Inconsistent spark plug firing; can't get all 4 to fire

[Todojo]

Hi, this is my first post here, as I wasn't able to find a thread on point.

I'm trying to resurrect our 1931 deluxe roadster bequeathed to me by my great grandpa. Recently while doing another test drive around the neighborhood it sputtered and lost power. We finally found that not all of the points were firing after installing a translucent distributor cap. What would cause two or three sparks to fire while the remaining don't? We moved the spark plugs around so it's definitely not the spark plugs. It seems to be something in the distributor because if we apply pressure to one side of the distributor pushing down it ignites more than one contact point but we can't get it on all four. Temporarily, to produce the same pressure, I pushed a small wedge of wood under one side of the distributor which helps it consistently have three sparks but the number two spark plug still won't fire. I even took apart the distributor looking for any issue but couldn't find anything. I have never taken the distributor off the head, however. We have a modern condenser installed.

I'm also curious how the spring end of the armored ignition cable is supposed to make contact with the lower plate of the distributor. I noticed that spring isn't very "springy" and found mention of it needing to make good contact but not too much otherwise it will ground out. I don't understand what that means or if it relates to my issue.

Any thoughts would be greatly appreciated. Thanks!

[Bigsnapper43]

Have you started the engine after dark? You may have a christmas tree. I have seen number three plug being shorted out many times. Get another distributor cap. I personally dont like the new caps!!! They are usually not round. The old ones were replaced when they were still very usable.

[Rob Doe]

The pathways inside the stock Model A, upper distributor body can fail, causing a misfire between them. 3 and 4 are common, but any can do so. Looking at the engine bay in a darkened garage, you might see the spark advance rod shorting #3.

Are the same 1 or 2 plugs misfiring randomly, or are all 4 misfiring randomly?

You might look for movement of the upper plate at its center. Push and pull on the spark advance lever horizontally. Slop in the center hole of the upper plate can move the points to and fro causing troubles.

Remove the rotor. Put your index finger and thumb on the distributor cam and attempt to move the distributor shaft sideways to check the bearings. There shouldn't be any slack if in good shape.

Run some fine, 400, 320 sandpaper through the points and then pass some clean white paper through them until no black streaks result.

[Rob Doe]

If you have the hand engine crank, crank the engine with key off, plug wires removed and turned 180 away from the distributor, trans in neutral. Check the gap using a gap gauge, between the rotor tip and each electrode of the upper distributor body. They should all be .025 thousandths. Also, remove the upper body and examine the inside wall all the way around for cracks or carbon tracking.

Check the bottom of the high tension lead at the coil and distributor cap for carbon tracks also.

[nkaminar]

If the ignition system is marginal, it may not fire all spark plugs. Check to see if the high tension wire from the coil to the cap is pushed all the way in at both ends. Clean the points as described above. Check or replace the condensor. Check all electrical connections to make sure they are tight. Measure the voltage at the coil while the engine is running which should be at least 6.5 volts (or 13 volts if a 12 volt system).

If the armored cable is screwed in too far the outer part will short out the distributor. The outer part is grounded. It does not have to be screwed in too far, just enough to make good contact with the center conductor pushed by the stiff spring.

The gap between the rotor and the contacts inside the distributor housing is specified at 0.025 inch but a smaller gap will provide a hotter spark. If you can move the distributor housing around with the wooden pusher then it is too loose and some contacts will have less gap than others. Bend the end of the rotor to change the gap but make sure that the rotor cannot crash into the contacts inside the housing if the housing moves.

[CT Jack]

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It sounds like you have a cracked distributor cap that is preventing the spark coming from the coil to reach the spark plug. Using a multimeter you should be able to verify there's electrical continuity between each of the 4 internal cap points and the external spark plug connections. The electrical connection between these 2 points is buried in the molded cap. If this connection is broken the spark from the coil will never reach the spark plug. If the cap has a fine crack that ends near the distrbutor base the spark from the coil will take the path of least resistance causing it to ground out to the metal base.
The simple solution is - install a new cap or another cap.

[old31]

x2 with Rob & NK!

