Poor Man's Distributor Test Bench - Long One

[grey beard]

Poor Man’s Distributor Test Bench

Many six cylinder flathead engines do not have proper working ignition advance mechanisms in the distributors. Here is a simple way our MoPar advance units can be tested and set without the use of a test bench. This method requires the purchase of a relatively inexpensive advance timing light and a little common sense. These are the steps I use to determine centrifugal and vacuum advance information for my own distributor:

If your vehicle is 6-volts, hook the 12-volt power leads from your timing light to either a 12-volt battery or to a 12-volt battery charger. If you already have a 12-volt system, just hook the light leads to your battery. Set the cylinder setting on your timing light to “6”.

Remove your distributor cap and rotor and connect the timing light to your coil and #1 spark plug leads. Select the “dwell” function on the timing light. With the key off and the engine cranking, read the digital dwell reading on the timing light. If the point dwell is not 38 to 40 degrees, reset the ignition points until you have this reading. If the reading is too low, adjust the points closer. If the reading is too high, open the points up a little. Get this dwell setting exactly where it should be before you proceed, because it will affect the timing readings you get later on. Dwell meters are many times more accurate than the best feeler gauge methods. When the dwell is set perfectly, put the rotor and cap back in place.

Climb underneath. Turn the engine over by hand with a breaker bar on the crankshaft, or have a helper bump the starter over gently with the key off, until you see the TDC timing mark on the pulley. Clean this area of the pulley very well. If it’s rusty or dirty, wire brush the area clean. Use either lacquer thinner or mineral spirits. Get it very clean, and then using a small brush, paint a white line over top of the TDC mark on the crank pulley. You will be using this mark for your reference point during the following tests. (You could also use a tire crayon, but the mark won’t last as long or be as easy to see.)

Unhook the vacuum line leading to the distributor vacuum advance chamber at the carburetor. Now start the engine and warm it up for twenty minutes or so. Make certain the choke plate is completely open and the engine idles at a comfortable hot idle speed. Set the initial timing with your timing light exactly on the TDC mark that you just painted or marked. Snug the distributor down carefully, without moving it. Double check the timing once more to make sure it is at the TDC mark.

Now switch your timing light to the tachometer mode and increase engine speed until it reaches 2850 rpm. Use your hand throttle while you watch the tachometer and set the engine as close to this speed as possible, so it will stay there.

Now switch your timing light to the timing function, and to the advance mode. With the engine running at 2850 rpm on the tachometer and the timing mark right on the TDC mark, read the digital advance readout on your timing light and record this number. This is the total centrifugal advance your distributor is producing. If your centrifugal advance is working properly, you should read 18 to 22 degrees at this speed. If your reading is less than this, you will need to do some work on the centrifugal advance weights and springs under the breaker plate in your distributor.

Without making any other changes, and with the engine still running at 2850 rpm, carefully reconnect the vacuum advance line at the carburetor. Shine your light on the crank pulley marks and once more advance the light until the TDC mark is aligned with the index mark on the pulley. Now read the digital readout on the timing light once again and write this number down. This is the total combined centrifugal and vacuum advance amount your distributor is serving to your engine when you drive the vehicle. Now slow your engine down to normal idle speed.

While the timing light is still connected, you might want to add 3 or 4 degrees of initial advance from TDC. When you set initial timing, always make certain the idle speed is low. If the speed is too fast, centrifugal advance may already be starting to come out and can give you a false reading.

If you have the correct total advance, this writer suggests you not set the initial timing any higher than these figures, or pre-ignition and detonation can occur. You may or may not be able to hear this when it happens. When it takes place, it can be very harmful to your engine. Engines will not tolerate more than 45 or 46 degrees total advance at rated rpm.

If you subtract the first (lower) number – centrifugal advance – from the larger number – total advance – you will find the total vacuum advance. If the vacuum advance reading is not within specifications, this can be changed by adding or subtracting washers from the end of the spring inside the vacuum advance unit. Disconnect the vacuum line at the distributor to get to this spring and the adjustment washers. The spring can also be changed if full vacuum advance is incorrect. If you find zero vacuum advance, your unit is likely leaking at the diaphragm and will need to be replaced.

I do not have the specs for every MoPar engine, but the L6 motors do not vary widely in this area of specifications. The 1953 Pilothouse Truck Shop Manual specs for half ton trucks are as follows:

Maximum spark advance

Centrifugal 18 to 22 crankshaft degrees at 2850 rpm

Vacuum 18 crankshaft degrees at 2850 rpm

Total Advance 36 to 42 crankshaft degrees at 2850 rpm

This method does not give you a detailed curve for either mechanical or vacuum advance. What it does give you is proof that these two systems are functioning as they should. If they’re out or whack, you will know how to address the problem when you are ready.

You can test for a bent distributor shaft or worn bushing by connecting your timing light to plug wire #6 and checking to make certain that the timing and dwell readings remain unchanged from the reading when hooked to #1 plug wire. They should be the same.

