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Discussion Starter #1
Not really a backfire but more of a put, put, put on decel. I think this has been happening since I bought the bike brand new. I did change the needle and jets per Schnitz Racing. Is this put, put sound normal?
 

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I have the same issue, it just developed at 24000 miles. It seems more pronounced when the engine is cold. I will be watching this thread closely.


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Timothys687
More miles and smiles for all!
 

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Decel popping is usually an indication of a lean condition in the pilot circuit.

Have you done the .22 mod? That will richen up the the low speed circuit (pilot jet and fuel screw) and should take care of most of the popping.

Decel popping can also be from a loose exhaust header to cylinder connection. Air will get sucked in and lean out the mixture.

Some popping is normal if you go from full throttle to no throttle. A vacuum will be created and suck fuel and hot exhaust gas into the cylinder and the mixture will ignite randomly.
 

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Popping on decel, usually result of fuel-lean idle mixture, as Spec says above; cured by removing the tamper-resistant plug, bottoming (gently) the fuel screw (underside of carb, engine side), and gacking it out 1.75 turns or so (for stock engine starting point).

Google, "Care and Feeding of the CVK40" for a readable primer on your carburetor.

A separate component, the "Air Cut Valve," or "Coasting Enricher," or "Backfire Preventer" handles mixture management on deceleration in a larger way, but . . . mild popping can often be cured, or greatly reduced, by backing out the fuel screw.

That said, mild backfiring upon deceleration remains operationally benign, IMHO. Don't hurt nothin'!
 

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I had the same condition, went away when I got rid of the air pump. You will need a block of plate. I made my own. I believe Eagle Mike sells them.
 

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Discussion Starter #7
It is not a lean condition as I was running very rich in the Colorado mountains and it still did it. It is just a light put put put when im slowing to a stop. Guess it is no problem.
 

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It is not a lean condition as I was running very rich in the Colorado mountains and it still did it. It is just a light put put put when im slowing to a stop. Guess it is no problem.
Respectfully, tcass, backfiring upon deceleration, conventional wisdom suggests, is symptomatic of a termporarily fuel-lean air/fuel mixture, possible even atop Mt. Everest.

The coasting enricher/air cut valve/backfire preventer acts to minimize this phenomenon, but adjusting the fuel screw plays its role also.

While the, "Back out your fuel screw to 1.75 turns" tip isn't addressed, the primer available at the link below explains how the mixture becomes fuel-lean upon deceleration, and how the coasting enricher/air cut valve/backfire preventer functions as a countermeasure to this condition:

http://www.gadgetjq.com/keihin_carb.htm

Finally, as you mention, slight backfiring upon deceleration ain't no thang!
 

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Mine hasn't popped once since I did the .22 mod and turned my fuel screw out 2-1/4 turns.
This fixed mine.

Only thing different was I did the .11c mod. Only used one washer.. I installed a mixure screw that you can turn with your fingers.
 

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IMHO, I can't see it making the bike fail the smog test but if it did, do you not have some time to get the thing fixed and bring it back for a free second test? This how it works here for cars...here. So IF it did fail you could easily turn the screw back in 1/2 turn for the test.
Setting the screw to2 1/4 turns out from seated is usually the setting needed. BUT, a loose exhaust header will result in a backfiring engine. The donut for the exhaust can get burnt up if left loose. Then it's new gasket time and retorque the exhaust bolts.:13:
 
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