Need help in diagnosing #3 cylinder problems

[grinchyclaus]

My 1924 touring is having problems in performance. The #3 cylinder is not working correctly. Have replaced all plugs, wires, cleaned all contacts in coil box, rebuilt good coils. Good compression in all cylinders (40 - 44). Yet when you short out each cylinder, #1, #2, and #4 will "short out" the engine, but not at #3. All 4 will spark with a strong spark when shorting out - but #3 will not respond to "slowing" the engine down. Carburetor has been rebuilt, so gas flow is fine. Yet when running, you can tell only 3 cylinders are working. Any suggestions how to address this situation ?

[Dan Hatch]

Check for air leak on intake manifold.

[TXGOAT2]

Air leak, maybe a valve spring issue or a valve with too little clearance. Try moving the spark plug to another cylinder and see if the problem moves.

[varmint]

I had a problem on #2 last year, all rebuilt coils, and fixed it by swapping #2 with a spare coil.

[Mark Nunn]

Move #3 coil to a different cylinder. If the misfire follows the coil, that's your problem.

[Mike Silbert]

40-44 PSI compression is down slightly from stock but should run ok.
Problems can easily hide and be tricky to find sometimes.
On a Model T Ford the easiest diagnostic tool you have is to swap the coils locations and see if the problem moves.
There is no mention of looking at the timer to see what it's condition is.
The wiring down at the timer (or anywhere) can crack, rub through, or short out.
It is very difficult to see down there and there are a lot of things to short against even cotter pins or the pan bolt under the timer reversed.

The coil box is also very important and on a '24 Touring looking inside is a challenging task.
It needs to be in good condition and I recommend a waterproof kit on "driver" cars.

A late firing coil can make a cylinder a poor performer.

While a vacuum leak will cause major issues typically it effects 2 cylinders but should be checked anyway.
How is the mixture set? You might have a cylinder that is more sensitive to mixture than others.

Normally I would say a plug or plug wire is the issue but you said you swapped them.
What is the plug gap set to?
When checking the plugs by shorting them what is the gap where the sparks start and stop?
Or are you checking this without the plug hooked up?
Electricity takes the path of least resistance and you have the screwdriver gap and the plug gap here.
These should be basically the same external gap and color on all cylinders.

Moving parts between the cylinders or "borrowing" parts from a good running car can help locate the problem.
It only takes Proper Compression, Good Spark (at the right time), and the proper air fuel mixture to fire a cylinder.
The trick is getting them all right at the same time.

Keep us posted on how it goes.
Mike

[Charlie B in N.J.]

I'm going with a vacuum leak too. I believe this is the correct answer. Everything is there: compression, spark, etc. The only thing that seems to be wrong is if enough fuel is getting to the cylinder. Essentially it's just "along for the ride". In other words it's not contributing (or making power), so shorting it out has no effect.You've described changing / replacing parts and all seems OK. I'd spray around the intake manifold at that cylinder and look for a change in the engines running. If #3 suddenly kicks in you'll know.

[TXGOAT2]

Mike makes a good point about checking the timer and the associated wiring. Sometimes, what you know is right is what's wrong.

[Jerry VanOoteghem]

A late firing coil can make a cylinder a poor performer.


Keep us posted on how it goes.
Mike

Bob,

Check your timing at each cylinder, not just at #1.

Also, you state that you see a spark when shorting #3. A spark at atmospheric pressure does not guaranty that a spark is occurring in the cylinder, at 40/44 PSI pressure.

[speedytinc]

A late firing coil can make a cylinder a poor performer.


Keep us posted on how it goes.
Mike

Bob,

Check your timing at each cylinder, not just at #1.

Also, you state that you see a spark when shorting #3. A spark at atmospheric pressure does not guaranty that a spark is occurring in the cylinder, at 40/44 PSI pressure.

Correct on both points. Switch plugs. Confirm its the cylinder vs that plug.

[Ron Patterson]

It could be a vacuum leak.
I have a some questions.
Has the car recently been wet? Did you get caught in a rainstorm or was the car washed just before this problem was noticed.
If so did you try and have trouble starting the engine because it was not working properly on all four cylinders?
Did the coil box or inside of the firewall get wet?
Think carefully about the time you first noticed this problem and what was going on just before.
When the car is running can you touch any bare metal part of the car and feel a tingling shock?
You may think I am nuts, but if you have ever trouble shot a wet coilbox it is an experience you will not soon forget, becasue the symptoms do not act at all normal.
Ron Patterson

[got10carz]

What Ron said is true. I experienced the tingle of spark leaning over a fender of a car with a wet coil box. Engine heat (fixed) it after 10 miles.

[speedytinc]

Remove the plug & confirm the valves are functioning. There is the possibility of a broken spring or retainer pin.

[Jerry VanOoteghem]

Remove the plug & confirm the valves are functioning. There is the possibility of a broken spring or retainer pin.

Add to that, check the tappet clearance, especially if you have the self-locking adjustable tappets, (which don't self-lock sometimes).

[Henry K. Lee]

I chase a pixie like this that damn near put me in the looney bin...., do as Jerry said and my lil pixie was a hard seat that was installed very crappy and was loose dancing around.