regardless to where that might bring the timing, try rotating the distributor one way or the other while the engine is running to see if the engine gets smoother at idle. also, while the engine is running at idle, try spraying brake cleaner between the intake and heads, and between the intake to carb. that will tell you if you have a vaccuum leak. be carefull when you do this, keep clear, it might catch on fire.if all this fails, try setting the valves to zero-lash, as mentionned before ,this will tell you if one or some lifters dont work properly. this might sound stupid, but you have ajusted the idle mixture screws, havent you?