[Trackrash]

What Hi Fi said.

But "pops when you lift off the gas'? Not sure what you mean. I am guessing you mean when you are decelerating with your foot off the gas it is popping out of the exhaust?

The thing about Zeniths, assuming it is a US car, is they were jetted very lean to meet emissions. If you make any changes to the motor you will have to adjust the carbs accordingly. First would be increasing the idle jet size.

Popping when decelerating usually is a result of too lean or vacuum leaks.

Just changing from the stock air cleaner or muffler can make this worse.

BTW, re-curving the distributor can help make these cars idle better. You want around 7* BTDC at idle instead of the 4* ATDC that these motors came with. But you don't want to increase the total advance, in fact you may want to limit the total advance to around 30 -32* with the CR you have.

Also, if you want to really wake up this motor change to E cams and use a sport muffler.