Hey everyone, the 302 in my 69 z28 has been acting up recently. It has had issues breaking up past 5k rpm. Ive done the basics such as replaced cap and rotor, replaced ignition coil, checked valve lash, and done a compression test. Nothing has cleared up the issue.

Besides breaking up at high rpm, it would also backfire through the carb if you give it gas when idling. I also discovered that while checking for compression using my bump starter, the coil was arcing from the output to the ground on the coil. Maybe that could be the issue? Hopefully someone can point me in the right direction.

 

Coils are cheap. Replace it and see what happens. years ago I was working on a 67 that had a bored out 283 to 302 with 350 heads on it (screwed up combo, I know) and the valve springs were breaking. Who ever put the engine together put in used valve springs. Looked like they sat in about 1/2" water and were pitted real bad, would break the last coil off of them.

 

you should never see any arcing, always not good

the best way to check for an arc is to look under the hood in the dark, seen several arcs under my dads 67. was jumping from boots to header pipes

 

Coils arent expensive - as mentioned before if you're seeing arcing i would look into replacing it. Also checking the ignition/spark plug wires for cracks in the insulation and/or around the protective boots. The breaking up could be cross talk as well as the coil.

 

As far as the carb goes, i would fix your electrical problems before blaming anything else quite yet. If symptoms still exist i have a hunch it is a lean backfire. Pull a few spark plugs and take a picture of one of them or look to see if they are soaked/black looking or white hazed or normal.