I have one of my 502 Motors backfing out the exhaust around 3000 RPM or higher every five to ten second. The motor also does not sound right while it is idling. I had a slight crack in the distributor cap I change it help a little but not much. I check all the spark plugs they look fine change the wire also. Just looking for some suggestions. I have a feeling one of the values is stuck? Or maybe a lobed cam. I can almost hear where it is. I am going to do a compression test tomorrow and see what happens.

i dont think a compression test will tell wheather its a wiped cam lobe or not, espaecially if its the exhaust side. have you tried killing individual cylinders at an idle and seeing if it is located in one cylinder or the whole motor?

I had a feeling that it was coming from the left side of the motor so I did a compression test on all the cylinders and the number 2 cylinder lost compression. So I took the head off and thank good it was just a value that was stuck so I replace all the value springs along with checking all the cylinders everything is fine and up and running.

a trick i use that works well is put a vacuum gauge on a vacuum port on that motor, it should read a steady reading{unless a big overlap cam is used} if the needle is moving a lot, possible broke valve spring, cam lobe or valve problem, good luck

I hate when she backfires under a load!

Thank for the information. I took the head off and replace the values and spring and everything is fine i had a bad value.

thanks