Popping/backfiring with Two Bros. exhaust.

First of all, let me apologise for not doing a huge search. - I only have web access through my iPhone and it is painfully slow.

Anyway, I have installed my new Two Bros. M-2 pipes (which sound AWESOME!) but there is a huge amount of popping under deceleration.
I have already done the PAIR mod (blocked the hose with a ball bearing and disconnected the plugs, as per the Johnny Chadda guide) so what else can I do to eliminate the backfiring. Or have I done something incorrectly?

 

Aftermarket pipes reduce back pressure which results in increased air flow. Increased airflow without adjustment to the fuel delivery can cause the bike to run lean unless it has been rejetted or remapped. The same thing can also happen if your exhaust system has leaks since extra air is being introduced into the system through the leak. What year is your bike and is it stock other than the exhaust?


Rollin

 

Popping is timing- period. Either correct the condition (finish your mod project with the appropriate ignition program) or undo whatever mod has created the condition before damage to other system components turns play money into repair money. Anytime you have ignition with an open intake valve (popping/backfiring) you risk damage ranging from fried sensors & blown intake gaskets to a full on fire.

 

@Rollin - It's brand new and it is stock.

@82nd - Hmmm...I found this in another thread:


Burn Baby Burn

The last thing I want to address today is the subject of deceleration backfire, or “popping”. This topic generates a lot of concern from inexperienced riders, or even from experienced guys who just hate the noise, so lets take a look at what causes it. But first things first, lets define the issue:

Deceleration Backfire is caused by fuel burning in the exhaust manifold or header.

No ifs ands or buts, that’s what causes it. But the bigger question is how does gas get there in the first place, and that’s a bit more complicated. Generally, there are a variety of ways it gets there, and a variety of things that can make the backfiring worse. But there’s a kicker, and something you should understand before we go any farther:

A motor in perfect tune will exhibit deceleration backfiring.

Therefore, just because your motor is banging it up, doesn’t mean there’s anything wrong. And consequently:

Getting rid of the noise means de-tuning your motor.

Yup. If you’ve jut got to eliminate that popping, you’ll have to accept the fact that your motor is going to be forced to run rich to do it, and that isn’t necessarily a good thing. So lets talk about what causes the problem.

Ok, so you’re riding along at some given rpm, and suddenly you decide to decelerate, and you reduce the amount of throttle. This causes an “overrun” – that is, the motors rpm is turning faster than the fuel provided can support, so the motor begins to spool down. This causes a couple of things to happen.

First, when you close the throttle, you are also closing the throttle plates. This reduces the air and fuel flowing into the motor, and increases the vacuum (lowers the pressure). This results in less air and fuel in the cylinder during the power stroke, which in turn results in a lower pressure in the combustion chamber. Remember I said earlier, that the A/F mix burns faster in proportion to the pressure applied? Well, when we reduce pressure this way, the mix burns slower. This results in two things happening.

1. The lower burning fuel generates less heat, and the cooling effect of the non-burning fuel tends to “quench” the flame front, or slow it down even further. Because the mix is burning much slower, the exhaust valve can open before all the fuel is consumed, and the unburnt fuel is ejected into the exhaust.
2. The engine designers, in order to promote smoother idling and better combustion, retard the spark when the throttle is shut, and this results in the mix being lit later.

So, now we end up with unburnt fuel in the exhaust, and burning fuel being ejected into the exhaust, and bang! Backfire. In addition, Honda has added a device called a “programmed air injection valve” (Pair Valve) that actually injects some fresh air into the exhaust to help this process along – since fully burning the fuel results in cleaner exhaust. So the backfiring is not only a normal part of the engines operation, it’s also intentionally amplified by Honda! Of course, normally, that massive bazooka pipe Honda hangs on your bike hides most of the noise, but it’s there, even when you can’t hear it.

So the bottom line, is: That backfiring is perfectly normal and expected. If you’ve just got get rid of it, that’s up to you. You’re entitled to set your motor up the way you want, and your goals are your goals. But don’t refer to it as “fixing” the popping. Rather, the correct way to think of it is “de-tuning a bit to get rid of the popping”.

There are a few ways you can do this.

