340 with som problems

my 340 drives mee crasy, today it has started to knock some havent found out from where yet but i will probably find it, it has started to run real poor and fires thru the exhaust during medium throtle checked all the sparkplugs to get an idee about what is going on and found them all too look ok a litle rich but nothing seriusly wrong except for the #6cylinder that is almost white wich is realy scary to see since i know that means that cylinder is running way too lean.. but i cant get why it is just that cylinder.... i have tried checking for a vackum leak att that port on my stripdominatormanifold and it seems to bee ok..i have also checked that i got ignition and my timing light flashes just like it should at all plugs and the ignition is set just as it has always been att 10degres at idle and 38degres at around 2500-3000rpms i have plenty of fuelpresure also cant find anything that seems to aply to what is wrong, any idees?
my backup plan is to put in my old 318 just to bee able to drive some more before it is to cold outside..