Unusual 'coughing' sound when engine braking

Got a 1991 yamaha sr125, and for some reason whenever I am slowing down using engine braking, what seems like a 'coughing' sounds comes from the exhaust. Only really happens when the engine is hot, but any ideas what this could be?

Thanks, 

Duncan

It's possible that it is unburnt fuel in the exhaust burning off, might be worth checking the valve clearances and timing.

Could also be down to a worn exhaust valve thats leaking.

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Yeah - what happens is that an air leak coming into the exhaust provides free oxygen so allowing unburnt fuel in the exhaust to ignite every few seconds. It happens quite a lot on highly tuned bikes on the over-run too, due to valve overlap and rich carburation - it's less common fuel injected bikes as there's less unburnt fuel about, but some modern bikes even have air injection into the exhaust on the over-run to burn this unburnt fuel and so clean up the emissions.

So as Doofle and I have said, it sounds like a bit of a rich mixture on the over-run, and /or a leaking exhaust....

Hi,

I'm new to the forum and was interested to se if this problem has been sorted - and what was cauing the 'coughing'... same symptoms exactly on my Enfield, only happens when warm.

Any advice would be appreciated!

Cheers,

Peter.