Missteps and misdiagnoses chasing a misfire

[Flivver]

I was following a thread here about rough running, sputtering, and backfires. Then it happened to me... Here is my tale. Hopefully someone will benefit from my missteps. (here is a Youtube video I made of the situation https://youtu.be/3zKL5BXE95w Or you can read below.)

My car was running well, and has been trouble-free through the summer. I’ve made a few driving videos, gone to some car shows: I’ve just been enjoying the car.

On my way home from a local car show a few days ago, the car started running very roughly. Sputtering and backfiring. Much worse when accelerating or climbing hills. So this car was out of commission until I got it repaired…

My first thought was that I was running out of gas. So I added a few liters…no improvement. My next thought was dirt in the fuel, or rust, from my old gas tank. Maybe the screen in the bowl was plugged-up. So I took the plug out of the bottom, and found that it was very clean – no sediment. Maybe there is rust or something plugging the fuel line? Not likely, since the screen was clear, but I disconnected the fuel line here, and found that fuel flowed un-restricted. So that wasn’t the problem.

Wishing I had a spare, working Carburetor (which I don’t) I convinced myself it must be an ignition problem, rather than a fuel problem. And I did NOT want to consider that it might be valve problems, I did a complete valve job on this car in the spring! So I used a screwdriver to ground to verify that I had strong spark at the top of each Spark Plug. There was; all four coils seemed to be working fine.

Now the good mechanics among you, (I do not count myself among your number) will notice that THIS is where I missed the opportunity to discover the true problem. REMEMBER THIS MOMENT, I’ll come back to it later.

Still not wanting to take the Carburetor apart, I continued my ignition trouble-shooting. I took the Timer off, and found it to be a bit oily inside. So I cleaned it all up and put it back on. The car was still running poorly. Next I swapped a good coil into each coil slot, one by one, and ran the car after each swap. No improvements.

Convinced that the problem was not ignition, I returned to the Carburetor. There must be some debris blocking a jet or passage… So I took the Carburetor off, and took it all apart. Pulled the float (looked good – not sinking) Pulled the float valve and seat, pulled the needle valve and main jet. All clean with no signs of trouble. Cleaned-out the passages, and put it all back together and re-installed it on the car. Started the car, and it still ran badly.

Reluctantly I considered valve problems. I REALLY didn’t want it to be valve problems. So I removed the spark plugs and checked the compression in each cylinder. It was fine. Relief – I guess; no valve problems. But I still hadn’t found the problem!

So I gave up, and went inside, and read some Model T Ford Club of America forum posts about similar problems. They listed all the things I had tried already. But I noticed one post, where the author described shorting each spark plug, one at a time, to see if the engine stuttered. If it DID NOT STUTTER, then that cylinder is the problem. I hadn’t tried that yet.

So next time I was in the garage, I started it up, and proceeded to short the spark plugs one by one. Sure enough, Cylinder 1 was dead. There was lots of spark at the top of the plug, but the cylinder wasn’t firing. Could it be as simple as a dead spark plug? Yep. That’s all it was. The spark plug, somehow, failed. It will not spark at all. It’s wet and black, but shows no other signs of trouble. I replaced it with another old plug I had that was working, and voila, misfire cured.

I took the car for a cold test drive (it was only 4 degrees) and pondered my mis-steps. When I was checking for spark at each cylinder, if ONLY I had also shorted each cylinder in turn, I could have solved this simple problem much sooner, with much less effort. But at least my Carburetor is all cleaned and inspected now, and my Timer is serviced. Live and learn.

[ThreePedalTapDancer]

I’ve found that to be a common culprit, along with coils that are out of adjustment. Most don’t realize that pulling a snug coil out of a coilbox by the point apparatus can make a coil misfire by changing adjustment setting.

[Scott_Conger]

Cylinder # 1 runs the coolest; a wet/black plug and 4C has all the earmarks of needing a hot air pipe and the resultant ability to lean the car out perhaps by as much as 1/2 turn...result: nice grey colored plugs in all cylinders for the rest of winter.

