Royal Enfield Owners Club,  Humberside Area
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A simple spark plug and HT system tester.

Most of us test for a spark by earthing the plug on the cylinder head and turning the engine over.
This doesn't tell you much, as the HT voltage needs to be about twice as much to produce a spark under compression so a spark in air may not mean a spark in the engine.
It also doesn't tell you if it's the plug or the ignition system at fault if there's no spark (or you just didn't earth it properly)
With this spark tester I can attach the plug lead to one end and earth the croc clip. If all is well there should be a spark, even when the gap is adjusted to it's widest. A good spark shows that the HT system is probably OK.
If the croc clip is then clipped to the plug and the gap on the tester adjusted to around 25 thou there should be a spark across the gap if the plug is OK.
The HT system should easily be able to jump two gaps.
It also can be used with the engine running to test for situations when the engine stops just after starting - just watch to see if the spark stops.
It's not foolproof I know but will tell me more than the usual method.
I know there are lots of these testers about but I'd like to think this is more robust than most and I just like making things.
Body is turned from a 18mm piece of delrin, contacts are 8mm brass rod threaded into the delrin. Plug cap connection has a removable terminal to cater for both cap types. A bit of shrink sleeving covers the rod to help prevent HT shocks.