Vanishing spark.

[davidwheeler]

My VIIIth has coil ignition, always worked well.  Just recently there has been difficulty in starting.
I have a set of neon spark indicators that go in series and demonstrate presence or absence of spark.    Coil to earth, plug lead to earth, spark plug to earth via jump lead all show a big fat spark with in the latter case the plug itself sparking nicely.   As soon as I put the (new) plug into the head however, the spark practically disappears  even if I still earth the plug body with a jump lead.   This defies the laws of physics.    I have earthed the head direct to the chassis, no difference.   I have four equal fat sparks from the distributor, advanced or retarded.  There is no leakage from plug leads to earth nor between plug leads according to my test meter.    I am baffled!

[Jaydub]

Hi David, if you mean the spark is weaker installed in the cylinder head when you are cranking it over. it could be due to voltage drop. I have encountered this a few times, lay the plugs on the head, crank, reasonable spark, put plugs in the head, no start! You might want to check battery voltage whilst cranking the engine, you need at least 9 volts on a 12 volt battery. Also check the voltage at the coil whilst cranking. A voltage drop at either of those 2 points can cause a starting problem. If you haven`t got a good low tension voltage at the coil. you wont have a good secondary output.
I hope that may help.

John

[davidwheeler]

Went into battle again this morning.   Jury rigged a different coil with a separate battery,  vroom.   Installed said coil,  vroom.    Bench tested original coil with an adjustable spark gap in the circuit and found that the neon sparker is absurdly optimistic, giving a bright flash with less than 1 Kv.    Original coil now firmly in the bin!     The increase in pressure (good compression!) is sufficient to increase the resistance across the plug gap sufficient to drop the voltage below the neon bright spark level.   Modern technology is not always helpful.

[Mikenoangelo]

That's good!

Going back to the connection issue, in the 1920's Rolls Royce used a separate positive feed wire to the switchbox from the battery rather than simply connecting to the positive terminal on the starter. The aim was to reduce voltage drop while the starter was operating. I was a bit sceptical about this but proved to myself that it did indeed allow an extra volt to get through.

Mike