Yeah, #1 & #3 look like crud build up from running too rich but maybe as you say it's caused by the hole size ?
Any time I've experienced even slight mis-firing the plugs (Stock Mazda) got changed and the problem cleared up.
For the past few years I've been running the BR10EIX as well and have had zero mis-fires. The only draw-back is the need to enlarge the access holes and buy/grind a special spark plug socket.
Jeff

Not sure I'm seeing the same thing as you in that chart. Actual plugs are darker than in the picture.

1st and 3rd are the leading plugs. Wonder if the small hole for the trailing plugs causes them to have less crud.

Also that crud makes me wonder if I should mix less oil. Still using about 1oz/gal. I think that Tracy mentioned using less for the Renesis. But then he's using a 50/50 mixture of MMO and 2 stroke oil.

I was going to replace them with the AR2592 but then got thinking about correct gapping.

Finn

Steven W. Boese SBoese@uwyo.edu <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: AR2592 gap
Date: Fri, 26 May 2023 03:38:14 +0000
To: Rotary motors in aircraft <flyrotary@lancaironline.net>

Indications were that sag was due to preignition caused by the spark plugs.  The hottest part of the spark plug is the tip of the insulator.  The temperature there is controlled by conduction of heat through the alumina insulator into the base of the plug and by conduction through the center electrode also to the base of the plug.  The vast majority of the heat is conducted through the center electrode which has a copper core to help do this because of copper’s high thermal conductivity.  

As the spark plugs age, salts form between the corroding center electrode and the tip of the insulator.  Those salts have a lower thermal conductivity than the alumina insulator so less heat is conducted to the center electrode.  

As the plugs age, the center electrode copper core becomes porous due to the cyclic temperature change with each combustion event.  This reduces the thermal conductivity of the copper core.

Those two processes result in the tip of the insulator becoming hot enough to cause preignition at high power conditions.  Throttling back stops the preignition.  Using leaded fuel appears to cause the preignition to occur with less spark plug time in use.

For the rpm decrease (sag) to occur, preignition must occur at both the leading and trailing locations simultaneously.  Preignition at only one location actually gives a slight power increase.

No amount of cleaning can reverse those changes.

Although the heat range of new stock spark plugs is initially appropriate, they essentially become hotter plugs as they age.

NGK plugs are colder with higher number.  Using stock trailing plugs in both the leading and trailing locations will help.  I’ve been using BR10EIX plugs in both locations which helps even more and they have been lasting well over 100 hours whereas the stock plugs would often develop sag at less than 20.

Steve Boese