Backfiring

[Samelak]

I have been getting a lot of backfiring on my 80 CBX during deceleration. I recently bought the bike from a motorcycle shop owner. He tuned the carbs right before I picked it up and supposedly rebuilt the carbs a couple of years ago. The bike starts very easily with choke and sometimes without it. Between 3500 and 4500 RPMS, the engine hesitates, but above that, the motor really takes off and sound great. The previous owner also installed a Stage 1 kit which may be causing some of the problems. What is causing my backfiring and what can I do about it? I am thinking of a carb rebuild by Mike Nixon if it is necessary, but are the carbs still good with the Stage 1 kit?

Lee

[zxbob]

"Usually" popping on decel is caused by a lean condition or your air cut off. Im sure someone with
more knowlage will chime in !

Im not an expert with these carbs . . . . . . ( yet )


Bob

[spencer]

"Usually" popping on decel is caused by a lean condition or your air cut off. Im sure someone with
more knowlage will chime in !
Im not an expert with these carbs . . . . . . ( yet )

Bob

Although I have never experienced it, the most common cause is said to be the air cutoff diaphragm. It's on the side of the #1 carburetor, and can be replaced in about five minutes. Just be careful of the little O-ring. (actually, it is not an O-ring. It has a flat side which I believe should be toward the carburetor.)

[Mike Nixon]

Lee -- The ability to start without the choke doesn't sound like a good thing. But popping on decel can be many things. I assume you don't really mean "backfiring" but rather afterburn in the exhaust. To some degree it's normal, but on the CBX it is pretty rare. First, you really need to get rid of the Dynojet kit. More performance problems have been associated with that product than anything else. Just as important, how clear are the idle circuits? You can confirm on a running bike with an exhaust gas analyzer (EGA), any good car shop and the few really good bike shops will have one. Carbon monoxide (CO) should be between 2 and 3 percent. If under 2 percent, and the HC is high, then you know you have leanness. Then I would ask if you are running an aftermarket exhaust, because they hold more air volume. Then I would ask if you have new exhaust gaskets, very important. Next, the aircut valve, surely, is a good and as already mentioned, easy thing to check. But its causing this problem is rare, usually it's richness when it goes bad. However, if someone worked on it and did not assemble it correctly, again as someone has mentioned, it could result in leanness.

[Samelak]

Lee -- The ability to start without the choke doesn't sound like a good thing. But popping on decel can be many things. I assume you don't really mean "backfiring" but rather afterburn in the exhaust. To some degree it's normal, but on the CBX it is pretty rare. First, you really need to get rid of the Dynojet kit. More performance problems have been associated with that product than anything else. Just as important, how clear are the idle circuits? You can confirm on a running bike with an exhaust gas analyzer (EGA), any good car shop and the few really good bike shops will have one. Carbon monoxide (CO) should be between 2 and 3 percent. If under 2 percent, and the HC is high, then you know you have leanness. Then I would ask if you are running an aftermarket exhaust, because they hold more air volume. Then I would ask if you have new exhaust gaskets, very important. Next, the aircut valve, surely, is a good and as already mentioned, easy thing to check. But its causing this problem is rare, usually it's richness when it goes bad. However, if someone worked on it and did not assemble it correctly, again as someone has mentioned, it could result in leanness.

I appreciate the info. Unfortunately being in Japan I don't know where to get access to an EGA and I really don't speak the language. I figured the Stage 1 kit was causing problems. The previous owner said he had to drill out the carbs for the kit. So are they repairable still? I would like eventually to send the carbs off to you for a resto. The bike does accelerate very well especially above 4500 rpms. It the lower rpms that causes the hesitation. The bike has a DG exhaust, but I will be putting on a brand new Mac exhuast soon. I still have the original pipes, but they don't look very good. I'll try some of your suggestions like exhaust gaskets and aircut valve in the meantime, but I think a proper rebuild is in order.

[Mike Nixon]

Dynojet kits generally instruct to drill out an existing hole in the slides, supposed to make the slide rise faster and thereby mimick a mechanical carburetor. In my experience it hardly makes any difference and going back to stock jetting can be done successfully even with the enlarged hole. Be happy to work with you.

[alimey4u2]

Leaking exhaust may also be a cause, also high ratios of gas stabilizer if used....