The 140 has 123 distributor and today, for the first time, it would not run on all cylinders. I checked the ign wires at both ends. Nothing was loose.
Tomorrow I will remove the plugs and perhaps change them to see what happens.
Any ideas….?
Start the engine and short out each plug in turn to see which cylinder is not firing.
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It turned out to be the plug on No 4 cyl, which had a pale look, and part rust, on the little screw on top part. Very strange!
But this prevented the ampere to create a spark and function properly. No spark and a very ugly bottom end of the plug. Crested somewhat of a ”bore wash”, during the 10 miles that I drove it.
With a new plug all is back to normal. It’s a 140 roadster with a C-head and twin 2” sand cast carbs. Very powerful and the lack of one cyl was immediately obvious.
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Excellent. Simple fixes are the best fixes. 
I know that I’m late to this discussion, but is the connection between the clip on the ignition wire and the head of the spark plug tight? The plug looks like there could have been arcing which corroded the cap on the spark plug, eventually causing ignition on that plug to fail.