recently when i'm at a stop i hear low sounding backfire.  does that mean i need a new valve? i looked them up on ebay. used $45 and new old stock $396 us.

There could be a multitude of various issues going on. What’s your diagnosis of it being a defective valve. Need some more in depth details.

my diagnosis is the muffler is back fireing when i'm stopped or slowing down. don't hear it when up to speed. some one said was the carbs . i don't want to go down that road.

Again there are a lot of options here beginning with checking the timing and go from there.

might have to go to my mechanic

Valve adjustment would also be low hanging fruit

Does your car still have the solenoid to keep the throttle from slamming shut when you get off the gas?

If it doesn't, then you have a sudden rich condition when you lift off the gas. 

How old is the exhaust? I had an 325is that would pop out the back downshifting. The muffler was bad but I liked it.

not 100% sure but i believe i still have the solenoid. all the anti pollution still on this 72.  can't recall how old exhaust is but i remember replacing muffler 10 years ago.

But is the solenoid working?

how can u test? and where is it located

Check the FSM. I’m sure it’s just jumping power to it and see if the solenoid engages.