Overcarbed is just what the name implies.........the carb has too much airflow and fuel being metered into the engine for it to all burn/be used. As an extreme example, if you put a four barrel carb on your lawn mower it would be overcarbed. The engine can only use so much, and will run very rich and badly if overcarbed.

Ideal for your engine is about 600 cfms. There is some mathmatical formula that I think Richard (techinspector1) has posted, but I forget what that is exactly. I just know the typical small block runs well on about 600 cfms.

Just for sake of asking, why do you want to lose the Rochester that is on there. Although they generally don't have the performance image of Holleys or Edelbrocks, they are actually very good carbs for a street engine. Mercruiser and all the other boat engine manufacturers have used this carb for years and years, and the nice thing about them is that they are very economical to operate due to the small primaries. You could have it rebuilt and probably be fine. The Jag isn't a drag car, so for smooth street performance the Rochester would not be a bad choice IMO.

Just a thought.

Don