Hitchcocks Forum

Failing iridium plugs.
I think it’s been on here before but it still seems to be a problem without an obvious explanation. It's a mystery.

Iridium plugs sometimes seem to ‘die’ once they get ‘wetted’ by carbs/injectors that supply an over-rich mixture.
(Please note - this isn't about plugs that get heavily 'oiled'.)
This doesn’t seem to happen with conventional plugs but seems to happen at times in very different engines with iridium plugs.
It’s not just that they sometimes fail when they are ‘wet’ but don't work after they have been ‘wetted’ like this.

Has anyone a reliable, sound, convincing, scientific, plausible, sensible explanation for this – ‘cos I can’t think of one!!
Please resist offering hairbrained theories!!!

Poor petrol induces greater heat from combustion and in some cases causes excessive wear to the earth electrode – not the iridium centre electrode. It also causes damage to the base of the porcelain insulator where it abuts the metal hexagon and threaded section of the plug.
I’ve no idea why this is more prevalent with iridium tipped plugs.
But this is of course the opposite of the plug getting soaked with petrol. So these comments may be considered ‘hairbrained’

I can offer no explanation, but from my own personal experience I have found NGK plugs are less susceptible to the problem. I've had two bikes long term that ran on Denso plugs that suffered from the problem, and it "disappeared" without doing anything other than changing to NGK. One of the bikes was a Ducati, and it was another Ducati owner who had suffered the same issues that made the suggestion to me. The problem in my case always happened from a cold start. Once the plugs were changed, the problem never returned .

Thanks 'Duke' (!!).
That's interesting because the Denso iridiums have an even finer centre electrode than the NGKs. I've used both and have never myself had this problem. In fact in 50 yrs have only ever had one plug fail - because I'd fitted a wrong heat range. Ah well.

Keep the info coming!

Assuming you have genuine NGK and and not a fake?
Personally I've never had any problems with ngk iridium plugs.

Probably not the cause of these problems, but it does say on the box of the one I had in the Bullet for over ten years;
Do not adjust the gap and do not clean.

Our host offers the iridium BR8EIX, this is with resistor,, shouldn't it be whithout?
A 'normal''R'NGK plug behaves badly in an enfield..

I gather ALL the Iridium plugs are resistor types (someone correct me if I'm wrong), they should be OK if you use a non-suppressor spark plug cap.

The lore is that resistor plugs AND suppressor plug caps will give you a weak spark, but EITHER a non-resistor plug in a suppressor plug cap OR a resistor plug in a non-suppressor plug cap will work. Unless you have a tired old magneto for the ignition, then you don't want any resistors or suppressors.

A.

I use ordinary spark plugs in all my bikes, including the racers. There is rarely, if ever, any trouble as a result and they are considerably cheaper, too, or is this just too simple?

I use iridium in my 650GS

For the very simple reason, I will always use iridium on a bike where it takes 4 years to change a plug.

Like you BW, normal plugs when you can see them.