I have a 302 from 73' 2bbl Holley, the PO said it has a "mild" cam. It has never idled right and I'm playing with plug heat ranges now, seems like it loads up at an idle, but wont idle when lean either. I have been over all the "normal" stuff, cant figure it out...

What # plugs are you running in your 302? How hot can I go?

 

A very good question.

 

Sounds like it might be carbon build-up, runing the engine at a fast idle, you can trickle (very, small) water or ATF fluid down the carb to dissolve/break it up. In regards to plugs, you can go up or down 1 step but timing & fuel mix will need to be adjusted. I'm betting it is either carbon or an electrical based issue though.

 

What are you setting the initial timing at ? And what ignition system are you running ? If you're running points ignition and/or are using the factory timing of 6*BTC, that's more likely your problem.

 

Autolite 45 is stock plug for that engine.

 

All good advise. Bought a vehicle about a year ago with same issues and black plugs. I did go up one heat range, but have slowly cleaned engine and carbon deposits running Seafoam and a little Marvel Mystery in the oil. Keeping a close watch on condition of the plugs, have had to go back to the colder plug and back off timing a couple degrees.