If the timing mark appears to bounce around at slow idle, it is likely that your timing chain has a lot of play, and the spark timing is erratic. This causes very poor engine performance.

Digital Advance Timing Light Sources

If you buy a new light, be sure to get a “digital advance” light. Not all lights have the dwell function, but I suggest you look for one that has this feature. I got mine at Pep Boys several years ago, and the prices have come down since then.

Autotoolexpress/timinglight.html

Actron CP 7528 $59.99

Amazon.com

Actron CP 7529 $73.12

Etoolcart.com

Equus INV 3568 $79.99

Autobarn.net

Equus EQU 3568 $89.00

[KJ's Dodge]

Greybeard,

Well done and thorough! I felt like I was back in my high school auto shop days.

KJ

[bkahler]

Dave,

Great right up! I appreciate you sharing your knowledge on the subject.

Who knows, I might be able to put it to use sometime later this year if all goes according to plan.

Thanks again!

Brad

[grey beard]

I have two distributors. Each is a different part number. Just out of curiosity, I pulled mine and stuck the second one in. With the intitial timing set right on TDC, the total advance at 2850 rpm was over FIFTY degrees - waaaay too much for the engine's safety. Had I run this distributor, and added a few degrees initial advance, as many of us are used to doing, I'd likely burn a hole in a piston dome from detonation in short order.

This is a good thing to check, and anyone can do it in a relatively short time. It is one of the most ignored parts of a tune-up, when done by hobbyists, because so few people understand how to check the advance.

When it's set correctly, it makes a world of difference in the way the engine performs and responds. Common sense says it will improve fuel economy, as well.

[Tom Skinner]

Dear Grey Beard,

Can you please clarify something in your disertation?

How can one crank the engine with the key off? How can one gently bump the engine around to TDC on the pulley with the key off?

I apologize in advance for my ignorance, however in my 1948 Chrysler 6 Cylinder C-38 Flat Head, with the key off I can't crank/bump anything.

I think you have a typo there and it should read with the key on.

I am only trying to understand and learn so I can aply this method to my car.

Thanks!

Tom Skinner

[bkahler]

Tom,

Pilot-house trucks use a foot starter. With the key in the off position push the pedal the the engine will turn over. Turn the key to the on position and push the pedal and hopefully the engine will start.

Brad

[Tom Skinner]

Dear B Kahler,

Thanks! my fault. I can check my dwell and do a similar test so this is still all good information.

Thanks Again!

Tom

[bkahler]

Just unplug your coil wire and use your key to turn the engine over and you're all set.

Brad

[Young Ed]

My plymouth with push button start will turn over but not start in the acc position. Try that on your chrysler

[DigmyP18]

I like this board more all the time. I'm going through my distributor as we speak because the vacuum advance is leaking. I have a couple of suggestions to augment your excellent instructions:

I'm sure it goes without saying but be sure to plug the vacuum line from the intake once you disconnect it from the distributor. Some newbies may not think of this.

Aso, since the L6 motors share a lot of parts. Does anyone have a favorite igniton parts supplier? I'm going to need a new vacuum advance for my IAP Distributor on my P18 and would welcome a recommendation of a trusted source. Thanks for the great post.

[Don Coatney]

I have a spare starter push button and I have attached wire leads with alligator clips. I attach thes clips to the starter solonoid and can spin the engine from under the hood with the ignition switch in the off position.

Terrell Machine in Texas can rebuild your vacuum advance unit. Not cheap but they do a good job. There contact information can be found in the links section of the main web page supporting the P-15, D-24 forum.

[grey beard]

Mr. Digmy,

Plugging the vacuum line is not really critical on our L6 engines because the source is venturi vacuum - vacuum taken above the throttle plates - and not manifold vacuum, like most GM vehicles. Wouldn't hurt to plug it, but it will not affect the process or the outcome in any way.

Don, you are correct. Terrell rebuilds vacuum pots, but both Roberts and Bernbum offer replacements at about $10 a pop less. Don't know why. However, your magical remote starter button works only on cars. Our trucks still use the old fashioned foot push rod - a mess sometimes. It is always in the way of the throttle and brake, and with my big feet I'm always stompin' on the wrong thing. Some day I'll find a car starter and a relay, and swap them out. Then I can use a remote button, too. Shahazammmmm . . .

Now I've checked two distributors using this method. Both were cranking out way too much vacuum advance. Mebby someone's had the end cap off and lost a few shims. Fortunately, flat washers work here, and they are relatively easy to adjust using them. I was surprised to find this. Thought there would be less than specs, not more. Goes to show you how important this check can be to your vehicle's performance.

[DigmyP18]

That's an interesting point that I hadn't thought of Grey. Thanks for the clarification.

Don, looking into Terrell machine now. thanks

[Jim Yergin]

I notice that Harbor Freight sells a timing light with advance for $29.99. Has anyone tried it?

Jim Yergin