Looking for some ideas on chasing down a frustrating intermittent engine skip or "stumble" that has developed in my car. It's a stock 1925 coupe. The problem occurs on battery and mag after the engine warms up. It's not a backfire or a pop, but rather like skipping a heart beat. Adjusting mixture and/or timing does not help. Engine is an older rebuild with 56 PSI per cylinder (warmed up). Car runs great, starts easily on battery or mag and has great power pulling hills.
Here is what I have eliminated:
Timer, coils, plugs and wires, harness, compression, switch, carb-mixture-fuel flow and intake/exhaust leaks.
Any ideas would be greatly appreciated.
My '12 does this occasionally, especially after running a long time with engine very warm, even to the point of being hot. Thought maybe the '20 Runabout was doing it the other day too, but didn't run long and hard enough to really tell. A friend suggested maybe a vapor lock..as the fuel line is awfully close to the exhaust pipe. Could be. Another thought I had was with all this ethanol they're cramming down our throats in our gasoline, and that ethanol attracts water, just maybe we're getting water droplets passing through the fuel. I'm going to start a steady dose of ethanol fuel treatment for a while to see if there's any difference.
Other than that, I'm at a loss.
Check throttle butterfly shaft for an air leak. Also check all mating surfaces for air leaks. With engine running at idle squirt some oil or gasoline at all connections to see if engine speed changes. Then correct the leak.
Pull coil box , check wood for carbon tracks to ground.
Timer really eliminated ? What type ?
Timothy... now that I decided to look at your profile, I guess a hearty "Welcome" is in order. As the veterans here say.. "welcome to the affliction". And as I've said in the past, Model T's are like Lays Potato chips...you can't have just one! You'll find lots of very valuable info from lots of guys here who know a whole lot more about this stuff than me, for sure. This is a great learning forum. And sometimes very entertaining.
Welcome to "the club", hope you get more than one T. I ended up with four!
Thanks for the quick responses!
Bob- I have been using a 1 year old Anderson style timer then tried an old one and finally put the new one on from Tony Wiltshire. These changes made no difference concerning the "stumble".
How do I spot a carbon track in the coil box?
Frank- I checked for leaks where the intake joins the head but never thought to check for leaks at the carb.
Very frustrating because the car used to idle so nice without missing a beat.
Can you hear it at idle? Or only at speed?
If at idle, see if you can determine which cylinder and then swap coils with another in the box and see if the skip moves. That elimiates coils if it doesn't.
Carbon tracking can be invisible. I had something similar on the '15 for years. Early morning start....fine. Mid-morning restart? Stumble and bumble. Late afternoon re-start? Fine and would run great all the way home! Just for grins and giggles, I decided to change the coil box to a FunProjects liner rebuild kit. Never happened again!
As I peeled the ply's on the old wood, there was a bit of luminescent green on the wood grain. It then dawned on me...I ALWAYS wash the car in the morning before taking it out! I may not have had a true carbon track...but let some moisture seep down the firewall at the hood former...and I guess sparks were jumping all over the inside of the wood plys.
Other than that....and if all else checks out...I'd look at a valve spring skipping a beat.
Remove the lower porcelains, look for dark, sometimes jagged lines in the wood that would be close to a metal part when coil is mounted on firewall.
George's response will help...George types faster
Also check the contacts of the ignition switch. I've had to make some adjustments to mine to eliminate a similar problem.
Timothy, In ten years of diagnosing/repairing/restoring T's Here are the top three things that I have found to cause the same issue:
1) It sure sounds like the timer is the most likely place for this to be coming from... I have also found this to be a problem in the Anderson style timers over the last few years. Some timers are okay, some are doing this right out of the box, some are starting to do this with 10-14 hours of run time. Just because you tried another timer does not mean it doesn't have the same issue. Find someone that has a T that runs flawlessly, and see if they will let you try their timer. But before you swap any parts, go for a ride in their T and see for yourself how it runs... The fact of the matter is that there is a great deal of variance in how well different hobbyists T's actually run. Even though most everyone considers their T to run great; One person may have an occasional intermittent miss and consider his car to run fantastic, yet another person may have the same issue and consider his car un-drivable. The T should never miss, or intermittently fire late. Having the front cover centered on the cam shaft is pretty important. If your front cover is not centered on the camshaft, then slight variances in timer quality that may not show up on other cars may be significantly compounded when installed on yours.
2) Second most likely thing to check is the coils. Even if your coils have been rebuilt, it is not safe to assume that they are not the issue. There are many folks rebuilding coils these days and every builders methods, quality control, setup procedure, parts, aptitude, experience is not the same (not to mention that the best quality parts that they are re-using are 85+ years old) and it is not safe to assume that anything "rebuilt" is a potential issue that you have eliminated.
3) Third thing to try is run the car at night in the dark and look for stray sparks around the coil wires, coil wire ends, coil box insulators to fire wall, spark plug porcelains, etc.
Sounds like you have done a lot of reasonable trouble shooting.
How did you eliminate the coils as a problem? Did you replace all four coils with some that you know are good?
Did you completely eliminate the switch as a problem by bypassing it completely? You can run a wire directly from the mag post to the coil box or from battery power to the coil box. If you run a wire from the battery be sure not to leave it on too long or you will melt down a coil.
Really if it was an intermittent ignition problem it would be likely that you would get a backfire when the spark started working after a miss. Maybe this would point toward a fuel/carburetor problem.
I had a similar problem with one of mine. I swapped coils with a car that ran good. I replaced the wood in the coil box. I replaced the spark plugs. I swapped carburetors with one that ran fine. I even replaced the cover on the New Day timer. None of these things fixed the problem. I checked the compression, it was fine. Finally I found the problem was the brush in the new day timer. It did not come out far enough to make a good contact with the cover. When the engine cooled off, it ran fine, but when warmed up, it started missing again. So each time I did something, I thought I had fixed it, and later it started happening again after it warmed up. I replaced the brush, and no more problems.
One thing for sure, your problem is always the last thing you check. Only harder problem to solve is when two things are wrong at the same time.
Dumb question, I know, but I see people doing it all the time.
Are you running your coil box with the lid removed? A perfectly good coil that is not held firmly in place can momentarily lose contact and cause a skip.
Adam's post about coils.... check the cushion spring travel.... If your coils were rebuilt about 5 years ago with the then current production coil points, I would suspect a wallowed cushion spring at the limiting rivit, thus increasing the travel of the cushion spring. Bad run of points.
If so, take your coils to someone who knows how to replace points and properly adjust the tensions to Ford specs.
Again, thanks for the responses.
I am using Ron Patterson coils from last year and I sent them to him last month and they checked out fine. I bypassed the switch and powered up the coil box directly with a battery but it did not solve the problem. Jim has a good point- there is no backfire and changing timers doesn't help so maybe it is not electrical but maybe fuel/carb. I am getting the proper fuel flow (fills a can and no problems under load) and have checked for leaks around the intake at the head and carb.
If this is just at idle I would look at possible worn camshaft lobes.
If you have not totally changed all four coils or at least one at a time, a coil could still be the cause of your problem. Even a properly set and adjusted coil can not be tested under all operating conditions. Maybe heat or the length of time operated could cause an apparently good coil to fail intermittently. The only way to totally eliminate the coils is to replace then and see if the same problem continues.
Every T owner should have at least one spare coil, just replace one at a time to see what happens. Or just borrow a whole set to see if it makes any difference