Topic: EXHAUST BACKFIRE

[Vagelis]

Hi everybody. Last week I rebuilt the carbs on my CB400/4, put them back, checked and tuned everything. From valve lash to synchronizing the carbs. But, everytime the bike is over 4k and I cut the gas it backfires from the exhuast. Checked the spark plug color at idle, 5k and WOT they are a little on the rich side (a dark brown). As allways!
Any suggestions?
Thanks for the help, ride safely. Vagelis.

[Bob Wessner]

Based on my own, albeit, limited experience, when I had the same problem it turned out to be slightly retarded timing.

[alfabeast]

Possible that you are sucking air into the pipes when you shut off the throttle, check the header gaskets and the clamps around the silencer to downpipes.

I have a CB750 that I put a small hole where the silencer joins the downpipe, it sounds like a machine gun when I shut off, I love that sound

[oldbiker]

You can only get backfiring in the exhaust if there is unburnt fuel getting in there. As Bob says, it could be slightly retarded timing or then again it could be one or more plugs are a bit dirty and not firing the fuel on over-run. This unburnt fuel gets into the exhaust where it is ignited by still burning fuel from another cylinder.

[alfabeast]

I agree with what's been said but if you add air to the unburnt mixture it becomes more volatile. and will cracklie and pop more on decelaration.

When you decelerate, you are creating a huge vacuum in the carbs that will syphon fuel producing a very very rich mixture, this gets dumped into the pipes and with the addition of air from a leak will create a more explosive mixture

[Vagelis]

 Finaly solved the problem. The linkage on the #1 carb was sticking. It was closing slowly the last 5mm before stop compared to the others. And the flange on the #3 exhaust header where it joins the lower manifold was rotted.
Thanks for all your help. My neighbors are talking to me again!
Happy ridding to everybody, Vagelis.