Ignition

[Smardo]

520 w/j13.  Starts ok, idles ok, then when driving sputters, stops.  Pull distributor cap, put back on, runs like a dream.  Any ideas?  D

[datzenmike]

Is it still running like a dream? 

 

Cause may be bad, damp or just old wires. Idle doesn't require much spark but when engine under load it's harder and the spark will look for an easier path to ground. Spark was jumping across to another wire.Taking the cap off moved them slightly out of reach. 

 

Spark will always look for the easiest path to ground. Could be another wire where the cylinder isn't being fired, could be across the dirty outside of the spark plug porcelain, could be across the plastic cap between nipples. Could even be the rotor inside or from dampness. Every effort should be made to make the easiest path to ground the spark plug by keeping the wires clean and checking their condition for damage or cracking with age. Keep the cap also clean and dry inside and out. Look for fine crack like marks caused by repeated arcing called 'carbon tracking'. This is where the spark has jumped along the surface before and it's burned a path that's easier to jump again. Look for this also on the outer porcelain of the spark plugs and the plastic of the coil tower and rotor. 

 

 

Small crack or perhaps carbon track where the spark would escape and arc to the ground terminal.

 

 

There are other causes that can explain backfiring through the carburetor like running lean when the engine is cold. Taking the cap off may have been coincidence.

[Smardo]

New coil, new plugs, new distributor.  Haven't replaced wires.  Will try.  Very insightful and educated response and much appreciated at my end.  Thanks.  D