excesive back-firing 640 adv?

**Andy White** · #1 26 Sep 2005

Maybe worrying for nothing (640 adv 04) but regardless of speed or gear whenever I decellerate I am getting excessive back-firing, not the throaty type growl one would expect from a single but rifle cracks that have ex vietnam vets diving for cover!! In europe at the mo so fuel is still good, and am about 2000km before next service is due. Before I start camping up and overhalling, Is it valve clearance?

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**Ian** · #2 26 Sep 2005

Difficult to diagnose, but both my LC4s (a 2000 and a 2002) have had similar backfiring when under engine braking. Maybe the size of the exhaust valves (pre 'High Flow' cylinder head) have something to do with it.

**JonStobbs** · #3 27 Sep 2005

Check to see if there's any small leaks in the exhaust joints/manifold joint,as even a very small one will make it pop back on the over-run.Also i've found that singles will often do this if the tickover speed is set too low.A quarter of a turn cured mine.

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Just going for a short ride on my bike....

**Kudi** · #4 27 Sep 2005

Backfireing could also indicate that the engine is running too "fat", means too much petrol in the air/petrol mix. Reasons can be wrong carb adjustment or, I had that once, a shut choke cable under the tank.

Cheers, Kudi

**JonStobbs** · #5 28 Sep 2005

Yes you could be right there Kudi.I once had to fill up with "super"98 octain petrol,and i seem to remember it did pop quite a bit on over-run afterwards.Seemed to rev a bit better too mind you....!

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Just going for a short ride on my bike....

**BMBaxter** · #6 28 Sep 2005

the symptons point towards an air leak in the exhaust system.

**Yuri** · #7 28 Sep 2005

I have the same bike, same problem and a friend too. It seems that they come like that from the factory. I dont think it is a problem since they all seem to be like that. Changing the exhaust and carb settings did improve this a little but didnt eliminate it. I dont think we should worry about it too much.

**matsplatuk** · #8 19 Dec 2005

I recently cured the same problem on my '99 Adv. It was a hole in the small header pipe where it joins the rest of the header pipe. It had rotted virtually right round, i think a lot of crap can sit on it.

Still backfires a little on run-over but nothing like as bad as it did.

Hope you get it sorted - Mat

**tifua** · #9 22 Dec 2005

If you're adv is fitted with the "secondary air system" and it has developed a leak it result in loud bangs when you come off the gas.
Check if there is a small pipe coming out from between the exhaust headers on the front of the engine (it's about 8 mm in diameter). If you do then you have an SAS fitted (is standard esp for bikes supplied in Germany). You can buy a little aliminium blocker which fits in under the header pipes. You can then remove the whole sas system an file it in the bin. Normally the sas leads to a breather on you airbox on the other side of it so if you remove make sure that the airbox is properly sealed.
Cheers
Nick