302 timing problem

[cobracosta]

I have had this kit for 10 yrs and it was a basket case when I first bought it ( it still is). My story is I usually have no time or no money and never the enough at the same time. The original owner literally threw this kit together but I wont go into all the issues. the engine has always run hot and at idle is very lumpy. The builder decided to run dual carbs and a large nitrous kit. (I never use the nitrous) I have always assumed it has an aftermarket cam and when setting timing it will not run between 12'-14' BTDC. It seems to like 20' btdc. I had a hydraulic valve go out so I replaced them all. Once I buttoned everything back up I was setting the timing and again it likes 20" btdc. In looking at the distributor the vacumn advance it turned so much Counter clockwise it is pushed against the carb spring bracket. I looked at how the spark plug wires are on the distributor and it all seems right. So the question is if you have an aftermarket cam would that cause you to retard your timing more ( from 12' btdc to 20'btdc). Im almost thinking I need a new distributor or my Harmonic balancer isnt accurate?