I use Iridium plugs, you never have to take them out so it isn't an issue

 

Iridiums will go for years and they are the best value .
I am a little confused about your post though so i will offer this...
Do not run any plugs that you are not sure of ?
The depth of the plug and its heat range must be compatible with your motor.
If i understand correctly , you are wondering why you have carbon on the last few threads of the plugs ? If this is true i suspect your plugs are incorrect for your motor and you are probably very lucky that the pistons did not hit the plugs and cause you a world of hurt .?
Many sites available to ensure you crossover the correct plugs for your bike.

 

You can measure the depth of the plug hole using a grip gauge. It's actually used for aircraft. If you can't find one PM me and I will get you one.

Here's a good one.

 

'86 EVO, stock heads. I run NGK BPR5ES and Autolite 4265 with carbon on the first 1/4" or so on both types of plugs. Curious, I googled "harley evo sparkplugs" under images and nearly every used set had the same signature. I'm not saying there is anything wrong as these are the plugs spec'd just curious why and how it would run with standard thread plugs and if anyone else has tried it?

 

Here's some good info on reading plugs, will explain the color you see on the threads.

http://honda-tech.com/showthread.php?t=3063102

 

Wow, now that's a great read, I've never seen an article on how to read spark plugs so informative, thanks for sharing. I've also been reading my heat range wrong and timming. I'm glad I made this post, I almost didn't.