I know this has been discussed before and for a while there has been concern that knock-off plugs were the source of premature failure. I get mine from a trusted source but have still had a new plug fail on me after just tens of miles. I was therefore interested to see a couple of letters in RealClassic talking about modern plugs being designed for the precise metering of fuel injection and " does not cope with the fluctuations of flooding, heat range, etc"........and, "for cost saving reasons, many spark plug manufacturers today mo longer glaze the centre electrode porcelain insulator.....modern fuels contain a range of additives that can accumulate in and around the centre electrode porcelain......reducing its insulating properties". To avoid this, one chap uses only pre-early 1970s NOS plugs and another uses Brisk brand plugs from F2 Motorcycles " made to heavy duty specs of t'olden days".
I got some Champions for a mate's old Trumph car. One failed. I've found that Champions start my WD 16h easier than my indian made Bosch plugs, though as they load up I think the Boschs may be a touch cold [5s]. On the seemingly correct Champion the 16H pinked. I've got new 6 and a 9 Champions. The 6 is working but I've yet to try and load it up by running it at relatively low revs with a lot of throttle. I'm dubious of the quality of the mag rebuild, done before I got the Norton. I've got a Lodge in the M21 that starts well and runs fine, but then it wasn't bad on the previous old Champion.
My Nuovo Falcone uses old NGKs and Bosch, and presently a new Lodge HBLN. Being coil ignition it seems not at all fussy. I hoard old gold thread NGKs for my aircooled DR400s.I never had a problem with their old style plugs, never had one fail or wear out. An old german made Bosch starts my old 1980 KLX better than Champion
It all comes down to EPA regulations and modern engine management systems.
Because of this the insulator on the center electrode is no longer glazed with a lead based glaze.
There is nothing that can replace it completely.
NGK's have no glaze at all.
Thus any deposits that are made on a plug stay there unless they are burned off.
Even worse is they build up in the bottom of the groove between the center electrode & the outer tube.
Now to really boil the stew the deposits from the unburned fuel are quite conductive at compression pressures.
This is why for instance every mower engine maker recommends replacing plugs at 200 hour intervals.
In a fuel injected engine the computer controlled lean as possible without siezing the engine burning keeps the plugs relative clean by making them so hot that the grunge can not adhere and what is there burns off.
In our carburettor controlled engines we can not do this so it is a case of always keeping a spare plug or two in the tool kit.
To a large extent you can protect yourself by always turning off the fuel & starving the engine off.
In the last dozen or so cycles the charge will go from lean to too lean to burn .
This drastically raises the temperature inside the cylinder and burns off the plug so it will be nice & clean thus start easily.
So may closed mind cretens that I associate with ( makes me one too ? ) called me every name under the sun because they would rather believe that there was a criminal substitution conspiracy than the plain truth so when they were stopping here for morning tea I asked who had a "dud plug" on them.
We put them in the vice & heated them till the center was red hot with the oxy.
Flame removed & they burned like candles with a nice short deep yellow flame, the longest for near 15 minutes.
I then wire brushed the threads popped a little copper anti-sieze on them shoved them back into their respective owners bikes and every one fired up first kick and remained working properly for the rest of the 200 mile ride.
They still think they are being ripped off but the cheap fake plug arguement is now dead & burried