Whats causing the backfire?

[xwhatsit]

Before re-jetting/needling, check other things first. I mean the carbs are set up like that, they're probably fine (you shouldn't really need to change jets and needles unless you modify exhaust/intake system). Boring stuff like check your air-cleaner isn't choking up the intake, check the choke is opening fully, there's no doormice in your exhaust, etc.

This is assuming it is running rich, which you'd need to check first.

[Kickaha]

So is it a leak in the pipe or is it running rich?

Has it always done it or has it just started?

[terbang]

Main jets take effect at full throttle at higher RPM whilst the pilot jet has more effect at lower throttle positions.
Air leaking into the exhaust system will cause popping on overrun and also an excessive rich mixture will as well. Dunno much about the Ducati but most 4 stroke bike engines are timed off the crankshaft rather than the norm of the camshaft that runs at 1/2 engine speed. This has the effect of a spark every stroke. This means that a lot of engines are actually firing on the exhaust stroke as well which is harmless apart from a bit of extra plug wear. If you have any unburn't fuel floating around due to rich mixture this will re ignite during the exhaust stroke which causes the characteristic 'pop' in the pipe. A moderate popping in the pipe can be healthy as a richish mixture keeps yer temps down. Well its better than too lean, which will fry your engine.

On Carbs with the throttle closed (butterflies closed, choking the air off) and high rpm of overun, the pump (your engine) is still demanding air. Here you have a situation where the fuel is still delivered through your Idle to just above idle enrichment (pilots and idle mixture ports) but with less air. A rich mixture! With a few extra sparks and just a lot of general hot spots on the exhaust stroke, Pop Pop Pop! It sounds real cool. The flames sound a bit excessive so I reckon you are in the rich scenario. You need a good tune up including a close look at mixture.

[McJim]

Took the fairing off and found the problem. I had lost a nut from where the pipe meets the cylinder. Fitted a washer and bolt (thanks Mitre10) and the problem is solved.

I was looking at the wrong end of the exhaust system. (I thought loose rivets were the prob).

[Matt_TG]

Glad you found the problem, but I always thought that backfires were a result of an engine running too lean, not rich? The unburnt gas builds up and ignites, bang.

On my Honda I had perforated air cut off diaphragms in the carbs. They work to shut off the air and richen the mixture when the throttles closed off, bloody embarrasing in a quiet street .. so I had to block off the air hole into the diaphragm area and screw out the mixture screws till I replaced the diaphragms. Richening it up worked in the meantime.

Honda Forum Discussion about that

Matt

[NighthawkNZ:Me]

but more of a loud huff.

Onions can do that

[The Lone Rider]

So whats the difference between backfire and the bike coughing?

I changed the ends of my pipes, and while they are the same lenght as the originals I've noticed it coughs a bit more then usual when I slow down.

[McJim]

Has it always done it or has it just started?

It had just started. Tightened up the bolt and it's now as smooth as a Ducati can be (i.e. still rough as guts but without the flamethrower and gunfire effects)

Cheers.

[McJim]

So whats the difference between backfire and the bike coughing?

I changed the ends of my pipes, and while they are the same lenght as the originals I've noticed it coughs a bit more then usual when I slow down.

The bike sounded like a large calibre handgun being discharged in an empty oil drum....turned heads on the motorway - had little trouble splitting through rush hour traffic

Most twins will cough a little - means nothing - this was LOUD.

[xwhatsit]

Took the fairing off and found the problem. I had lost a nut from where the pipe meets the cylinder. Fitted a washer and bolt (thanks Mitre10) and the problem is solved.

I was looking at the wrong end of the exhaust system. (I thought loose rivets were the prob).

I see you're finding out about Italian bikes

Singles do the same things as Italian bikes, even Honda singles. Mine dropped a footpeg -- and also, early on, one of the headers came undone much like yours and caused a backfire on over-run. Not as loud as you say yours was, but my cylinder is 50cc less than yours are.

So whats the difference between backfire and the bike coughing?

I changed the ends of my pipes, and while they are the same lenght as the originals I've noticed it coughs a bit more then usual when I slow down.

It's possible that your engine is now running lean, after putting those new `silencers' on (just open pipes, now, right? So way more free-flowing). You might need to re-jet your carbs, most people seem to need to do that when they change exhausts.