Forum

[Mark M]

I thought the resistor in spark plugs was to control, by suppression, RF (Radio Frequency) interference with other devices. You used to actually be able to hear this on old Long Wave radios as a regular "click" as an unsuppressed vehicle went by and also see it on UHF televisions as a white line that appeared across the picture. But thinking about it, spark plug caps were then fitted with a resistor (known as a suppressor, you'll often see old caps with the word Suppressor on them,) to fix the problem so why would you need one in the plug as well? A look on the NGK website gives the answer, apparently modern vehicles fitted with electronic engine management systems can suffer from interference too, and presumably being much closer to the source of the "noise" need the extra suppression.

REgards, Mark

[Dennis C]

Yes, resistors very confusing, three types as far as I know, resistor plugs, resistor plug caps and resistor leads, with magneto ignition use non of the former, with the coil ignition Enfields use one but only one.

[Leon Novello]

I would check the high-tension lead, it might be a resistor type which has a carbon core that can be broken by ham-fisted bending creating gaps along its length. These leads should be handled with great care and should never be bent. When you are fitting the new plugs you might be disturbing the carbon, temporarily connecting the breaks in the lead.
Otherwise, I would suggest an Iridium plug ; it won`t make any difference to the bike`s performance apart from making starting easier. The main advantage is they will last a lot longer, 60000kms in a car, so you could expect to fit and forget one in your bike for
maybe,20000kms or more, so justifying the cost.
They are all resistor type, so be sure you don`t have a resistor cap or high-tension lead as well, as others have said, too much resistance will cause problems. Only one resistor is needed and that is in the plug.

[Presto]

Certain [‘R’ type plugs in NGK codes] are resistor type plugs. The resistor is a 5k ohm ceramic element, used to suppress ignition ‘noise’. Resistor plugs reduce electromagnetic interference and are recommended for use with on-board electronics – e.g. electronic ignition systems. The use of a non-resistor plug can, in certain applications, but by no means in every case, produce an erratic idle, high-rpm misfire, engine run-on, power drop-off at certain rpm levels and abnormal combustion.



The use of resistor type caps with resistor type plugs is not recommended. The combination of two resistors (one integral to the plug and one in the plug-cap) may reduce spark efficiency, especially in older systems, and thus significantly affect fuel efficiency. It is claimed that the combination of two resistors may reduce fuel efficiency by as much as 10%. However, the use of a resistor cap with a resistor plug on engines that use electronic engine control may not lead to these problems, as the plug cap may reduce unwanted electronic noise that could interfere with electronically regulated engines.



I have a hunch that many of these plug ‘failures’ are due to the wrong heat grade being used. The ideal operating temperature of the central electrode is between 400° and 800°c. If the plug tip temperature rises too high, say above 950°c, the tip becomes incandescent. The glowing plug may then ignite the mixture before the spark occurs. This effect is referred to as ‘pre-ignition’. Where pre-ignition occurs the fuel charge does not burn progressively and the combination of the extreme heat, typical of pre-ignition, and the shock wave from the uncontrolled explosion of the fuel charge may result in serve damage to the piston crown.



Plugs are designed to run at high temperatures. Failure to reach these temperatures often results in the plug ‘oiling-up’. A plug tip temperature below the optimum, below 400°c, will not allow the plug to self-clean: to burn off carbon and fuel deposits. Allowing an engine to idle for long periods may also result in plug oiling, for the level of heat generated, though comparatively high, will not be high in relation to the plug’s ideal working temperature.



Classic bikes are frequently ‘nursed’, often run at only very moderate speeds, and often over relatively short distances. This may prevent the plug from reaching its optimum temperature and is one common reason why plugs ‘die’ after only low mileage. This is not, as is often supposed, a fault of the plug. In these circumstances the heat range originally recommended by the manufacturer may be too ‘hard’ (or ‘cold’), and thus the recommended plug, being subjected to less heat than the manufactures envisaged, is unable to reach a self-cleaning temperature.

[Nettshubby]

"Not too oily" there shouldn't be any oil on it, there's your problem!

[hagis]

What colour is the spark plug when removed from the engine, Plugs come out of engines with either grey electrodes or light brown or black electrodes, if it is light brown then your engine is running correctly, if grey then its lean to much air, or black sooty electrodes its running rich to much petrol, an engine with grey or black plugs is running at the wrong temperature,which of course makes the plug run at the wrong temperature leading to premature plug failure, there have been a lot of threads regarding failing plugs and I wonder if the cause is elsewhere and not the plug.

[jefrs]

Resistors, suppressors whether in the plug, the HT cap or the HT lead, whilst they do suppress radio interference are actually there to place a load on the coil (or magneto) to compress the spark i.e so we get a nice fat juicy spark at the right time.



Plug fouling - if the plug is fouling and dying too soon try using a slightly hotter plug for better self-cleaning. In NGK terms from an "8" to a "9". Much depends on how you use the bike, for example if you thrash it a lot you may want to go colder. A plug dying can often be it being unable to burn its electrodes clean enough. Part of the problem is unleaded fuel, always use the best stuff available for cleaner burning. And unleaded doesn't 'show' on the plug colour like leaded did, there's guesswork.



NGK, Bosch and Champion are different designs despite heat equivalent tables. So if one is not working, switch to the other make.

[Presto]

I can anticipate the response (!!) - but I don't agree that resistors are there to produce a 'fat' spark. A fat spark is produced by any plug that is operating as it should. And two resistors do not produce a 'fatter' spark than one resistor, one resistor does not produce a 'fatter' spark than no resistors. They are there to 'suppress' (!!) electronic noise.

[hagis]

If you go into www.ngksparkplugs.com and select videos this will tell you all you need to know about colour/problems/ heat ranges ect, this is the spark plug bible.

[Beezabryan]

Fitment of resistor plug and/or resistor plug cap is not recommended for magneto