Pop Bang Crackle
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Since I've had my S4 it has exhibited excessive popping and banging on over run. Whilst I do like a little burbling this has always been excessive and has recently got worse, with visible flames from the exhaust at times I'm told.
I have checked:
Ignition timing
Vacuum leaks
Cam timing (Only as far as checking the timing marks and that the cam wheels are on the correct sides)
Carb float levels (DHLA)
Leak down test results all good.
and haven't been able to find anything amiss, any ideas?
I do currently have the head off to change a noisy water pump, and did note the the carb studs appear to be drilled incorrectly leaving a small step between the carb throats and inlet posts (60 thou max) but a little port fettling will resolve this.
I have checked:
Ignition timing
Vacuum leaks
Cam timing (Only as far as checking the timing marks and that the cam wheels are on the correct sides)
Carb float levels (DHLA)
Leak down test results all good.
and haven't been able to find anything amiss, any ideas?
I do currently have the head off to change a noisy water pump, and did note the the carb studs appear to be drilled incorrectly leaving a small step between the carb throats and inlet posts (60 thou max) but a little port fettling will resolve this.
1968 S4 DHC heavily patinated
- grumbly
- New-tral
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- Joined: 20 Oct 2015
Your engine runs a bit rich on initial overrun, so there's unburnt fuel in the exhaust. If there's any possibility of air entering the exhaust system, that can cause the fuel to ignite with results as you describe.
Have you checked the exhaust for leaks?
Have you checked the exhaust for leaks?
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RogerFrench - Fourth Gear
- Posts: 637
- Joined: 01 Dec 2009
Popping also happens with too lean carburetion (or one spark not giving...) : the mix does not burn in the chamber, but if the exhaust is hot conditions for it to burn there can be met from time to time, hence the occasional bang...
S4SE 36/8198
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nmauduit - Coveted Fifth Gear
- Posts: 2045
- Joined: 02 Sep 2013
Seems you've checked most obvious things, so it might be worth looking at less likely suspects such as the rotor arm, coil and the points. I change the points first whenever I adjust the timing as a matter of routine. It sounds more like an ignition fault to me based on experience. A failing condenser can also cause the same symptoms.
Nigel F.
Nigel F.
1970 S4SE/1760cc big valve/SA-AX block, L2s, 45DCOEs, 1978 Jensen GT, 1962 AH Sprite, Alfa-Romeo 159, 1966 Bristol Bus, 1947 AEC Regal bus.
- nigelrbfurness
- Fourth Gear
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- Joined: 04 Apr 2008
Thanks for the input guys. The car runs Luminition but I'm not sure if I have put a proper rotor arm in and will check.
The exhaust manifold gaskets do show some signs of leakage, although no undue noise was evident. The manifold face doesn't look to good so I will try to face this up a bit which will hopefully give a better seal and report back once the cars back together.
The exhaust manifold gaskets do show some signs of leakage, although no undue noise was evident. The manifold face doesn't look to good so I will try to face this up a bit which will hopefully give a better seal and report back once the cars back together.
1968 S4 DHC heavily patinated
- grumbly
- New-tral
- Posts: 11
- Joined: 20 Oct 2015
Update, the frustration continues. All back together and running but the popping and banging persists.
Whilst the head was off I relapped all the valves and reshimmed to spec. Had a brief moment when I thought inhad found the problem when timing the cams. The exhaust cam was retarded 6 degrees. Sorting this and cleaning up the inlet ports has certainly perked the engine up, but the issue remains.
I also rebuilt the dizzy whilst it was apart, but couldn't find anything amiss.
The crackles are quite violent, but dissapear with only a hint of throttle, not much left, carbs?
Whilst the head was off I relapped all the valves and reshimmed to spec. Had a brief moment when I thought inhad found the problem when timing the cams. The exhaust cam was retarded 6 degrees. Sorting this and cleaning up the inlet ports has certainly perked the engine up, but the issue remains.
I also rebuilt the dizzy whilst it was apart, but couldn't find anything amiss.
The crackles are quite violent, but dissapear with only a hint of throttle, not much left, carbs?
1968 S4 DHC heavily patinated
- grumbly
- New-tral
- Posts: 11
- Joined: 20 Oct 2015
Grumbly
Well done a process of elimination you are now a happygrumbly
Well done a process of elimination you are now a happygrumbly
John
+2s130 1971
+2s130 1971
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Hawksfield - Fourth Gear
- Posts: 601
- Joined: 14 Jul 2004
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