque range. Timing of the cam speed to the distributor speed must be done under a load. This is impossible when the distributor is out of the engine. We can use 'rule of thumb' just to get in the ball park but fine tuning must be done by you after a set of trial runs.

Basically, you want distributor advance to follow engine speed and cam torque all the way up the power curve, just behind pinging and knocking while the engine is loaded.

CLICK HERE for my distributor section. I hope it will give you a better idea of what's going on.

Are you using a Pertronix ignition? <--this makes a difference.

Tom is right about how you set your initial timing, it needs to be done with NO vacuum advance. So when you start the engine there is no vacuum and spark is retarded for easy starting. As soon as it fires, vacuum is produced, causing the pancake to advance the spark. As rpms increase, the centrifugal weights advance the spark to the max, which should be 'all in' at ~2,500 rpm.

There are deviations. At wide open throttle, your vacuum drops. You could be climbing a steep mountain or racing. Too much advance while climbing mountains can cause detonation (pinging & knocking), so you want to back off spark advance under extreme load. The pancake (vacuum advance) should be adjusted to do just that. When your gas pedal eases back up and the engine produces vacuum, the vacuum advance adds more spark advance, which is correct because the air-to-fuel mixture is leaner.

Let's talk about numbers. Again, they cannot be specific to your application but we can separately determine how much advance the vacuum pancake adds and how much advance the centrifugal advance adds. There are adjustment parameters (a range) on the distributor reluctor. (Refer to Picture #12) The springs counteract the weights according to engine speed.

Having a 'hot' cam, you probably need maximum advance (~38) to happen at higher rpms, because that's where your cam's torque curve maximizes. You can see this with your timing light provided you extend the numbers on your damper pulley.

Coming back down to idle speed is simply a matter of how easily things slide. Any binding with the vacuum advance linkage or plate, or binding of the center shaft in your distributor will make things 'stick'. Of course, throttle and choke linkage must work smoothly as well.

Many months back Richard Hord had a similar problem. It wouldn't kick back down to a slow idle. He noticed his linkage caused the holes to elongate. When he gave a little tap, the linkage 'let go' and the engine idled properly. - Dave
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Old 01-08-2014, 10:46 AM
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I had this same problem. "Many months back Richard Hord had a similar problem. It wouldn't kick back down to a slow idle. He noticed his linkage caused the holes to elongate. When he gave a little tap, the linkage 'let go' and the engine idled properly"

I purchased a used bellcrank assembly from Bob's Bird House...
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Old 01-08-2014, 11:02 AM
davidmij davidmij is offline
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Thx Dave and Tom, your distributor section is really helpful!

First, to answer your question, I am using manifold vacuum. I initially tried the ported vacuum as the Edelbrock install DVD said to use, but I could barely get it to even run. Others on here and in articles I read on line said to use manifold instead. That worked great.

Secondly, I am using a Petronix ignition just like in your distributor section.

The engine had (and has) no pinging on any of the many steep hills around here, even while flooring it. It runs really well while accelerating, even in 4th gear at low speeds. The builder did ask me how much my car weighs before using his curving machine to set the distributor. Question; So if the car has to be under load why do they make a distributor machine that helps adjust the curve while out of the motor and on the machine? Is it just for a close base line setting? And then you need to adjust it by trial and error to fine tune it?

I didn't reset the timing after getting the distributor back. I will definitely start there! Would you guys recommend the ported vacuum or the manifold? Remember, I have a hot cam and somewhat performance motor.

If that doesn't help I will try disassembly and make sure all the moving parts of the distributor are moving freely with no binding.

One more thing; can you tell me what the adjustment of the vacuum module does? I read that it is adjusted by turning an allen wrench screw in the tube of the vacuum module. That is really all the builder adjusted - he said the rest was very close and didn't need any adjusting.

By the way, the distributor was new about 500 miles ago and has worked fine until now. So I would guess that it isn't worn anywhere. Then again, never take anything for granted.

Thanks for your help and patients with me. I know you guys are more used to helping people with a better understanding of cars and motors.

Regards, Dave J
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Old 01-09-2014, 01:12 AM
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David, the real question is, what is your total advance? When does it reach 'total advance'? You can answer this by using a simple timing light with a tachometer.

Why use machine? Because you can measure the above by simply attaching the points wire to a variable speed machine. It needs no timing light because it already knows the position of the drive gear. The machine cannot simulate an engine load, which is the most important part of spark advance.