Been there; done that.

[TXGOAT2]

It's also a good idea to avoid very short trips any time, and especially in cold weather. Avoid needless idling. Avoid starting the car unless you are going to drive it at least several miles, preferably with some open road driving. If your car is equipped with a water pump, it needs to have a thermostat installed. The thermostat ought to be modified to allow some limited bypass flow to permit more uniform warm-up and to allow any trapped air to escape the system when the thermostat is closed. I'd use a 180 F thermostat with a 3/16" to 1/8" hole drilled in the valve plate to allow some bypass.

[TXGOAT2]

A heat pipe arrangement is very advantageous under cool, damp operating conditions.

[speedytinc]

I had a similar problem with a customer's T. I shorted the plugs & found the bad cylinder. All signs pointed to the spark plug as in your situation. I pulled the offending plug & checked it for spark resting on the head. Good spark. Thats a head scratcher. Intermittent??

I happened to have acquired a champion plug tester a few weeks earlier. Upon testing I found that the plug would not spark under pressure.
But, by changing the gap from .026" to .024", The plug sparked consistently. How's that for odd?

[TXGOAT2]

Not so odd, really.

[George House]

Good sequence of events diagnostic tale Steven. I actually enjoyed reading it Glad you made correct determination.

[John kuehn]

Shorting out a plug to diagnose a problem is probably one of the oldest tricks when working on older gas engines. Old tractors, old cars and anything that has more than 1 cylinder is a candidate for this simple test.
Model T’s are probably the best known old car that’s been used one since 1909. No kidding!

No strobe testers, amp probes, and etc. are required. Just your screwdriver.

[Dan Hatch]

Did not read all threads so you may have looked into this.
Is head gasket on correct? If not will make Front cly run cold. There is a front and back to head gasket. If all ready discussed, sorry for repeating. Dan

[Jim, Sr.]

If my Model T is missing, sputtering, and does not run right, I do it a little differently.
I take off the coil box cover. Start the engine and set the throttle to a fast idle. Then, hold the points down on 3 of the coils so that it has only 1 cylinder firing. It will run like a 1 cylinder "hit and miss" engine. Then, do the same to the other 3 cylinders. You will find out real quickly which cylinder or cylinders are weak and most likely the problem. If it will not run on that 1 cylinder, and dies, I will pull out that coil and try a known good coil and see if it improves. Often, that is the problem. If that did not fix it, I will check the wires, spark plug, coil box contacts and timer.
If you did not find a problem in the ignition system, you have identified the weak cylinder and can check for low compression, intake manifold leaks, and other possible causes. If the problem is with the carburetor or the fuel system, it will not run good on any cylinder.

[Flivver]

Good suggestion - would have saved me a lot of time! Thanks.

If my Model T is missing, sputtering, and does not run right, I do it a little differently.
I take off the coil box cover. Start the engine and set the throttle to a fast idle. Then, hold the points down on 3 of the coils so that it has only 1 cylinder firing. It will run like a 1 cylinder "hit and miss" engine. Then, do the same to the other 3 cylinders. You will find out real quickly which cylinder or cylinders are weak and most likely the problem. If it will not run on that 1 cylinder, and dies, I will pull out that coil and try a known good coil and see if it improves. Often, that is the problem. If that did not fix it, I will check the wires, spark plug, coil box contacts and timer.
If you did not find a problem in the ignition system, you have identified the weak cylinder and can check for low compression, intake manifold leaks, and other possible causes. If the problem is with the carburetor or the fuel system, it will not run good on any cylinder.

[Moxie26]

Steven.. your method to find your problem in my opinion is a lot better than what was just stated. It rules out faulty coils and narrows the problem to a non-working spark plug. Thanks again for your video.

[Norman Kling]

That was a good diagnosis. Too bad it was not the first test you made. It can, however, be deceiving, sometimes they will run at idle or level, but will misfire when pulling, because the compression is higher when the throttle is open, which it is on a pull. So that one is even a bit harder to find.
Norm