During our trip to Cypress Hills the engine in our T pickup began to lose power. The sound it made was reminiscent but not as intense as the sound the engine made with we lost a valve keeper a few years ago. To diagnose the problem I shorted out the coils, three at a time, and found number three cylinder pretty much dead. I then checked the number 3 plug wire connections and discovered that the contact on top of the plug was loose. The porcelain was also loose. When I pulled the plug it was missing the center electrode. I assume the electrode can now be found in the muffler.
Fortunately I had two backup plugs under the seat.
Champion X spark plugs are popular among T enthusiasts because they are about the right heat range and they are less likely to foul than modern style plugs. At least that is the prevalent thinking. I have used them with much success. Though I have used Champion 25s with good success I am inclined toward Champion Xs. I fancy that they are easier to start and run smoother. On the other hand my modern car seems to run smoother after I wash it or change the oil.
Note the shape of the grounding electrode in the illustration at the left and compare it with the A-25. Murray Fahnestock in “The Model T Ford Owner” points out that with this design (X) oil will tend to accumulate on the elbow of the electrode rather than in the gap between the electrodes (A-25).
The plug in the number 1 cylinder is most likely to foul because that cylinder is the natural depository for non vaporized fuel which runs along the front edge of the intake manifold and into the 1-2 intake port.
I set the gap at .030 and that seems to work well. A finer gap is more easily fouled and will put out a less powerful spark. A weak coil may not be able to push the spark across a wider gap in the cylinder’s high compression environment. Also, the greater the distance the spark has to travel the more likely it is to find an easier route, either arcing in the coil or through the coil box wood. Once that happens the coil or wood must be replaced.
Tighten the plugs just tight enough so that they don’t leak. It is best not to tighten them when the head is hot because it can contract and make the plug difficult to remove. For example, when I screwed the relatively cool plug from under the seat into the relatively hot head, I was careful not to over tighten it. If a plug is very tight in the head, run the engine for a while and when the head heats up the plug may be easier to extract.
A little anti-seize on the threads will reduce the chance of the plug and head corroding together. Especially when using an aluminum head.