Setting Timing on a old school points distributor w vac advance.
07-19-2008, 08:45 AM
#13
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Join Date: Oct 2005
Location: Ohio USA
Posts: 4,616
RE: Setting Timing on a old school points distributor w vac advance.
How to Set Ignition Point Gap
By Glenn Goodspeed
July, 1998
[hr]
Setting the ignition contact point gap may eventually become a lost art, since the points are replaced in modern cars by electronic ignition. Even some owners of classic cars find it more practical to replace the points with an aftermarket electronic system.
I'm sure the procedure for setting points may be found in many workshop manuals, but having done this job countless times in the twenty-odd years I have been fooling with cars, I thought I might be able to offer a worthwhile perspective and maybe some tips you won't find in every workshop manual.
How Points Work
The ignition contact points serve as a trigger for the ignition coil. A small current flows through the points momentarily when they close, and this is what tells the spark coil exactly when to release the jolt of electricity that powers each spark plug.
To see how the points work, turn off the engine, remove the distributor cap and pull off the rotor. (The rotor is keyed, so you will be able to replace it correctly by making sure the key fits into the slot in the
distributor shaft.) Don't remove the plug wires from the distributor cap. If you get them hooked up wrong, the engine won't work right.
Now you're looking inside the distributor, and you can see the points. One of them is attached to the baseplate and the other is attached to the end of an arm. Midway down the arm is a small brown block that rubs against the central distributor shaft. At the base of the arm is a pivot and some sort of spring, usually just a curved strip of metal.
Notice that the distributor shaft has humps on it where the rubbing block rides. The humps are called lobes, and all the lobes together are called a cam. As the shaft turns, the arm is pushed in and out by the spring and the cam, and thus the points are closed and opened.
If you think about the relationship between the points and the cam, you can see that placing the stationary contact point closer to the arm will make the points stay closed longer when the rubbing block is between lobes. How long the points are closed is measured by how far the shaft turns while they are closed. The measurement is called dwell.
Evaluation and Cleaning
If you're working on your ignition points, chances are it's because you're having trouble with them. When points get old, they often become burned, pitted, pimply or just plain ugly. In many cases, you can clean them up and use them for a few thousand more miles. With one or two cleanings, a good set of points should last at least 5,000 miles, and possibly as many as 8,000.
If you don't want to spend the time doing this and you'd rather buy a new set, the cost is not prohibitive. If you purchase new points, I recommend sticking with the stock brand, which is Bosch.
Cleaning the points is simply a matter of filing them down until they're shiny. The best way to do this is to remove them from the distributor and put them in a vice if it's possible to do this without damage. The arm on the point set used in my distributor is too delicate to grip in a vice, so I put my best flat file on the workbench and rub the point on it until it's clean.
Small pits in the surface of the points are not cause for concern. Craters comprising more than thirty percent of a point's surface are a good reason to buy new points. Pimples can be filed off nicely. Large craters, pimples and burned, blackened surfaces indicate that the points have been set too close together. When properly set, used points will have gray surfaces with small pimples and no soot.
After initial filing to evaluate and restore the surface, it's a good idea to use a fine file to smooth the surface. If you're very fastidious and you want to make them as shiny as new, you can follow the fine file with 600-grit sandpaper and then crocus cloth.
While you have the points out, it's a good idea to wipe all dust, dirt and grease out of the distributor and cap with a clean cloth, paying special attention to the shaft and cam. Before re-fitting the points, apply a very thin smear of clean distributor cam grease to the cam. This special grease may be purchased at some auto supply stores or through the J.C. Whitney catalog.
Setting the Point Gap
Getting the best gap between the points is essential for engine performance and reliability. Set the points too wide and the spark plugs don't get enough juice your engine whimpers. Set them too close and the engine works fine for a few miles. Then it stops because the points are so badly burned they can't perform their function.
The Haynes manual recommends a point gap of .016-.020 inches for engines used in the P1800. I used to set my points at exactly .018", but they didn't last long, and once I had to stop by the side of the road and file and reset them to get the engine running. Now I set them at .019". All engines are different, so you have to look for signs that your points are out of adjustment before you decide where your best setting is.
To measure the point gap, you need a set of feeler gauges. These are flat steel blades marked with their various thicknesses in inches and/or millimeters. A set
of feeler gauges usually folds into a handle like a pocketknife, and one or more blades may be unfolded at a time.
Adjusting the point gap is a simple process, but it takes some patience to get the hang of it. First, make sure the rubbing block is on the high point of one of the cam lobes. If it isn't, you will have to turn the engine a little bit in order to turn the cam.
Some shop manuals recommend using the starter to do this, a frustratingly imprecise method. Instead, I recommend turning the crankshaft by fitting a socket wrench to the crankshaft pulley bolt. Put the transmission in neutral and turn clockwise to avoid loosening the bolt. If you can't see the cam and rubbing block while you are doing this, have someone else watch them and tell you when they are properly lined up.
