Thanks for all the replys.. I will perform a leakdown test first and see if there is anything unusual. The other checkups might not be a job for myself so I may need to seek professional assistance.

I tried swapping the plug with one from another cylinder with no luck. I have also tried running the engine a bit warmer recently because the plugs are a bit sooty. If this does not work I will go to a hotter range.

The carb is a 750 HP DP Holley, the engine is 409cid making 500hp and 500ft/lbs so I think the carb might be ok.

Thanks heaps!