Hitchcocks Forum

Why do we have suppressors and resistors in spark plugs and plug caps? Just replaced a magneto in a qualcast mower which looked burnt out only to find a suppressed plug cap.Do old motorcycle magneto ignition systems have these caps? Also spark plugs can have resistors.The resistance on my enfield 535cc plug cap is more than the mower! Why kill the power/spark.

It's to prevent radio interference like you get with lightning. It doesn't affect the intensity of the spark to any great degree although some magneto systems may suffer with them fitted.

As Frank says. If you have experienced a static noise on a radio or the picture on your TV starts ghosting with wavy lines, it`s a sure bet that someone is driving past your house with no suppressors on the ignition system. A good way to antagonize the neighbours.

Thanks for your replies,i`m going to be a devil and fit a non-resisted / non-suppressed cap to see if i get a better spark and a nicer coloured spark plug.

I occasionally listen to the BBC World Service on AM but most of the world listens to FM bands and watches digital telly and I've never or heard seen spark interference on either of those media. I have no resistors on either the Bullet with its SR1 or the Ducati with its rather crude Ducati Electonica CDI ignition. Resistor leads and caps fry the primary coils in a mag and are definitely to be avoided. So what if some box watcher has to cope with a brief flicker in front of the latest reality show horror, they should feel lucky to have witnessed a phenomena of a bygone era.

Good answer simon,resistor leads! I`ve forgot about them,i`ll test the lead as well.Thanks.

I'm fairly sure that a suppressor cap is required if you have a Boyer electronic ignition. For anyone who has, check their website before changing your plug cap/lead to make sure.

Taken from the Q@A section of the Boyer blurb....



Why Do I Need To Use Suppressed Plug Caps?
MKIII and MKIV do not require suppressed plug caps for operation although we recommend using supressed 5000 ohm plug caps. Micro Digital and Micro Power units must be fitted with supressed plug caps. Plug caps fitted with suppression resistors are usually fitted to prevent radio interference. Radio interference (noise) can cause more complex electronic circuits like radios and computers to malfunction.




Our Micro Digital and Micro Power ignition systems contain a small computer operating at high speed that can be adversely affected by radio frequency interference. Symptoms can range from refusal to start to intermittent engine stalling symptoms. Use of non-suppressed plug caps has been known to cause permanent damage to the ignition unit.

The High Tension side of the coil develops a few thousand volts as the points close. It has to be high enough to jump the plug gap because air is quite a good insulator.
However, once the spark has started, the ionised air is a good conductor and the plug circuit includes a high resistance to limit the current and hence reduce the radio interference. So if you have a plug with built in resistor, you do not need a suppressor cap and vice versa. Magic ain't it.

Cheers TrevorCH

Hi guys-------- Sorry TREVORCH, the HT is generated as the points OPEN and the magnetic field that had just been in existence collapses around both the primary AND secondary coils, thus inducing the initial HT current AND a back emf that the capacitor multiplies many 1,000's of times to give that fat, blue spark that we need................ I'm sure that was just a "Typo" on your part ??