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?

Excuse my ignorance, but is that a bad thing??

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Tim
Dusk Blue
1964 850

What sort of exhaust are you running?

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If it's not a Cooper S, the mini was designed to be modified!

Basic 3 into 1 and RC40 system.

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!

Cheers Shane

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If it's not a Cooper S, the mini was designed to be modified!

Usually is due to an exhaust leak somewhere.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

No obvious exhaust leeks - block up the exhaust and the engine stalls.

I just bought an A45 AMG Mercedes... It does the same thing in sports mode!... Cost a lot of money to hear that sound

Timing not optimised?

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"Show me the Mini!"

I read somewhere that can be caused by running a bit rich - basically unburnt fuel popping off in the exhaust

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did I tell you that I won a trophy?

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.

Thats a great sound why change it

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I have an Addiction just not the time.......
1966 Morris Mini Deluxe http://www.ausmini.com/forums/viewtopic.php?t=80459
1968 Morris 1100 viewtopic.php?f=3&t=101555

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.

MAnifold gasket ?? Blown not sealing brothers car use to back fire out the exhaust when he took foot off accelerator

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.