New exhaust pops and bangs.

[Sparkrite]

My tcs was finally running okay,but as the silencer box was rusted I replaced it with a small straight through exhaust box that I cobbled up to fit. The diameter was almost the same as before at about 50mm. However there is now popping and banging mostly on overrun. What's the best way to stop this.

[4129R]

If it were my car, I would check the timing with a strobe, 35 BTDC at around 4500 rpm maximum advance.

[BDA]

I agree with 4129R to check your timing. If that's good, I'd check your carbs. I'm assuming you're running Dellortos or Webers and your float level is set correctly...

Popping is normally a symptom of a lean condition. I am NOBODY's carb tuner but I'd turn one or more of your idle mixtures screws out a little. Which one? Since this is a new situation, I might turn out one idle mixture screw a half turn and see if it helps. If not, I'd turn it back and go to the next one.

HOWEVER, what I consider the real method for setting your idle mixture is in the attached file (which must be done with a hot engine). I have not been able to actually do that because my ear is not good enough to detect a 50 rpm difference reliably but I'll continue working on it. In the mean time I have gotten my car where it idles at a steady 1000 rpm and not pop or spit so I'm at least not embarrassed with the way it runs! I'm really proud of that!  But as I say, I'm nobody's carb tuner so I have a lot of room for improvement.

[SwiftDB4]

Is there an exhaust leak? Even small leaks cause overrun pops as outside air is sucked into pipe.

[jbcollier]

All you’ve changed is the exhaust?

Then you fit one with lower back pressure and have leaned out the mixture.  Might be an easy fix.  If you have original Strombergs, it gets complicated.

[Sparkrite]

I just pulled the plugs for a quick check before adjusting the Weber 40s and apart from the slightly richer rear two plugs they don't seem too bad for town driving with quite a lot of traffic.

[4129R]

What choke size, main jet and air corrector jets are you running?

I am on 32 chokes, 115 mains and 150 A/C jets.

[BDA]

Your plugs don’t look bad but remember that plug color is pretty imprecise indicator for mixture.

Can we assume you’ve found your timing to be correct since you haven’t mentioned it?