Spark plugs and backfire

[dave914]

I have Bosch platinum plugs in my 1.7 D-jet 914. I have replaced them twice at 3500 miles, and each time they have been:
#1 Clean, white core, some grey (basically ok, but maybe a bit hot, no melt down)
#2 Clean and grey/brown, text book perfect
#3 Grey core, and a bit black film elseware
Not oil. Some very hard white deposits.
#4 Same as #3, but with much more white deposits.
What is the hard white deposits. They apear fluffy, but are rock hard. The fresh plugs improve performance, but why are #3 and 4 doing this after 3500 miles?
Also I have had some popping (backfire) on overrun (let up gas between shifts, or coasting down hill in gear) I have cranked the brain mixture knob clock wise a few clicks as suggested in the Haynes manual, and it did improve some, but still happens.
What can I do about this?
Also the engine has been a bit sluggish at times. Compression is good, and i have a Pertronix pointless system.
thanks dave

[rich]

Leaking dribbling injector causing excess fuel on deceleration?

[Conrad W Peden]

Check your intake manifolds where they bolt to the head and the boots that attach them to the metal box thing under the air cleaner and throttle body(I don't know FI so I don't know the correct word for it.)If you have a leak at any of these points it can screw up the air mixture and and cause the engine to run lean(I think its lean maybe rich),that can cause the backfire, not sure about the plug problem though(if it is a problem).

[This message has been edited by Conrad W Peden (edited 06-11-99).]