Weak Spark on No 1 Plug
 

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?

Re: Weak Spark on No 1 Plug
 

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

Bob

Re: Weak Spark on No 1 Plug
 

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

Re: Weak Spark on No 1 Plug
 

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.

Re: Weak Spark on No 1 Plug
 

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...

Re: Weak Spark on No 1 Plug
 

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.

Re: Weak Spark on No 1 Plug
 

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.

Re: Weak Spark on No 1 Plug
 

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

Re: Weak Spark on No 1 Plug
 

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?

Re: Weak Spark on No 1 Plug
 

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

Re: Weak Spark on No 1 Plug
 

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

Re: Weak Spark on No 1 Plug
 

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.

Re: Weak Spark on No 1 Plug
 

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?

Re: Weak Spark on No 1 Plug
 

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

Tom Endy

Re: Weak Spark on No 1 Plug
 

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??

Re: Weak Spark on No 1 Plug
 

Could there be a crack in the distributor cap body?

Re: Weak Spark on No 1 Plug
 

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

Re: Weak Spark on No 1 Plug
 

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

John

Re: Weak Spark on No 1 Plug
 

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

Re: Weak Spark on No 1 Plug
 

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.