[nkaminar]

CT is also correct. Try a new or known good distributor housing (cap).

[Todojo]

You all are wonderful; thank you so much! I'll give it all a try and report back.

[BRENT in 10-uh-C]

Originally Posted by Todojo

You all are wonderful; thank you so much! I'll give it all a try and report back.

My vote is going to be a little different. Based on past experiences, when the distributor bushings are excessively worn, -or when the base plate is and/or distributor housing is worn, it does not keep the points in alignment with the centerline of the distributor camshaft. When this happens, you will have erratic point dwell and point gap settings which can cause symptoms just as you have explained.

As Jack and others mentioned above, first use a Volt/Ohm meter to check continuity however remember that even if you find one terminal not making continuity, it should still run reasonably well because with a good coil and saturation time, the spark will jump the broken gap to complete the circuit. To check the distributor itself, first remove the rotor and then see if you can move the distributor cam fore & aft in all directions. Also check to see if the distributor base can be moved fore & aft. This is not to be confused with rotating the plate, but instead I am speaking about any other movement. If so, this needs to be corrected.


Also, seeing as this is your second post, Welcome to Fordbarn!!

[ModelA29]

Originally Posted by BRENT in 10-uh-C

My vote is going to be a little different. Based on past experiences, when the distributor bushings are excessively worn, -or when the base plate is and/or distributor housing is worn, it does not keep the points in alignment with the centerline of the distributor camshaft. When this happens, you will have erratic point dwell and point gap settings which can cause symptoms just as you have explained.

+1 - If it's random this is the answer.

[The Master Cylinder]

Originally Posted by BRENT in 10-uh-C

My vote is going to be a little different. Based on past experiences, when the distributor bushings are excessively worn, -or when the base plate is and/or distributor housing is worn, it does not keep the points in alignment with the centerline of the distributor camshaft. When this happens, you will have erratic point dwell and point gap settings which can cause symptoms just as you have explained.

Isn't that about the same thing Rob Doe wrote back in post #3.

Originally Posted by Rob Doe

You might look for movement of the upper plate at its center. Push and pull on the spark advance lever horizontally. Slop in the center hole of the upper plate can move the points to and fro causing troubles.

Remove the rotor. Put your index finger and thumb on the distributor cam and attempt to move the distributor shaft sideways to check the bearings. There shouldn't be any slack if in good shape.
.

I was going to suggest the same, but Rob beat me to it.

[rotorwrench]

The breaker points open and close the full induction coil circuit. The condenser is in that circuit to aid in the induction process and prevent arcing when the points open. It has to have good capacity for charge with slow leak down rate or the coil won't induct a high tension spark well at all.

I first use a spark check tool such as the Nu-Rex spark polarity tester or their regular spark test tool. The high tension lead from the coil is disconnected from the distributor top cap and the tester is plugged in between. A person can then make certain that is has a spark from the coil each time the breaker opens. If it does then the problem would be cap, rotor, spark connetor strap, or spark plug related. If the coil spark is intermittent then it would be in the induction coil circuit which would include the coil and all the little parts of the distributor breaker assembly as others have mentioned. The way that Ford wired the electrolock switch up to the distributor and coil is complicated when compared to the more modern points ignition systems. Any one of those parts can be suspect with intermittent operation. Loose distributor shaft bushings can allow the points cam to wobble as was mentioned. Erratic point opening and closing may be the problem. The modern type points use a modern type condenser and condensers are not what they used to be. Many are bad right out of the package.

Many of the electrolock "pop out" switches can have intermittent problems as well as the modern replacement switches that folks replace those with. This would likely be more erratic though and not affect just one or two specific spark plugs. Even the modern wireless lower distributor plate can develop problems so nothing in there can be discounted until all is fully tested and confirmed. The old lower plates with the wire can short out eventually due to movement of the spark control if the wire rubs something in there.

[BRENT in 10-uh-C]

Originally Posted by BRENT in 10-uh-C

My vote is going to be a little different. Based on past experiences, when the distributor bushings are excessively worn, -or when the base plate is and/or distributor housing is worn, it does not keep the points in alignment with the centerline of the distributor camshaft. When this happens, you will have erratic point dwell and point gap settings which can cause symptoms just as you have explained.

