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Exhaust backfire/crackle - what can it be?
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Author:  IndigoBlueCooperS [ Sun Mar 30, 2014 6:45 pm ]
Post subject:  Exhaust backfire/crackle - what can it be?
Put a new engine in the car, but it has a backfire/crackle from the exhaust when you coast down hill in gear.
1275, 1&3/4 SU (OA6 needle, yellow spring), 256 Megadyne cam & A+ electronic disy, timing on 7 degrees and steady as.
Engine runs well, starts easy, does not run on, good power / torque, fuel mixture seems OK, no obvious exhaust leeks.
It just crackles as you coast down a hill in gear.
Any ideas what I should check next?
Author:  MrFail [ Sun Mar 30, 2014 6:50 pm ]
Post subject:  Re: Exhaust backfire/crackle - what can it be?
Excuse my ignorance, but is that a bad thing??
Author:  MiniVLT [ Sun Mar 30, 2014 7:05 pm ]
Post subject:  Re: Exhaust backfire/crackle - what can it be?
What sort of exhaust are you running?
Author:  IndigoBlueCooperS [ Sun Mar 30, 2014 7:13 pm ]
Post subject:  Re: Exhaust backfire/crackle - what can it be?
Basic 3 into 1 and RC40 system.
Author:  MiniVLT [ Sun Mar 30, 2014 7:21 pm ]
Post subject:  Re: Exhaust backfire/crackle - what can it be?
I had a 3 into 1 with a 2" lukey rear muffler on a 1330 with a 731 cam & twin 1 1/4". It went like stink but I had the exhaust crackle you mentioned when the weather was warm & you backed off at high revs. I just put it down to the 2" pipe being too big for the engine....but I didn't change it....I liked it! 8)

Cheers Shane
Author:  drmini in aust [ Sun Mar 30, 2014 7:53 pm ]
Post subject:  Re: Exhaust backfire/crackle - what can it be?
Usually is due to an exhaust leak somewhere.
Author:  IndigoBlueCooperS [ Sun Mar 30, 2014 8:32 pm ]
Post subject:  Re: Exhaust backfire/crackle - what can it be?
No obvious exhaust leeks - block up the exhaust and the engine stalls.
Author:  Scrimo [ Sun Mar 30, 2014 8:35 pm ]
Post subject:  Re: Exhaust backfire/crackle - what can it be?
I just bought an A45 AMG Mercedes... It does the same thing in sports mode!... Cost a lot of money to hear that sound ;)
Author:  9YaTaH [ Sun Mar 30, 2014 8:35 pm ]
Post subject:  Re: Exhaust backfire/crackle - what can it be?
Timing not optimised?
Author:  simon k [ Sun Mar 30, 2014 8:40 pm ]
Post subject:  Re: Exhaust backfire/crackle - what can it be?
I read somewhere that can be caused by running a bit rich - basically unburnt fuel popping off in the exhaust
Author:  IndigoBlueCooperS [ Sun Mar 30, 2014 8:45 pm ]
Post subject:  Re: Exhaust backfire/crackle - what can it be?
I will check the mixture with the AF meter, but I don't think it is that far out.
Unless it is a combination of the mild cam & a little rich.
Author:  surtees [ Sun Mar 30, 2014 8:57 pm ]
Post subject:  Re: Exhaust backfire/crackle - what can it be?
Thats a great sound why change it
Author:  gtogreen1969 [ Mon Mar 31, 2014 12:19 am ]
Post subject:  Re: Exhaust backfire/crackle - what can it be?
The old rule of thumb

Backfire from Carby is lean
Backfire from Exhaust is Rich

Could also be dozens of other things but start there first.
Author:  i got a mini [ Mon Mar 31, 2014 4:17 am ]
Post subject:  Re: Exhaust backfire/crackle - what can it be?
MAnifold gasket ?? Blown not sealing brothers car use to back fire out the exhaust when he took foot off accelerator
Author:  Wayne King [ Mon Mar 31, 2014 5:07 am ]
Post subject:  Re: Exhaust backfire/crackle - what can it be?
It is most likely getting air in the exhaust somehow.
Could be past the manifold gasket, past the slip joint if there is one or even past the valve guides if they are worn enough.
Manifold gaskets somtime leak under vacuum and not under pressure.
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