There is another method, and it is the one I use, but it works only on cars with manual transmission, and must be done with the car on a flat, level surface. Put the car in fourth gear, release the handbrake, and with your eye on the distributor cam, manually push the car forward or back a few inches until the rubbing block is on top of a lobe. Don't roll over your feet!
Once you have the rubbing block on top of a lobe, you can measure the point gap. Although it is possible to set the point gap using one gauge, it is much easier and often more accurate to use two. Let's say you want to set the point gap at .019". Unfold the .018" and the .020" gauges. When you can insert the .018" gauge between the points without touching both of them, the gap is wide enough. To make sure it is not too wide, remove the .018" gauge and insert the .020" gauge. If the wider gauge can only be inserted touching both points, you have the correct gap. This is called the "go-no-go" method. One gauge goes through, the other doesn't.
How do you adjust the gap? Loosen the screw that holds the stationary point bracket to the base plate. Not completely just enough so that you can move the bracket by inserting a screwdriver tip and twisting it. Different distributors have different places to insert the screwdriver tip for this purpose, even on P1800s, so I can't tell exactly where this will be. Look at the stationary point and how it is held to the base plate, and you will find a way to do this.
Adjustment is a matter of trial and error. Move the stationary point out a bit if it was too close, tighten the holding screw (not too tight), and measure the gap. If it still isn't right, try again. Most distributor baseplates have a pattern stamped into them to make it easier to judge how far you are moving the stationary point during adjustment. You also might find that the bracket moves a little when you tighten the screw, and you have to take this into consideration, too.
When the point gap is close to correct, it is difficult to tell whether the gauges are touching both points or not. When inserting a gauge between the points, make sure it is lined up straight by moving it to and fro a little. To be sure the smaller gauge does not touch both points at once, listen closely while you move the blade from one point to the other. If there is space on either side of the gauge, you will be able to hear a tiny click as the blade hits either point when you move it.
The larger gauge, of course, should not produce this sound. You will still be able to insert it between the points, because the spring pressure will allow it, but you will be able to feel the friction produced by the points against the blade.
Many mechanics like to check the dwell measurement with an electronic meter after setting the points. If you set the point gap properly, this is not necessary, but given the nature of the task, it might be a good idea for the inexperienced to check their work with a dwell meter. However, you should remember that the ideal dwell, like the point gap itself, varies from one engine to another.
By Glenn Goodspeed
July, 1998
[hr]
Setting the ignition contact point gap may eventually become a lost art, since the points are replaced in modern cars by electronic ignition. Even some owners of classic cars find it more practical to replace the points with an aftermarket electronic system.
I'm sure the procedure for setting points may be found in many workshop manuals, but having done this job countless times in the twenty-odd years I have been fooling with cars, I thought I might be able to offer a worthwhile perspective and maybe some tips you won't find in every workshop manual.
How Points Work
The ignition contact points serve as a trigger for the ignition coil. A small current flows through the points momentarily when they close, and this is what tells the spark coil exactly when to release the jolt of electricity that powers each spark plug.
To see how the points work, turn off the engine, remove the distributor cap and pull off the rotor. (The rotor is keyed, so you will be able to replace it correctly by making sure the key fits into the slot in the
distributor shaft.) Don't remove the plug wires from the distributor cap. If you get them hooked up wrong, the engine won't work right.Now you're looking inside the distributor, and you can see the points. One of them is attached to the baseplate and the other is attached to the end of an arm. Midway down the arm is a small brown block that rubs against the central distributor shaft. At the base of the arm is a pivot and some sort of spring, usually just a curved strip of metal.
Notice that the distributor shaft has humps on it where the rubbing block rides. The humps are called lobes, and all the lobes together are called a cam. As the shaft turns, the arm is pushed in and out by the spring and the cam, and thus the points are closed and opened.
If you think about the relationship between the points and the cam, you can see that placing the stationary contact point closer to the arm will make the points stay closed longer when the rubbing block is between lobes. How long the points are closed is measured by how far the shaft turns while they are closed. The measurement is called dwell.
Evaluation and Cleaning
If you're working on your ignition points, chances are it's because you're having trouble with them. When points get old, they often become burned, pitted, pimply or just plain ugly. In many cases, you can clean them up and use them for a few thousand more miles. With one or two cleanings, a good set of points should last at least 5,000 miles, and possibly as many as 8,000.
If you don't want to spend the time doing this and you'd rather buy a new set, the cost is not prohibitive. If you purchase new points, I recommend sticking with the stock brand, which is Bosch.
Cleaning the points is simply a matter of filing them down until they're shiny. The best way to do this is to remove them from the distributor and put them in a vice if it's possible to do this without damage. The arm on the point set used in my distributor is too delicate to grip in a vice, so I put my best flat file on the workbench and rub the point on it until it's clean.
