The following comes from memory of something read...and I haven't found it repeated in a short search. Correction with facts or by someone of respected stature appreciated.
There are two places to check for a/f mixture..one being the base of the insulator, the other being the flat of the threads.
IF that is ineed so, it doesn't look like you have a huge lean problem to worry about.
Still...the mixture ring at the base of the insulator is where you need to look. There are inspection tools that allow you to see that ring without cutting the threads off.
RR notes such a tool
here.
Note that the mixture ring may not be visible all the way around the base. It starts 'under' the ground electrode (that metal 'tip' bridging thingy) and goes further around the insulator the longer the run. On 1/4mile runs of only a few seconds, this ring may
not be all the way around the insulator.
Twenty seconds is a bit different. Should be plenty.
'Can't find an -eg..' ??
Got 'em in
most shops 'round here..and certainly available thru rockymountain. Good plugs!
...change the plug once in awhile, though. At least annually (sez me....no bibliography notation available).
No matter how good you're jetted a spark plug leads a tough life. Just 'cuz it still works doesn't mean it shouldn't be replaced.
Post a pic of your mixture ring when you get those threads off! ;)