i have a stock 72 240z. after the engine is warmed up and left to idle (700 rpm) every 30-15 seconds it will backfire out of the exhaust. i have searched but did not find the problem i am experiencing. can someone explain what is causing this and how to fix it?

I always check the easiest things first.

Vacuum leak (big or small whooshing sound from hoses and connections), plug condition and gap, points (and cap and rotor) condition, ignition timing and dwell. If all OK check air cleaner and carb mixture setting and balance. If that's all perfect you need a better advisor. When my truck was doing that it had a leaky vacuum hose.

More info needed. How loud is the backfire? Has it recently had a good tune-up? Plugs & wires, valve adjustment, timing, points, condensor , advance diaphram & base plate functioning, carbs tuned & float levels checked? What color are your sparkplugs? Whew, I think I got it all. Oh Yeah, Confirm that your carbs are round top SUs. My uneducated guess is, it's caused by either wrong timing or rich mixture or both but you have to rule out everything else to be sure. I vaguely remember my 71 stumbling at idle now & then but I don't remember it being a backfire or at least a loud one. How strong does your engine run from 700RPM-5000RPM? Any stumbling, low power, backfiring on or off the throttle? OK I'm done. I think.

Mark in Portland

round top su carbs. i replaced spark plugs & wires, ignition coil, cap, rotor and points 2 months ago. is not on the road yet. also when i let off the gas while driving it backfires as the car's weight pushes the car forward. the backfire when idle is just a loud pop about as loud as the car at 1000 rpm

This still sounds like a possible combination of timing & mixture but Stanley is correct. Check for vacuum leaks. Easy to find with a piece of hose held to your ear. What color are your plugs. Also have you checked the other items on my rather lengthy list?

Mark in Portland

i am still learning as i go. what is "advance diaphram & base plate functioning"? from my experience and my neighbor's comments my engine sounds good and is running strong.

Easy check. Pull the distributor cap & the vacuum line that runs to the diaphram on the side of the distributor. Suck on the vacuum line & watch the base plate that the points are mounted on. It should rotate a few degrees as you apply vacuum & should return after the vacuum is released. I'd still like to know what color your plugs are. That will tell us if your air-fuel mixture is close.

Mark In Portland

I'd still like to know what color your plugs are.

i just posted a photo of one of the plugs.

I couldn't access your photo. We will need photos of all six or a description of each. Keep the plugs in order so we know which cylinder they came from. The reason being that each carb runs 3 cylinders. 1 plug will only tell us about 1 carb. Just when you thought the list couldn't get any longer, as long as the plugs are out a compression test would be a good idea too.

Mark in Portland

Could be many things, but my guess is look at this :

- Ignition timing

- Overal condition of the ignition system

- Valve clearance not right

- Mixture to rich.

Edited March 8, 201411 yr by bartsscooterservice

Maybe leak in exhaust manifold connections, or rusty header.