Weak Spark on No 1 Plug

[Greg Jones]

Well this is a new condition for me. My engine is running a tad rougher than I like, so was shorting out each spark plug as part of my check out. Cylinders 2, 3. and 4 were shorted out against the cylinder head and showed a healthy, snappy blue spark. Cylinder #1 showed a somewhat wimpy (and apparently intermittent) much weaker spark. So I thought, must be a defective distributor body and switched with another. Same results. Switched with another, with same results. The distributor is a rebuilt from Berts about 5,000 miles ago but kept oiled. Using standard points set to .018" on a Snyders 6:1 head using 3X Champions with gap set at 0.030". Using the Stipe machined distributor cam, lubed lightly. I also tried two other plugs (one new 3X and one used, same weak spark). The issue seems to be the distributor, however, not the plug itself.

Then I thought maybe plug is not properly grounding. There was some paint on the cylinder head so cleaned it off. Seemed to improve a tad but still not the same strong spark when I short out the plug as cylinders 2, 3, and 4. About the only thing I can think that would cause this is if indeed the bushings were worn such that when the distributor shaft comes around to cylinder #1 the point gap closes up (or I suppose it could expand too, if worn, widening the gap). Checking the bushings for wear, I am not visually seeing any but of course maybe a couple thou wear is hard to discern. Anyway, anybody run into this sort of thing, i.e., one plug receiving a wimpy spark when all the others were good and strong?

[Bob C]

I'd switch the plugs around and see if it's still #1.

Bob

[1955cj5]

Is the rotor to contact gap inside the cap consistent on all four?

[Greg Jones]

Yes I checked that carefully and it is 0.020" But of course that is cranking it over by hand. I am thinking maybe the bushings might be worn (although I sure can't tell it) such that when the shaft comes around to #1 it throws the point gap off. When the engine is running there is centrifugal force at play that is not present when the engine is not running (obviously). Need to pull out my spare rebuilt distributor and give that a try. Normally I am not just a "parts changer" but the Model A ignition is fairly easy to work on.

[1955cj5]

Good opportunity to test that spare distributor!

Let us know what you learn.....

For what its worth, I replaced my old dist with a rebuilt from a fellow on ebay and it does make a difference....small but noticeable....the old dist is now the spare...

[George Miller]

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Maybe you have low compression on number 1 cylinder. I would take the plug wire off one and try shorting it again to see if you have a strong spark that way.

[Ed]

Make sure the distributor cam is not flat on one lobe. I just dealt with this same problem on my CCPU. I found one lobe was flat. The points were barely opening the worn lobe.

[Tom Wesenberg]

This is where a clear cap comes in handy, so you can watch the spark gaps.

[SeaSlugs]

Originally Posted by Tom Wesenberg

This is where a clear cap comes in handy, so you can watch the spark gaps.

couldnt you make a "clear" cap but cutting off the ends of a standard cap and just leaving a strip across just enough to latch it down? or drill 4 big 1/2" holes in the cap directly over the contacts in the body to see them?

[denniskliesen]

Check your points gap at all 4 positions. I was talking to a club member yesterday that had a bent shaft.

[Tom Wesenberg]

Originally Posted by SeaSlugs

couldnt you make a "clear" cap but cutting off the ends of a standard cap and just leaving a strip across just enough to latch it down? or drill 4 big 1/2" holes in the cap directly over the contacts in the body to see them?

I've seen guys do that, but I made a few clear caps out of thick Lexan.

[CarlG]

Originally Posted by Tom Wesenberg

This is where a clear cap comes in handy, so you can watch the spark gaps.

Originally Posted by SeaSlugs

couldn't you make a "clear" cap but cutting off the ends of a standard cap and just leaving a strip across just enough to latch it down? ...

Several years ago I bought one of these "strip caps" made by NuRex.

I have seen the clear caps advertised by one of the vendors, Mike's if I'm not mistaken.

