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Old 04-26-2009, 08:37 PM   #1
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Default Exhaust backfire, popping when letting off throttle

I notice that when I am accellerating, if I let off the throttle, my exhaust backfires and pops.

What causes this?

My muffler is not packed and has no baffles. Idle is 1500rpm. I never have popping or backfire during idle, cruising speeds or under accelleration. Only time I'm getting it is if I open up throttle to speed up rapidly, then let off throttle.

Is this damaging to my engine?

It's like a rapid succession of pops/bangs... like 3 to 5 of them.. Or is this its normal sound?
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Old 04-26-2009, 11:45 PM   #2
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I have the same problem. However, I've also noticed the backfiring when I decelerate from highway speeds without pulling in the clutch lever, just letting the engine spin down.

I talked to a friend of mine that knows a LOT about engines n stuff and he said it was not damaging to my engine.....just some extra gas that didn't get burned and ended up in the pipes and due to the heat of the pipes ignited making the POP.

I'm not sure how true that is but sounds reasonable.
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Old 04-27-2009, 01:06 AM   #3
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Check your plugs to be sure the fuel mixture is correct. You can damage the engine by running too lean or rich.
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Old 04-27-2009, 01:59 PM   #4
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Quote:
Originally Posted by Dodsfall View Post
Check your plugs to be sure the fuel mixture is correct. You can damage the engine by running too lean or rich.
x2 I also found this posted on the forum you might find it useful its a breakdown about spark plug wear.http://www.dansmc.com/Spark_Plugs/Sp...s_catalog.html
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Old 04-27-2009, 06:25 PM   #5
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this happens in bikes with carburetors, not fuel injected, maybe you need to see if it's tuned right...
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Old 04-27-2009, 06:39 PM   #6
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Quote:
Originally Posted by Prodigy View Post
this happens in bikes with carburetors, not fuel injected, maybe you need to see if it's tuned right...
uh oh... my FI does this too.
I was told something similar to what Jethrol said... which makes sense to me. I heard that MC's are a little more inefficient at burning gas? no?

popping only started to occur after I removed the baffle from my stock muffler. I also told that the "popping" was always there and that the muffler (in its entirety) muffled the "popping".
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Old 04-27-2009, 06:54 PM   #7
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It's a matter of degrees. Large pops can signify a mixture that's way off, while some bikes may pop a little at peak efficiency.

Removing the baffles will generally require a jet change or remap. Performance is hindered or engine damage can occur if it's not done properly.

The easiest way to gauge is by checking plugs. If you want to get it perfect, you should use a dyno or use data taken from a dyno with your exact setup from a reliable source.
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Old 04-27-2009, 07:08 PM   #8
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Ok my mistake, I thought popping was due to bad fuel mixture... since FI motorcycles are controlled by a computer I thought that it wouldn't apply.... well I've learned something!
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Old 04-27-2009, 07:38 PM   #9
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Changing things like reducing the exhaust back pressure or reducing the intake pressure will confuse the computer. It's assuming a certain fixed amount of air in and exhaust out in order to mix the fuel correctly.
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Old 04-27-2009, 09:59 PM   #10
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Diagnosing problems via the Internet can be almost impossible at times, but Dodsfall nailed it right on as far as I can think. Remove the baffles and you increase airflow through the engine, and you have a lean condition. Once you re-jet, (assuming we're right) and things are running right, do what he suggested and check your plugs AGAIN. You may need to change the plugs to another heat range.

Also, for what it's worth, I've seen a similar type of popping symptom that was the result of a defective electronic ignition module on a carbureted bike.
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Old 04-27-2009, 10:01 PM   #11
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I've heard so many bikes pop on deceleration that I thought it was normal. As long as it doesn't hurt the engine, I say keep it... I kinda like the sound anyway.
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Old 04-28-2009, 12:31 PM   #12
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Quote:
Originally Posted by Eye_m_no_angel View Post
Diagnosing problems via the Internet can be almost impossible at times, but Dodsfall nailed it right on as far as I can think. Remove the baffles and you increase airflow through the engine, and you have a lean condition. Once you re-jet, (assuming we're right) and things are running right, do what he suggested and check your plugs AGAIN. You may need to change the plugs to another heat range.

Also, for what it's worth, I've seen a similar type of popping symptom that was the result of a defective electronic ignition module on a carbureted bike.
So, this "re-jetting"... is that expensive? difficult? specialized work? or is it a DIY thing over the weekend?
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Old 04-28-2009, 12:34 PM   #13
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Quote:
Originally Posted by super32 View Post
So, this "re-jetting"... is that expensive? difficult? specialized work? or is it a DIY thing over the weekend?
It involves removing and at least partially disassembling the carbs. Depending on the bike, it can be fairly expensive to have done. If you are comfortable around wrenching, it's totally do-able over a weekend.
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Old 04-28-2009, 12:38 PM   #14
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I probably should add that the ECM needs to be reprogrammed or replaced in an EFI bike, since there are no carb jets involved with those. Aftermarket modules such as a Power Commander are fairly popular for this, since they can be bought for about the same price as a single adjustment to the computer at a dealer. (About $200-$300)
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Old 04-28-2009, 09:49 PM   #15
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"So, this "re-jetting"... is that expensive? difficult? specialized work? or is it a DIY thing over the weekend?"

