Misfire mystery

[katy]

What happens if you increase the idle speed a little bit? And then a bit more?

[Fairlane514]

The higher the rpm, the less evident the misfire is. At cruising speed you dont notice it, or its no longer there

[Fairlane514]

Bypassed the ammeter, no change.

Have no resistance in the "power cable" to the condenser.

Top plate of distributor has good ground

6.40 volts at coil (engine off)

[Fairlane514]

Tried a new terminal box (old one was slightly suspicious) , no change

Rod Doe suggested a touring cam causing this......Havent been able to find much about the cam and the characteristics

[Fairlane514]

Any votes for a possible loose timing gear? would it be a random misfire? noisy?

[Fairlane514]

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Trying to upload a video but its too large apparently....23 seconds long.

Im not good with computers...

[Fairlane514]

Pulled my spark plugs, pretty black at this point.

Checked my wiring from beginning to end for continuity issues etc.

I have 6.40 volts at the power cable to condenser. Engine off, key on.

Started to look at the carb again... I get response from the idle mixture screw, not so much from the GAV (seems like nothing)

I noticed on this particular Zenith 1, I have a jet in the bore of the GAV that can be removed for cleaning etc.

[Big hammer]

A fellow posted on another place of his nice idling A after he timed it the Paul Shinn way and installing Renner’s flowed carburetor jets, it’s sweet to hear the exhaust note of a well tuned and running A tick tick tick tick

[Fairlane514]

Yeah, thats what Im trying to get......Im going to talk with Renners tomorrow.

[Fairlane514]

Received the Renners jet kit and rebuilt a third carburetor. No change in the misfire.

The carburetor is more sensitive to adjustment now with the Renner's jets.

Tried closing the spark plug gap to .030, thinking maybe a weak coil or ignition system having a hard time jumping the gap. No real change....

Its just so random, I cant say its a mechanical issues such as cam lobe etc.

My coil is an Oreilly Master Pro 6volt.

I did check my air gap at the distributor cap contacts and rotor. As the rotor first approaches I have just under .019, as the "blade" of the rotor passes to the end, I am over .025. Pretty consistent on all four.

Cap is new and I tested for carbon tracks or "bleed over" from one contact to another.

[Rob Doe]

"Checked my wiring from beginning to end for continuity issues etc."

Fairlane, I hear members of the local Model A club and occasionally others make the same comment. You might already know this, but just in case you or someone else doesn't:

Continuity is checked with an ohm meter. The ohm meter passes milliamps through the portion of a circuit bounded by the positions of the two test probes. Using an ohm meter, the circuit may test good when it is not capable of passing a legitimate load test. Milliamps are .000s (thousandths of one amp) In operation, the ignition on the Model A will draw ~ 4 amps, or ~ 4000 milliamps.

"Voltage drop" test methods are used to determine the quality of a circuit. You can find a series of voltage drop test procedures that will verify that your car's wiring is in good shape, marginal, or poor condition in Les Andrew's blue book, "Troubleshooting and Diagnosis", Chapter II.

[Fairlane514]

Rob, I agree with what you are saying, and that is something I have not done.

I was giving the benefit of the doubt to the new wiring, etc.

As random as it is, it seems a weak spark or ignition system may be more plausible.

Im going to focus more on that.

Thanks

[SHEC]

A thought - my coupe will run out fine and idle very smoothly when first started and a short drive. When warm the engine will sound like a slight misfire but no knock or backfire etc. The suspect in my case is carbon in the cylinder head - with clean plugs. Engine does not smoke and pulls strong. It was rebuilt in 1979. As noted it only does this when warmed up.
Just a thought- you have many qualified suggestions and good support. All the best with your search .

[Big hammer]

Originally Posted by Fairlane514

Trying to upload a video but its too large apparently....23 seconds long.

Im not good with computers...

I’am out of ideas, a shorter video ? sure might help

[Fairlane514]

I had the head off and cleaned the pistons etc of carbon.

My friend has an oscilloscope which Im going to attempt to use.

I also was going to try an epoxy filled coil in place of the oil filled one I have installed now.

[Fairlane514]

Here's today's find......

The only component of the ignition system I had not replaced was the small armored cable from the ignition switch to the condenser.

I had 6.4 volts at the ignition switch and to the brass contact at the end of the wire. So I wanted to attempt a voltage drop test. Results were nothing definitive.

As a last resort, I performed a Mr. Spock mind meld and asked the car what was wrong. No immediate answer....

I kept staring at the armored cable (mine is the small aftermarket one).....I started to wiggle it around and the next thing I know I got a smoke signal from deep in the tunnel under the dash ( I had the dash out ). Turned off the key and disconnected the battery.

So.....my theory, it was able to carry voltage with key on engine off, but had voltage drop while running, causing a weak spark.
Pulled the cable wire out and found a black burned looking area about and inch and a quarter long. There are places in the "conduit" that feel crispy or stiff.

I will attempt to download some pictures.

[Fairlane514]

https://www.fordbarn.com/forum/attac...1&d=1712536977

[Fairlane514]

First picture is the armor unwrapped to show the black cloth then white plastic insulation

Second picture is the "burned area" I found after removing the cable

Third is the armor unwrapped at the burn site.

[noboD]

Persistence pays off.

[Rob Doe]

X2 on congrats. Your post describes a failed ignition cable. I also like your theory. The post implies that the misfire condition, which you have been looking for, is now gone.

I'm hoping this is true????