Motorcycle Forum banner

Exhaust backfire, popping when letting off throttle

337K views 66 replies 31 participants last post by  Trials 
#1 · (Edited by Moderator)
I notice that when I am accelerating 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 to pop or backfire during idle, cruising speeds, or under acceleration. The only time I'm getting it is if I open up the throttle to speed up rapidly, then let off the 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?
 
#2 ·
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.
 
#6 · (Edited)
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".
 
#7 · (Edited)
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.
 
#10 ·
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.
 
#14 ·
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)
 
#15 ·
"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.
 
#42 ·
"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.

You just nailed one of the main reasons kept my Carbed Dyna .. :)
 
#16 ·
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.
 
#17 ·
popping

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
 
#21 ·
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)
 
#19 ·
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.......
 
#25 ·
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.
 
#29 ·
Hello, I'm new here.
My dad has a 1992 CBR 900rr, and he got a Yoshimura slip-on on it (I think its a rs3, but I'm not sure, as I've never seen one like that, at least not with the same plate
" Yoshimura
R&D something
Not for road use")
The shop where the exhaust was fitted, my dad told me, they fiddled" with it ( "afinaram-na" in portuguese), but I don't know what they did to it, but they sais it gave the bike 7hp more.
The bike doesn't have" cats, never had.
My dad bought it when it came out, rode it some years, left it asside for arround 20 years, and now, I'm grown, and he wanted me to experience it. We took the gas out of the tank, as it was left there, and we sent it to a shop to make it work. It came back working, but idk what they did to it. When he took me for a ride, I noticed that everytim we deaccelerated the bike made lots and lots of pops from the exhaust, barely audible when riding thaks to windo noise, but one time I took my phone with me ans recorded the sound. Its not just 3 or 5 as someone sais before, it make pops as long as we are deaccelerating, and it makes them various times a second.
I thought it was pretty cool and didn't worry about it, as the mechanics said they had "fiddled" with it (idk if fiddle is the right word to use, as I want to say tune, but idk if I can say that with a carburated engine). I just started worrying about this right now, as I read all the entries in this thread when I was looking for a way to make my frien's duje 125 do exactly that.
So, I just want to know if those pops and bangs (which I love) hurt the engine in any way. Not because of the bang itself but, as said before, if the engine is running lean or something.
My dad almost doesn't ride it, and when he does he takes me with him, ans we go for about a 15 min. ride.
I love the pops, the bangs and the great deep loud sound the bike+exhaust procuce, so if it doesn't harm the engine, I'm not foing to change it.
I'm sorry if I was to boring explaining things that might be irrelevant for you, but I want to make sure that everything that has happened is understood and so maybe someone can give me an answer.
 
#30 ·
One way to tell if it is lean: put in fresh spark plugs, and ride at a steady speed for several miles, then look at the color of the spark plugs around the inside tip. If they are still white, it is lean, if a smooth gray-brown, lighter near the tip, it is OK. What type of fuel you use will affect the color, but there should be some color coating around the tip.
 
#32 ·
Ans one more thing, how can it make pops while not being delovered enough fuel? Shouldn't the pops occur when the engine is running rich, where the excess fuel may go into the exhaust?
Oh, and the bike still uses the stock headers, and they are black, idk what they are made of, so they don't have any particular colour
 
#33 ·
You asked if it might be lean enough to damage the engine, so I gave a method to find out. If the plugs are too white, the operating mixture is lean, and, if lean enough, can cause problems.

When the mixture is too lean, it doesn't burn properly, so fuel goes into the exhaust and explodes there. When you close the throttle, the mixture tends to become lean. Most newer carburetors have a valve that increases the fuel when you let off the throttle, to help prevent after fire pops. The pops can also come from fresh air being drawn back into an open exhaust, disturbing the exhaust extraction and causing any unburned fuel to ignite; leaking header seals can cause this too. The pops could also be normal for this engine, and a louder exhaust just allows you to hear them.
 
#34 ·
Sometimes they pop via the air injection also, I know the ex500's Kawi's when you took the stock cans off you would remove the air injection also to prevent it from popping. Just thought I would throw my two cent out there on that.
 
#35 · (Edited)
Sorry, I like this stuff, engines, etc but I still have mostly everything to learn when it comes to carburators and many things, as you may have already noticed.
And I don't know what that air injection thinggy is that you talked about. I just googled it, and, in a chevy forum, they said it was a way for the catalytic converter to work better...? If it is , then I don't believe the bike's got that, as I can't see any cats on the exhaust system, just 4 into 2 into one piping and then the slip on.
And thanks for the fast response you're giving, and again, sorry for not knowing what certain things are.
And thank you, WintrSol, for explaining why that happens.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top