[Mike V. Florida]

Originally Posted by denniskliesen

Check your points gap at all 4 positions. I was talking to a club member yesterday that had a bent shaft.

That is what I was thinking. He could also rotate the cam one lobe and see if it follows the lobe or stays with the number one position.

How is the internal rotor gap to the body gap on all 4 positions?

[Tom Endy]

I would try a different distributor upper section. There may be a burnout of one of the imbedded wires.

Tom Endy

[Mitch//pa]

To see the integrity of the spark to each plug i use a spark tester. Many times ill hook up four of them..
When shorting out a plug i take a wire with two alligator clips attached, connect one end to the screwdriver shaft and the other to a good ground such as a head nut. Then all u need to do is touch the plug wire to check for a good cylinder contribution drop. This method also eliminates the spark backing up into the cap creating havoc with another cylinder.

I use the same method to find leaky ignition wires on a more modern car. Especially when its a wet or damp misfire condition. Spray the wires down with a squirt bottle of water, then run the screwdriver shaft around with the wire attached as mentioned above . Any spark leak fron the wires will jump to the shaft


Im wondering if it actually is a weak spark??

[jm29henry]

Could there be a crack in the distributor cap body?

[Mitch//pa]

Originally Posted by jm29henry

Could there be a crack in the distributor cap body?

Stated in post #1. ,,"""switched with another"""

[aermotor]

Originally Posted by Mitch//pa

Stated in post #1. ,,"""switched with another"""

Mitch, You forgot the ""defective distributor body and "". To me this doesn't mean the cap was changed - am I missing something?

John

[holdover]

bought one of these great inexpensive tools years ago, works on everything that has a spark to test spark intensity via the length of the spark and at the price you can't beat it. To see how strong your spark is you increase the distance with the adjustment button on the top. Over the years mine has diagnosed defective wires, coils, coil packs on modern vehicles etc. No tool box should be without it, and NO I am not the one selling it.

http://www.ebay.com/itm/Adjustable-I...gAAOSwzJ5XXnxw

[BILL WILLIAMSON]

Greg,
Do you actually SEE a weak spark or just a weak reaction, when you ground out #1? "Might" be a manifold vacuum leak?
Shorting out in the distributor body always effects #3 & #4.
Study the ignition on modelabasics.com & you'll see WHY. That's a GREAT WEBSITE!
Bill W.

[Tom Wesenberg]

Originally Posted by aermotor

Mitch, You forgot the ""defective distributor body and "". To me this doesn't mean the cap was changed - am I missing something?

John

Not sure I understand this statement, but he did state that he tried another distributor top, although it might have been defective also.

I was thinking he could have a tight gap on #1 plug, and this would show a small spark when using the screwdriver short test, but he changed the plugs also.

If he was close by, I could do a scope check to eliminate the ignition.

[aermotor]

[QUOTE=Tom Wesenberg;1438662]Not sure I understand this statement, but he did state that he tried another distributor top, although it might have been defective also.

Oh well, I guess it is just me that I can't find in the OP where the distributor CAP or top - what ever it's called was replaced. I'm a newbie and just try to troubleshoot in my own KISS way.

John

[Greg Jones]

I should have been clearer in my description. I replaced the distributor top as Tom says and not the distributor body. I am intrigued by George's comment on compression (and in a pm from Tbird). In theory compression should be ok but will run the test to be sure.

[Mitch//pa]

Yep a proper diagnosis needs to be performed so u dont go chasing ya tail
Keep us posted

[Tom Wesenberg]

Originally Posted by Greg Jones

I should have been clearer in my description. I replaced the distributor top as Tom says and not the distributor body. I am intrigued by George's comment on compression (and in a pm from Tbird). In theory compression should be ok but will run the test to be sure.

"So I thought, must be a defective distributor body and switched with another. Same results. Switched with another, with same results."

This can be confusing, as Ford called the bakelite part the "BODY". A lot of us think of the body as the cast iron main part of the distributor.