No pinging or knock? You may actually need more advance (or you may not) depending... Where are you at?

Some vacuum advance pancakes are adjustable, most are not. If you are using a bone stock setup, there is no real reason to adjust. And exactly what are we adjusting? Stroke, of that little arm that pulls on the points plate. You may want more or less advance under NO load (full vacuum).

Air-to-fuel ratio (I'll call it, 'fuel mixture') is very different:
When the choke is engaged,
When the car is cruising at constant speeds,
At WOT (wide open throttle),
At normal temp idle speed,
When you press the gas pedal which squirts a shot of gas, etc.

Today's electronic fuel injection systems compensate for all that plus altitude, humidity, knock and more (without a distributor or vacuum advance module). Our classic setups are very limited in their mechanical abilities. The best we can do is tune the mixture and spark advance with the cam's torque curve at engine load. Simply called, 'tuning your engine'.

If not done right, you won't unleash all the HP your engine can produce (called, efficiency). - Dave
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Old 01-09-2014, 02:17 AM
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Quote:
Originally Posted by davidmij View Post
Thx Dave and Tom, your distributor section is really helpful!

First, to answer your question, I am using manifold vacuum. I initially tried the ported vacuum as the Edelbrock install DVD said to use, but I could barely get it to even run. Others on here and in articles I read on line said to use manifold instead. That worked great.

Would you guys recommend the ported vacuum or the manifold? Remember, I have a hot cam and somewhat performance motor.


Regards, Dave J
Stick with manifold vacuum, poerted vacuum has no place on any engine. (Except as an aid in reducing emissions).
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Old 01-09-2014, 10:17 AM
davidmij davidmij is offline
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Thx guys, I'll try to work on it this weekend weather permitting. It looks like Saturday will be the only day that it's semi-warm and not windy. I'll have my wife help me with the tachometer reading and find out what the RPM's are when it reaches total advance. I'll also note what the total advance is.

I just have to do a LOT of trial and error to feel the difference in the way it runs. I have a hard time feeling any difference in the way the car runs under load. (a "newbie" thing no doubt) Unless it's real obvious, like the way the motor would almost stall as I go from a dead stop before I had the distributor re-curved. Before the re-curve I could not let the clutch out without using any throttle. After the re-curve it was super smooth like a modern vehicle.

I'll stick with the vacuum manifold and start fresh.

Thx for the detailed info Gents, I keep learning more and more. It's fun to be gaining a better and better understanding of how everything ties together.

regards, Dave J
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Old 01-09-2014, 11:37 AM
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Quote:
Originally Posted by davidmij View Post
...Unless it's real obvious, like the way the motor would almost stall as I go from a dead stop before I had the distributor re-curved. Before the re-curve I could not let the clutch out without using any throttle. After the re-curve it was super smooth like a modern vehicle...
You just told me that you got more HP after giving the distributor more advance at idle.

How? When you depress the gas pedal, your vacuum goes down which relaxes the vacuum advance. Letting the clutch out stalled the engine.

"After re-curve" you can let the clutch out under full vacuum advance, and it's smooth.

I'm anxious to know what your total advance is set to. - Dave
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Old 01-10-2014, 06:29 PM
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OK, the latest.
It's cold and windy but I couldn't wait to figure out what is going on so I got after it.

I reset the timing and got the idle down to 600 which is as low as I can go without it sometimes stalling at stops. I think that's to be expected with the cam. I took my tools and timing light with me and drove all over the place. Adjusting, readjusting, I went from 8 degress all the way to 12 and everywhere in between. The builder told me to set it somewhere between 10-12. As it turns out, 11 degrees seems to be the best. The high idle problem has gone away. When I come to a stop it drops back down to 600 RPM like it should. Hurray!

As best I can tell the total advance is right around 36-38 degrees. The light gets a little shaky when I rev it past the the rpm point that the advance maxes out to. (Hope that makes sense) I was alone so I don't know the exact RPM it stops rising at, but I'm pretty sure it's in the 2500-2800 range by the sound. I'll check it again tomorrow with my wife watching the tach for me.

I still don't understand how or why after setting it set it to around 680 rpm in the driveway, it would idle around 1100 rpm when I had drove it a bit and came to stops. Anyway, getting it down to 600 rpm and 11 degrees initial timing seems to have done the trick. Hopefully it stays happy.

I may drop it down to 10 degrees just because the builder said I should NEVER go over 38 total.

Any final thoughts or explanations would be appreciated.

thx a ton everyone!

Dave J
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