Our Model K six cylinder has a "chronic" problem on the rear, number six cylinder of running too rich. All five other cylinder plugs look good with a gray/brown light ash color. Our other Model K touring engine did the same thing.
Trying to find different brands "hottest" plug has proven difficult, with several trips to different auto supply stores. Each time, the first question has been, What engine is this for?" Of course, we know the answer isn't going to help find the prescribed plug.
I tried Autolite #66, Champions (two different plugs) and NGK. Finally, I found this simple drawing and explanation online:
So, according to the pic above, a plug with less insulation and more exposure or depth (space between the threaded base of the plug and the electrode) means a hotter plug.
After discussing this with an old car friend in Minneapolis, he said "why not use an old Champion/Ford plug?" He suggested that old, original plugs have a bare electrode, so they may be much hotter than a modern plug. Furthermore, he reminded me that originally, our early cars didn't have oil rings, so fouling do to oil burn required very hot plugs.
With that, I dug out a few nasty old plugs taken from our 1924 Model T Tudor (now residing in Arkansas). These plugs were well worn, and I didn't clean them up, except to knock the dust off. Below is the result. The old Champion is on the right (now with the valve journal firmly clinging to the plug ). On the left are two "modern hot" plugs I tried, only running a few miles before becoming carbon covered. The "old Champion" is actually cleaning up, and looks better than when I put it in. The electrode is bare well into the base of the plug, so I bet the electrode is red hot:
The "old" plug is running cleaner, and the car doesn't miss. Before, I had to lean the carb out to bring #6 on line, then lean the car as much as possible to keep #6 firing.
Below is the second plug I'm running in #6 (dual ignition). It's a "modern plug," and showing more carbon deposit than it's running mate. Not a great pic, but the tip is barely clean, and it looks like this plug is headed toward fouling out:
If you are experiencing fouling problems, you want to try NGK Iridium plugs. They seem to stay cleaner in less than optimal chamber conditions.
It could have uneven heat on the intake manifold or head, causing #6 to run rich. It sounds like a built in problem. If you could transfer some heat to the intake manifold at #6 it might help. Does your engine run cold? Your water pump might be circulating to much water. It should run close to 200 deg. Just thinking....
Les torque when tightening down a spark plug will make it run hotter.
In extreme cases using two spark plug washers will help a lot, that is if it uses washers.
I would think a model A thermostat could benefit an old engine that has a water pump.
Using resister type spark plug wires should help the fowling problem some.
Thanks guys. The cylinders all run around 205 degrees (gun). The old plug seems to be doing well. I drove 30 miles with one stop late this afternoon, then pulled both #6 plugs. It looks like both are cleaning up a little. I think I'll stick with the old Champion for a while:
"The old Champion is on the right (now with the valve journal firmly clinging to the plug"
Wot's a valve journal? (Google didn't help.)
Ken, the Model K was made before Ford developed the removable cylinder head, so the cylinder castings had threaded caps over each valve to mount and service them through. The spark plugs were also threaded into these caps.
Many motorcycle engines kept this design until the mid 20's.
Ah so, thanks, didn't realize those cylinder access plugs were called journals.