Carb jetting with K&N

My car ('81 1500) has a K&N air filter, however I don't think the carb has been rejected to suit and I think it's running lean. The engine is otherwise standard. Anyone have any experience of what jets are needed to correct the increased airflow of the K&N?

Because the weber carbs work on air flow and manifold vacuum not just air flow you will need a degree of correction to the mixture. The exact nature of this is not so simple. We can take a guess but the only way to get it right is a visit to the local friendly rolling road. It typically takes them about 30 minutes to an hour tops and the really good places are even quicker.

The problem is that each engine is different despite the obvious "basic" standard, likewise each air filter design is different. In your case, with just a filter change you probably aren't getting much more gas flow but you will have lost manifold vacuum at lower revs and loads. The difference to the jets is going to be pretty slight - possibly only half a size in which case you can't just buy the correct jets as they only come in a range of whole size steps. Any adjustments inbetween the steps requires the jet to be drilled out.