Hi there,
I have a 400 cui in my 74 newport. Last year I redcurved my distributor to 18 initial and 36 total, without vacuum connected and a lighter spring in it.
This year I rebuild my Thermoquad and tuned it to a lambda of 0,91 in idle.
I have a dual exhaust with no connection between the pipes and just cherry bombs as mufflers. So it is pretty loud.

I just noticed that the exhaust makes a little "plop" in idle and while driving slowly, when the engine is cold. I currently have no choke (I am waiting for parts) so I thought, it is because of that.
But the plopping in idle (neutral) is never going away. It is pretty quiet if you compared it to the overall exhaust noise.

But when I step on the gas in neutral to rev it up and then take my foot away it bangs pretty lound two times.

Is it the carburetor or the ignition?

When the engine is warm and I step on the gas it accelerates very good. High rpm is no problem. And I wanted more performance. MPG is not important.
I use 98 ROZ (88 MOZ) fuel which would be 93 in the US?

Somebody told me that true duals with no muffler could cause some banging, because of the pressure changes etc...
But I never noticed it.

Sounds like an exhaust leak. When it's rich exhaust (as in coasting down from revving) the extra air allows the fumes to reignite in the exhaust. Usually in a muffler as they are the likely places for the excess fuel to collect.

Check for leaks at all connections. Could be a loose clamp or bad header seal or..

Should I also lean out the idle mixture a little bit? As I richened a bit much after rebuilding the carb for more performance.
Keep in mind that I also recurved the distributor quite a bit because of my low compression.

I'll also change my old very dirty air filter for a new one. Maybe that also leans out the mixture just a tiny little bit. I have one of those aftermarket cleaners as I can not find an original one here in germany.

Only time I ever had that happen I had an intermittent plug wire bad issue, it's not fire the plug, and the next cylinder on that side would fire the mixture on it's exhaust stroke. Sounded like a putt inside the car, but more like a gunshot outside and a flame would shoot out the exhaust.

Crossed plug wire?

It's not that extreme and began after rebuilding the carburetor.
I first thought, that the carb ran rich while leaking. But it seems it ran leaner, because it only leaked on the outside.
So now it runs rich, without leaking. lambda of 0,91 in idle.
I wanted to gain a little bit of power. Maybe it is too rich when stepping on it.
But I don't know if that could cause the banging.

Lean provides more power than rich, rich provides a safety margin.

A miss is causing your "banging", it's raw fuel igniting in the exhaust, not the engine.

I got that it is in the exhaust. I thought, that a rich (or too rich) mixture would cause the fuel not to burn completeley, flow into the exhaust and bang.

What I also learned is that a richer mixture will provide more power.
You want to have all the oxigen to be part of the burning process. To achieve that you want to use a bit richer mixture. So that every oxigen molecule has it's fuel.
If you run lean, not every oxygen molecule has a "partner" and goes out unused.
And how much air goes in, is regulated by your foot. So you realy want to use every bit of it.
Running lean just saves fuel.
That is what I've learned and it makes sense.

I hope I got that right and I am understandable. My english is far from perfect.


EDIT:
I forgot to write something else.
I also learned that a too lean mixture can cause misfire, because the mixture will detonate early. But I think then it would go out through the intake.

Hello again,

I got rid of the banging while idling and stepping on the gas.
I wanted the car to have the best acceleration possible and I think that is now achieved. I goes of like never before.

But I have a question. When the car is driving normally and not accelerating the exaust is "plopping".
I cannot describe it correctly. It is not a loud sound. I don't even know if it is detonating. It is more in between of plopping and a "missing" sound.

If you have that constant sound and then in between is this sound-stop.

Is this ok? Since I wanted maximum power and not smoothest driving.
Idle, stepping on gas while idling, accelerating are good.
WOT is awesome.

Or do I need new metering rods?

No, you need to get away from the idea that your car needs lots of fuel at all times.

You most likely have unburned fuel, re-igniting in the exhaust tubes.
That, or a vacuum leak at the intake or carb somewhere.

How do the sparkplugs look like? I would presume they are all pitch black.

Also, for 'best' performance, you don't want an aftermarket (open) airfilter as your motor now only ingests its own hot air.
Those cherrybombs aren't helping matter either.
A crossover tube between the exhaust pipes would help in acceleration.

For your information, Lambda 0.91 is about a 13.3 AFR.
You would only want this number during moderate acceleration.