Small pits in the surface of the points are not cause for concern. Craters comprising more than thirty percent of a point's surface are a good reason to buy new points. Pimples can be filed off nicely. Large craters, pimples and burned, blackened surfaces indicate that the points have been set too close together. When properly set, used points will have gray surfaces with small pimples and no soot.
After initial filing to evaluate and restore the surface, it's a good idea to use a fine file to smooth the surface. If you're very fastidious and you want to make them as shiny as new, you can follow the fine file with 600-grit sandpaper and then crocus cloth.
While you have the points out, it's a good idea to wipe all dust, dirt and grease out of the distributor and cap with a clean cloth, paying special attention to the shaft and cam. Before re-fitting the points, apply a very thin smear of clean distributor cam grease to the cam. This special grease may be purchased at some auto supply stores or through the J.C. Whitney catalog.
Setting the Point Gap
Getting the best gap between the points is essential for engine performance and reliability. Set the points too wide and the spark plugs don't get enough juice your engine whimpers. Set them too close and the engine works fine for a few miles. Then it stops because the points are so badly burned they can't perform their function.
The Haynes manual recommends a point gap of .016-.020 inches for engines used in the P1800. I used to set my points at exactly .018", but they didn't last long, and once I had to stop by the side of the road and file and reset them to get the engine running. Now I set them at .019". All engines are different, so you have to look for signs that your points are out of adjustment before you decide where your best setting is.
To measure the point gap, you need a set of feeler gauges. These are flat steel blades marked with their various thicknesses in inches and/or millimeters. A set
of feeler gauges usually folds into a handle like a pocketknife, and one or more blades may be unfolded at a time.Adjusting the point gap is a simple process, but it takes some patience to get the hang of it. First, make sure the rubbing block is on the high point of one of the cam lobes. If it isn't, you will have to turn the engine a little bit in order to turn the cam.
Some shop manuals recommend using the starter to do this, a frustratingly imprecise method. Instead, I recommend turning the crankshaft by fitting a socket wrench to the crankshaft pulley bolt. Put the transmission in neutral and turn clockwise to avoid loosening the bolt. If you can't see the cam and rubbing block while you are doing this, have someone else watch them and tell you when they are properly lined up.
There is another method, and it is the one I use, but it works only on cars with manual transmission, and must be done with the car on a flat, level surface. Put the car in fourth gear, release the handbrake, and with your eye on the distributor cam, manually push the car forward or back a few inches until the rubbing block is on top of a lobe. Don't roll over your feet!
Once you have the rubbing block on top of a lobe, you can measure the point gap. Although it is possible to set the point gap using one gauge, it is much easier and often more accurate to use two. Let's say you want to set the point gap at .019". Unfold the .018" and the .020" gauges. When you can insert the .018" gauge between the points without touching both of them, the gap is wide enough. To make sure it is not too wide, remove the .018" gauge and insert the .020" gauge. If the wider gauge can only be inserted touching both points, you have the correct gap. This is called the "go-no-go" method. One gauge goes through, the other doesn't.
How do you adjust the gap? Loosen the screw that holds the stationary point bracket to the base plate. Not completely just enough so that you can move the bracket by inserting a screwdriver tip and twisting it. Different distributors have different places to insert the screwdriver tip for this purpose, even on P1800s, so I can't tell exactly where this will be. Look at the stationary point and how it is held to the base plate, and you will find a way to do this.
Adjustment is a matter of trial and error. Move the stationary point out a bit if it was too close, tighten the holding screw (not too tight), and measure the gap. If it still isn't right, try again. Most distributor baseplates have a pattern stamped into them to make it easier to judge how far you are moving the stationary point during adjustment. You also might find that the bracket moves a little when you tighten the screw, and you have to take this into consideration, too.
When the point gap is close to correct, it is difficult to tell whether the gauges are touching both points or not. When inserting a gauge between the points, make sure it is lined up straight by moving it to and fro a little. To be sure the smaller gauge does not touch both points at once, listen closely while you move the blade from one point to the other. If there is space on either side of the gauge, you will be able to hear a tiny click as the blade hits either point when you move it.
The larger gauge, of course, should not produce this sound. You will still be able to insert it between the points, because the spring pressure will allow it, but you will be able to feel the friction produced by the points against the blade.
Many mechanics like to check the dwell measurement with an electronic meter after setting the points. If you set the point gap properly, this is not necessary, but given the nature of the task, it might be a good idea for the inexperienced to check their work with a dwell meter. However, you should remember that the ideal dwell, like the point gap itself, varies from one engine to another.
07-19-2008, 09:32 AM
#14
2nd Gear Member
Join Date: Jan 2007
Posts: 308
RE: Setting Timing on a old school points distributor w vac advance.
Man that brings back memories! I had a 72 Chevelle when I was 18 and it was squeaking from inside the distributor cap. I had know idea what it was, a friend of mine told me to use somedi-electric grease on the dist. cam, who knew. Dualpoint system was thebomb back then.
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