A newly rebuilt coil- one of 4 in a set - runs good at first. After a few miles a miss develops. All normal "swapping" tests are performed to determine that the coil is defective. Replace coil with spare and the car runs fine.
Next morning, re-installed the defective coil- runs good. After a few miles the same scenario. Miss develops, installed spare and the car runs fine.
In a freshly rebuilt coil box- what would cause this intermittent firing?
By coil box, you mean the box that holds all 4 coils, correct, not the coil unit? If you rebuilt the box, and replaced all the metal including the spring contacts, I'd suggest checking the tightness of the connections. If you didn't replace those then make sure there is no corrosion on the contacts of the box and coil.
A break in one of the windings. The wire is close enough when cold, but when warmed up the gap becomes too great and it causes them to misfire.
Chris- the problem is with one of the 4 individual wooden coils that sit within the metal coil box which incidently been rebuilt with the Fun Projects "1926-1927 Coil Box Rebuild Kit" which has plastic parts.
Bad coil. Throw it out and ask the rebuilder to rebuild you another.
It is important to remember that these coils are 90+ years old. An experienced rebuilder knows to you look for the things that can cause trouble. But, if you rebuild enough coils invariably something will elude you.
I rebuild about 1000 coils per year and get 2-3 back each year for latent problems like the one you described.
I simply rebuild another and replace it free.
Ron the Coilman
Contact the person who rebuilt your coils. I expect he'll take care of the problem.
Does the coil have plastic parts, like the case, or does the rebuild kit have plastic parts included?
Coils normally require the capacitor be replaced and the pints normally need more adjusting that just installation will provide.
I bought 5 new coils, with recycled center coil parts, from Fun Projects and the StroboSpark Coil tester to insure they would function as advertised.
They all passed inspection!
Just to set the record straight, this is not a Ron Patterson rebuilt coil, it was "rebuilt" by an individual w/a hand-cranked coil tester.
Your statement, "Throw it out", does this mean "dumpster" or is it repairable? If so, what should my friend look for?
No dumpster. I think most of the rebuilders pay a few bucks for rebuildable coils.
Sorry, I should have provided more detail.
Not to worry, I would have provided this same advice had I rebuilt the coils.
Some faults are repairable:
Have the rebuilder check for intermittent cold/failed solder joints at the bolt heads inside the case. This is the most common failure I see on returned rebuilt coils.
Closely check the solder connections on the center of the three box contacts. If something looks amiss here simply re-flow solder on the center contact hole.
Check the two piece contacts on the case, these commonly have corrosion between the two parts. Again this can be repaired by again re-flowing solder on the entire contact. These two faults are the second most prevalent fault I see.
If the problem is, as Andy Loso suggested, pitch it out to get it out of circulation. No coil rebuilder want it except for usable top hardware. Secondary winding faults or inter windings faults cannot be economically repaired.
The primary winding itself rarely fails.
Remember they made 75 Million Model T coils and find another for rebuilding is easy.
Ron the Coilman
You are a good man, Ron Patterson! Thanks for the advice.
I had a stack of 29 coils, all of which had failed the simple continuity test. Most of them had simple fixes like Ron P described. (A wonderful and very helpful fellow, by the way)
Any good coil re-builder could miss one once in awhile. Several of mine passed the simple continuity test later after I had written "open" (in pencil) on the side of the coil. Most, I was able to determine which contact had gone intermittent.
Drive carefully, and enjoy, W2