Originally Posted by The Master Cylinder

Isn't that about the same thing Rob Doe wrote back in post #3.

Yeah, but Rob & my thoughts are somewhat different than others thoughts. Which side of the fence do you want to sit on??





.

[Rob Doe]

Todojo,

Having just read down through this thread again, I seem to remember a unique post by a senior member that described a problem with a dist. rotor that was causing a miss or cross firing problem. (long term memory)

Also, check the continuity and tightness of the rivets of the insulators on the plates. The rivets should not have continuity to their respective plates. If loose enough they may be vibrating and intermittently grounding. I was shown an example of this on a failed dist. recently. It was a good catch by a senior club member.

Good luck

[Chris in Paradise]

"If the armored cable is screwed in too far the outer part will short out the distributor. The outer part is grounded. It does not have to be screwed in too far, just enough to make good contact with the center conductor pushed by the stiff spring." Great bit of advice! Following a gas tank replacement, I had no voltage at the points, even though each individual part checked out good with a volt-ohmmeter. A friend told me about this issue, I backed the armored cable out of the distributor about 3 threads, and she starts and runs fine!

[Todojo]

I appreciate the added comments and insights.

Here's what I've found and done so far:

- Checked gap between distributor rotor tip and points:
Found it was way over .025. Even bending it I couldn't get it within that range. The points on the inside of the distributor body appeared to be worn and sanded short over the years. But, as share below, that doesn't appear to be the primary cause. I didn't see any cracks in the distributor body but ordered a replacement just in case.

- Checked for any play in the top of distributor shaft:
No movement right/left or forward/backward; it is quite firm.

- Installed new distributor body:
The gap between rotor tip and distributor points was then at the .025 range. But, it did not fix the problem. The issue is clearly points 2 and 4 are not consistently firing, and most pronounced is point #2. Point 2 never fires.

- Checked distributor parts:
I removed the distributor upper and lower plates to check for any issues. Tightened it all back down and retimed the distributor. No change.

Based on the comments about the distributor bushings, I decided to inspect what I could and remove the entire distributor/shaft. Due to manifold heater, I have to take entire exhaust manifold off to access the securing bolt below the distributor for removal. Luckily, the distributor lifted out without much effort, but it was very corroded. I've attached a picture of when first pulled out and then after cleaning it up as much as I could. Also added a picture of the armored cable tip.

After putting it all back together, the distributor body was more firm (due to tightening the securing bolt more firmly). I was able to start the engine but number 2 point never fired. Also, when I put any load on the engine--second gear or mild incline--the engine faulters.

The modern condenser is new as of about 3 months ago. If it is faulty, would it exhibit this type of problem? Consistently firing #s 1 and 3, sporadic 4, and never 2?

I'll keep digging, but I haven't found the culprit or fix yet.

[Rob Doe]

Put a drop of oil on the points post where they pivot open and close? Crank the engine with the starter, key off, rotor off, and make sure the points aren't hanging up. The distributor bushings look dry. Maybe the points post is dry too and the points are sticking open???

Your spring loaded tip on the cable is crushed and likely not operating as it should.

With the modern upper plate, you can run a jumper from the coil directly to the points. You could also hook a test light to the switch side of the coil and clamp the light to a ground. When cranking the engine, the light should go off when the points cross the flats (points closed) on the cam and come on when the points open at the 4 high lobes of the cam.

A close up picture of your dist. points from directly above would be nice.

[1stWoody]

Check the distributor cam to see if all the lobes are consistent.
Here's a video that uses a dial gauge to check the cam and point gap accurately.

https://www.youtube.com/watch?v=y2R-ljyf9eg

[nkaminar]

Try a different coil. There is something unique with #2. Some added resistance somewhere or a path to ground. If the coil is weak it may not be able to overcome whatever problem is with #2. Also check to see that the high tension wire leading from the coil to the distributor top is inserted all the way at both ends. Replace the wire if in doubt.

Because you said that you moved the plugs around in your original post then it is unlikely a problem with a plug.