Help! Backfire on decel.

[bug klr]

I have a 2006 with 685 kit uni filter and staintune exhaust and I cant get it to stop poping when engine braking, it will also do it when reving the engine in neutral.
Things I have tried, I replaced the headpipe gasket, tried stock exhaust, I cleaned the carb (which looked perfectly clean already) I have the thumbscrew for easy adjustment to the air/fuel screw? And I have tried adjusting that anywhere from 3/4 turns out to 3 1/2 turns out.
The carb has the dynojet needle with the clip third from the top, I tried changing the main jet to 140 from 136 and that did not help. Oh the slide is drilled to 7/64 as well and the valves are set to the open end of specs. Anything I am missing here? I have another carb that I think I will try next and if that one does not do it I have the other klr I can swap carbs with but I would rather figure out the problem and fix it. Thanks and any help would be great

[SgtMarty]

There is an air cut-off valve on the side of the carb that can sometimes fail and this often leads to backfiring. I don't know the details of it, but it's another place to look.

[bug klr]

Originally Posted by SgtMarty

There is an air cut-off valve on the side of the carb that can sometimes fail and this often leads to backfiring. I don't know the details of it, but it's another place to look.

Hey Thanks Marty I will try swapping carbs and see if it fixes it, if it does I will try and research the valve you speak of and fix/replace it in the current carb. Thanks again and I will let ya know what happens

[jeffsaline]

I'm gonna bet it's your exhaust sucking air.

Good luck with your troubleshooting.

Best,

Jeff

[jhampshire]

Turn the idle adjustment to the high setting, I think 1400 RPM. Less suction is created in this case.

[tofire409]

Mine was doing that due to a missing heat shield muffler bolt.

[bug klr]

Originally Posted by jeffsaline

I'm gonna bet it's your exhaust sucking air.

Good luck with your troubleshooting.

Best,

Jeff

Thanks, I thought this to that is why I replaced the headpipe gasket but it did not seem to change.

[bug klr]

Originally Posted by jhampshire

Turn the idle adjustment to the high setting, I think 1400 RPM. Less suction is created in this case.

I do have my idle set to between 14 and 1500 rpm, when you say less suction what does that mean?

[tofire409]

I'd check for leakage from the head to tail. There could be a hole ( very loose screw) in the pipe (where it's not supposed to be). Other then that, I can't offer any more ideas.

Good luck, I hope you find the problem.

[mudflats]

Originally Posted by jhampshire

Turn the idle adjustment to the high setting, I think 1400 RPM. Less suction is created in this case.

Ditto.

idle setting made a big difference in minor backfiring on my stock bike.
And it's free!

[jhampshire]

Originally Posted by bug klr

I do have my idle set to between 14 and 1500 rpm, when you say less suction what does that mean?

Well, if what you are describing is an exhaust popping rather than backfiring throught the carb, raising the idle speed just allows more air into the cylinder when the throttle is closed. Carbs tend to go mixture rich when vacuum is high but air flow is low. This is what happens when you chop the throttle at speed. Your cylinder is still asking for a large volume of air but the carb is restricting air flow.

You can test this theory by holding the throttle open enough to prevent popping.

Now...if you truly have a backfire situation going, a flame is regurgitated back into the intake and you will get a pop in the carb. The main duty of intake valves is to keep the flame in the cylinder. If there is ignition for any reason while the valve is open, you will get backfire. Extreme lean mixture, incorrect timing, leaking intake valves, can cause backfire. Usually backfire occurs under heavy loading or acceleration. I don't think you are having this in an engine that runs properly in all other respects.

[Chuck B]

Originally Posted by bug klr

I have a 2006 with 685 kit uni filter and staintune exhaust and I cant get it to stop poping when engine braking, it will also do it when reving the engine in neutral.
Things I have tried, I replaced the headpipe gasket, tried stock exhaust, I cleaned the carb (which looked perfectly clean already) I have the thumbscrew for easy adjustment to the air/fuel screw? And I have tried adjusting that anywhere from 3/4 turns out to 3 1/2 turns out.
The carb has the dynojet needle with the clip third from the top, I tried changing the main jet to 140 from 136 and that did not help. Oh the slide is drilled to 7/64 as well and the valves are set to the open end of specs. Anything I am missing here? I have another carb that I think I will try next and if that one does not do it I have the other klr I can swap carbs with but I would rather figure out the problem and fix it. Thanks and any help would be great

I'm fairly new to KLR world but being that you are in Calif per chance your KLR has AIS? I know V2 KLR's come equipped with it but not sure on V1 Ca models. AIS, particularly when mods are done, can cause exactly what you are describing.

[bug klr]

Originally Posted by jhampshire

Well, if what you are describing is an exhaust popping rather than backfiring throught the carb, raising the idle speed just allows more air into the cylinder when the throttle is closed. Carbs tend to go mixture rich when vacuum is high but air flow is low. This is what happens when you chop the throttle at speed. Your cylinder is still asking for a large volume of air but the carb is restricting air flow.

You can test this theory by holding the throttle open enough to prevent popping.

Now...if you truly have a backfire situation going, a flame is regurgitated back into the intake and you will get a pop in the carb. The main duty of intake valves is to keep the flame in the cylinder. If there is ignition for any reason while the valve is open, you will get backfire. Extreme lean mixture, incorrect timing, leaking intake valves, can cause backfire. Usually backfire occurs under heavy loading or acceleration. I don't think you are having this in an engine that runs properly in all other respects.

I should not say backfire, it is popping out the exhaust, if I turn the idle up to 2000 then it is harder to make it pop but the problem is still there.

[bug klr]

Originally Posted by Chuck B

I'm fairly new to KLR world but being that you are in Calif per chance your KLR has AIS? I know V2 KLR's come equipped with it but not sure on V1 Ca models. AIS, particularly when mods are done, can cause exactly what you are describing.

It is a California bike but I dont think it has the AIS and whatever it did have for smog crap fell off a while back

[harleymav]

I have the same problem and trying to come up with a fix as well. I have been told, not yet tested though, I may be running to rich. The extra fuel is what is actually backfiring on decel.... what color is your plugs? They say its a quick way to check for richness....

[HenryJ]

I had the same back firing problem. I followed all the tips above with no help for my condition. Enriching the mixture, checking head pipe bolts, idle speed. All worthless.
I finally found the problem!

Mine too is a CA emissions bike. The canister has been replaced with a tool kit. The "tee" vacuum line back at the carb had been capped off and the rubber cap was cracked. Now it all make sense. During high vacuum conditions created under deceleration it was going lean. Enriching had helped the condition a little as did raising the idle speed, but it never did stop it.

I would suggest looking for an intake side vacuum leak. Spraying carb cleaner around and listening for a change in rpm can be a good diagnostic tool. Dry rotted rubber parts are suspect. Check the intake adapter very closely too.

I replaced that cap with a tight fitting nylon replacement. Now I can not get it to backfire. What a relief. I have been able to turn the pilot down and recover some fuel mileage too, I hope. It just runs right now. I can make it "putter" a little on deceleration, but it will not backfire. That is how it should be.

Don't give up. There is an answer.