JXP, you stated the plug had a black residue. This leads me to think you are just to rich and after a while (updraft carb!) the engine just floods out. Once it sits the excess fuel evaporates out of the engine and repeat.
Check all 4 plugs and if they are black or sooty check to see that the choke is fully open. If an open choke is confirmed you have a carb problem. Your blue spark and dirty plugs lead me to this.
When you were running the engine at 1450 were you in gear or not?
Dave Neptune