The body can short out internally, and it's usually between #3 and #4. You say you changed it and same results, so that's why a scope check might help now. Otherwise, as Bill said, it could be an intake leak. Just open an unlit propane torch and move it around the intake runners to see if the speed picks up.

It could also be a slow to close valve, and a vacuum gauge can show this.

[Mitch//pa]

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Yes the terminology on a model A can be confusing...

Distributor cap is where the coil wire goes into.
Distributor body is where the plug straps go onto


Correct?

[cpf240]

I'm a bit confused here... the OP said the problem was a weak spark when shorting out the plug when compared to the others. I don't see how compression, or intake, or anything else other than the electrical portion of the ignition system or the operation of the distributor would affect the 'strength' of the spark.

What am I missing?

[JDupuis]

I have a distributor body (wires go on to) that is shorted at #1 and #2. Anything is possible. Jeff

[Bob C]

You got me thinking Mitch so I had to look it up in the Ford parts book.
Body a-12105, Cap A-12115, Distributor Base A-12130.

Bob

[Bob C]

Originally Posted by cpf240

I'm a bit confused here... the OP said the problem was a weak spark when shorting out the plug when compared to the others. I don't see how compression, or intake, or anything else other than the electrical portion of the ignition system or the operation of the distributor would affect the 'strength' of the spark.

What am I missing?

The higher the compression the higher the voltage necessary to fire the plug.

Bob

[Mitch//pa]

Originally Posted by Bob C

You got me thinking Mitch so I had to look it up in the Ford parts book.
Body a-12105, Cap A-12115, Distributor Base A-12130.

Bob

Thanks Bob

[cpf240]

Originally Posted by Bob C

The higher the compression the higher the voltage necessary to fire the plug.

Bob

But he was shorting it out to observe the spark... the spark itself doesn't care about the compression.

[George Miller]

Originally Posted by cpf240

I'm a bit confused here... the OP said the problem was a weak spark when shorting out the plug when compared to the others. I don't see how compression, or intake, or anything else other than the electrical portion of the ignition system or the operation of the distributor would affect the 'strength' of the spark.

What am I missing?

with low compression the spark would jump the plug easy, so there would not be much left for the screw driver test. That is why I said take the plug wire off.

[cpf240]

Originally Posted by George Miller

with low compression the spark would jump the plug easy, so there would not be much left for the screw driver test. That is why I said take the plug wire off.

Ok, I had assumed that the angry pixies would take the easiest path, ie the short, rather than also try to jump the plug gap at the same time.

[George Miller]

Originally Posted by cpf240

Ok, I had assumed that the angry pixies would take the easiest path, ie the short, rather than also try to jump the plug gap at the same time.

Well it would, but we do not know what the easiest path was. How far did the spark have to jump with the screw driver? The plug was probley around .032 if the screw driver was more than that it will jump the plug.

[Mitch//pa]

Quote BobC ''The higher the compression the higher the voltage necessary to fire the plug"""

Bob can u elaborate
Are we talking flameout?

[Greg Jones]

I am going to try George's recommendation after work tonite if time permits. This work stuff gets in the way of Model A mechanicing.

[wensum]

I had an all new distributor from Snyders and always set the points gap on number one whilst checking the timing.
The engine ran poorly until I discovered that the points gaps on the other three were way out,
Either the cam had been ground off centre or miss-shapen
A new Stipe cam solved the problem and made a big difference.
Keith

[George Miller]

Originally Posted by Mitch//pa

Quote BobC ''The higher the compression the higher the voltage necessary to fire the plug"""

Bob can u elaborate
Are we talking flameout?

Yes that is the way it works.

[Mitch//pa]

Originally Posted by George Miller

Yes that is the way it works.

Do you think we have to worry about flame out with the compression ratios of the A?

[Mitch//pa]

When using a spark tester in line to a good plug what would flame out do to the spark intensity when looking at the tester?