Spark Plugs Colouring

[sump]

Hi all me again

Yesterday I went put in a new set of plugs BPR6ES as recommended by my LPG guy into my 1.9 DG all gapped as suggested at
27 thou.

Looking at the ones I removed I'm puzzled though Three are really quite black but one is what I would think was about right 
a sort of whitish brown colour. 

How can 3 be black and 1 be OK?

All answers carefully considered  Could it be a leak on inlet manifold and carb too rich??????

 

[ajsimmo]

It's rare to get black plugs at all on LPG, let alone just on 3. Have you been running on petrol a lot?

Sent from my ONEPLUS A3003 using Tapatalk