失火（ミスファイア） [編集]
点火の際には燃焼作用によって煤（カーボン）が発生するが、これが電極間に付着するとカーボンを伝って電気がリークしてしまい、火花を作れなくなる。この状態は、俗に「かぶる」と表現される。プラグには自身が高温になることで電極に付着するカーボンを焼ききる（燃やしてしまう）自浄作用の機能が求められるが、反面、温度が高すぎると早期着火（プレイグニション）による不正燃焼やプラグ自身の焼損を招く。

1. air-fuel mixture - if you don't get it right what happens to the plug is the issue here.
2. if too much fuel - (sorry, it's a link to google book...)

http://books.google.com/books?id=p3ZHkkm4QU0C&pg=PA259&lpg=P...

Plug could get [black and wet] or [black and dry].

Then, 焼け気味 is - http://www.theultralightplace.com/sparkplugs.htm
"too much air" thus "a white colored plug". It's burnt - but carbon/oil etc (black stuff) are burnt off, and plug itself is burning too hot - thus looks white-ish.

That's what that sentence is saying, although, I did have fun looking at google: plug color chart, and seeing how things could go wrong in so many ways (as Joyce A. said).

Then hard part is how to translate 「焼け色はかぶり気味になったり」「焼け気味になったり」.

'it will look black [could be either wet or dry fouling] or it will look white [burnt].' is very straightforward and simple but - you can't back-translate...