'80 GT B21F Guesstimate 275k-350k Km on engine.
Changed plugs to avoid future removal problems. Plugs 1-3 were a perfect tan
brown with no deposit. #4 cyl is a noticeably lighter color and has deposits
on electrode,tip,porcelin and between the porcelin and metal body. Ran new plugs for 50 miles and cyl 1-3 look new while 4 has little specs on it.

I interchanged injectors between cyl 1 and 4 and still cyl 4 seems odd. Noticed
the injector hole wall had black buildup on cyl 4 where the injector seal
would seal against head but spraying the area with water didn't seem to have
any great affect on engine speed at idle. Maybe a bit more carbon buildup
looking down #4 injector hole. Also the limited view one gets in the #4 spark
plug hole gives the impression of similar lighter color and deposits in the combustion chamber/top of piston.

No noticable loss of coolant and no fouling of plug with moisture is ocurring.

Now I am wondering which of following might be causing problem:
a) valve clearance and or cam lobe.
b) high tension lead
c) plugged fuel distributor
d) bent timing tab causing different timing for cyl 4.
e) burnt valve
f) worn guides/rings
g) timing belt stretched.
h) slow coolant leak

Engine seems to have lots of pep and had new head gasket/timing belt about
20k miles ago.

Anybody seen this before?? And/or have suggestions....thanks in advance.... syd