On my bike (HD) it's about a 30 minute job if I stop for a cup of coffee and a cigarette. (Not counting running checks and plug evaluation.) I have heard that some of the metric bikes however, can be real nightmares to get the carbs off, but I have no experience working on them. The concept itself, is very easy.
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Old 04-29-2009, 05:48 AM   #16
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Backfiring happens all the time, in cars and bikes, you just don't hear it because the muffler/exhaust. Since you've modified it, now you do. What it is, is incomplete combustion, fuel is left over, and shot into the exhaust pipes. The heat from the pipes then ignites the vapor and boom, backfire. Not uncommon, should happen when decelerating in gear, the explosions aren't as violent, and don't get as much gas burned, so left-overs happen.

If you have it happening at times other than when you are decelerating, then yes, you are running to rich and need to adjust that, or you should perhaps check your timing, you might be experiencing detonation, which can sometimes not burn allt he fuel up, as it fires to early preventing the peak and optimal explosion.
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Old 05-04-2009, 10:41 PM   #17
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my vulcan has always popped a little on deceleration since the day I brought it home from the dealership im pretty sure some popping is normal since im all stock and the bike is still running well after 8 or 9 years that's aweful im questioning how long I've had the bike oh well its popped for ar least eight years and yes it does sound cool
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Old 02-19-2014, 02:15 AM   #18
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check the spark plug..
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Old 02-19-2014, 07:18 AM   #19
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So many things it could be, a lot depends on what model motorcycle.

Too lean, as mentioned, is generally the cause. A vacuum leak is not uncommon.

Plugged slow jets not uncommon (and that can cause other major problems. If the bike never did it, sat a long time and started popping, that's more of a possibility).

Incorrect air screw settings can sometimes cause it, I've been able to tune it out (or hide it..) with an air screw adjustment in some cases.

Many bikes have a emission control system (PAIR, for example) that injects air into the exhaust, sometimes removing/blocking these corrects the issue.

Then, some people like it, and learn to get really good at making the POP loud, optimized. It can be a lot of fun at other people expense, which in the big picture, is not true fun at all.......
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Old 02-19-2014, 08:45 AM   #20
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Popping on decel is a lean condition. Many things can contribute to a lean condition as noted above, but two other possible causes not mentioned are

1. exhaust gasket leak
2. a free flowing (performance) exhaust
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Old 02-19-2014, 08:58 AM   #21
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Quote:
Originally Posted by mhdchehade View Post
check the spark plug..
I'm hoping "5 years later" the guy who originally started this topic has figured out the problem

But yeah if you alter the exhaust backpressure by either swapping out the pipe for an aftermarket one or by removing the factory baffles/restriction, then you need to either rejet (carb) or adjust the ECU (if fuel injected)
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Old 02-19-2014, 09:53 AM   #22
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"I'm hoping "5 years later" the guy who originally started this topic has figured out the problem "

LOL!
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Old 02-19-2014, 10:33 AM   #23
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And no one mentioned that the spark plug also fires on the exhaust stroke too. That also contributes to the popping.
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Old 02-19-2014, 03:38 PM   #24
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of course he did but there some people that are googling for solution just like me
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Old 02-20-2014, 10:06 AM   #25
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There is no adjustment for a FI ECU. The OE ECU is preset and runs according to OE build of engine. You can get a power commander and have it programmed to correct settings or get a power pro commander (CVT) continuous variable tuning which self adjusts to changes in air/fuel mixture. Nice unit made by Cobra but does cost more then a power commander. Easy install all plug-n-play.
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Old 02-20-2014, 10:36 AM   #26
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Quote:
Originally Posted by super32 View Post
So, this "re-jetting"... is that expensive? difficult? specialized work? or is it a DIY thing over the weekend?
I did a ninja 250 a couple years ago, if you know what jet sizes you need (pilot and main usually) and whether or not you need any needle shimming you can order just what you need and that is often $5/cylinder or less. If you dont know what you need you may have to buy a set of jets and experiment ultimately not using most of the set. That can increase the cost.

http://www.dynojet.com/jetkits/ has a lot of kits for a lot of bikes that come with several jets so you can get exactly what you need.


The jets themselves are often (always?) brass the only specialized work is uncrewing them without marring the brass. This is not hard if you use the correct sized screwdriver and take your time. Oh and inspecting to make sure you dont have any insect parts in the carb bowls before final reassembly :P

You might have to adjust the idler screws which on some are capped because of emissions controls. This is not hard to do either, usually you drill the plugs out just go really slow with the drill to avoid punching through to what is underneath.

Since you will probably remove the carb to do this work you might as well sync it or at least check the sync to see if it needs any work.
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Old 02-20-2014, 10:38 AM   #27
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Quote:
Originally Posted by Eye_m_no_angel View Post
"I'm hoping "5 years later" the guy who originally started this topic has figured out the problem "

LOL!
I am bad about reading meta data I just recall seeing this thread pop up a couple times and figured I would read it. doh!
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Old 02-20-2014, 11:02 AM   #28
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No worries. Like the guy said, somebody else might get something out of it even though the original poster solved his problem long ago.


(Unless, of course, he's still staring at it and scratching his head.)
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