First, use the stock pipe. It will hide the sound, by absorbing it into mass, and masking it with the larger baffle space. Second, you can add more fuel during deceleration. This has the effect of raising the chamber pressure slightly, which burns a little more before the exhaust valve opens. Lastly, you can remove the Pair valve, which reduces the amount of available oxygen in the pipe to burn the unburnt fuel.
http://www.vtxoa.com/forums/viewtopic.php?t=28918


Now, if I understand this rightly, some popping (due to unburnt fuel in the headers) is completely normal. This is not the same thing as detonation.
The writer of that article is saying that ALL bikes pop/backfire under deceleration; we just don't hear it because the OEM exhaust systems do a good job of muffling it.

I'm not sure what to think now.

 

Brand new GSX or VFR ?

 

Thanks for posting that explanation, Rainbow7.

 

Duccman - new VFR. (I can't update my details; the site won't let me access them, for some reason.)

 

When I first got my 2009 VFR800 the PO had put on the Two Bros Slip On cans and I was experiencing the same thing. I left the PAIR system intact but changed to a K&N air filter, de-snorkeled the air box and removed the air box flapper vacuum actuator (not to be confused with the PAIR valve), plugged the vacuum hose and secured the flap to stay open. This seemed to calm the backfiring down, or maybe it was just my imagination. I have since put in a Power Command V with an off the shelf map (soon to be officially Dyno Tuned) and I do not notice any back firing now. I assume that you are still running with the stock air filter and still have the air box snorkel in place? What year is your VFR?

 

Stock filter and snorkel intact.

2009 VFR but brand new from dealer.

 

Sounds like the same setup I started with. Personally I would de-snorkel the air box and either remove the flap or secure it in the open position and get a better flowing air filter, BMC, K&N, etc. and see if that doesn't calm down the back firing. Worse case scenario your bike will breathe a little better and you'll have a reusable air filter but be mindful in heavy rain fall. (NOTE: You can do these with non-destructed means. i.e. You can return it to stock rather easily)

When you said you disabled the PAIR system, were you talking about the vacuum actuator on the air box? Just so you know, that is NOT the PAIR system. The PAIR system is a whole other animal. Some people remove it others leave it. I chose to leave it.

Right now, apart from having my bike bike Dyno Tuned with my PCV installed, I'm pretty satisfied with just the 2 Bros and air box mods.

 

OOTVS, can you please do me a favor and post a picture for me of your 900SS, had one and sold it awhile back and still regreting it . Would love to see one again-Thanks

 

Just for Duccman

Here's a couple for you...

 

Although i believe you have identified the popping source, how about doing a little test to verify that you have closed-throttle lean-popping?

To do this, make a run down the road in 2nd gear to get the revs up, then chop the throttle closed, but don't let it shut all the way--leave it slightly open and off the stop so that the TPS (throttle position sensor) still indicates a POT (partially-open throttle) condition. Try it in 3rd gear or higher too.

If the TPS senses a completely closed throttle position on overrun then the mixture is adjusted leaner than if the TPS senses POT, and you will probably be able to hear it in the exhaust note. i'm betting that the popping will be reduced on POT overrun.

 

Rainbow7's post is the best write up I've seen on the subject. With that said. It doesn't mean you may be to lean when off throttle. This will also create more heat in the engine. Richening up enough ,pilot circuits, to keep the popping towards the minimum side is a good thing for a street bike.

Not sure how the FI works pilot circuits for adjustments. It may be worth experimenting with. I know it is with carbs.

 

Im a new member and hope Im doing this right. I just bought a 2002 VFR with 14500K on the clock. Great bike. Only mod is a Two Brothers Slip on. I took her out for the first time yesterday and she ran good until she got warmed up at 200F or so, then began to get harder to start unless I bumped the throttle as I was hitting the starter button. I also felt like her power went away a bit as well as the exhaust seemed not as smooth at idle. Not super knowlegable of these powerplants. I also recieved the stock exhaust cans with the bike. Any ideas?

 

Welcome racn,

Might i suggest that you will get better help if you copy your post above, then paste it in a new thread of your own in either the mechanics garage or the 5th generation forums, and add some additional words about the age of the battery, the battery voltage at rest, at idle and at 5000 rpm, etc. After you do that then come back here and delete this post and monitor/subscribe to the one that you start.

 

:welcome: to the madness Racn45
++++++1 to what Kenny said :thumbsup:
Create a threat introduce yourself and ask question many will chimp in for suggestion

 

Rainbow7, thanks for finding the post that explains the deceleration noise. My car does this (Dodge Neon SRT-4), and honestly I love the sound, it's beastly, and after 120k miles it still does it and has been a non issue. Would love to hear it on your VFR, got any clips or video by chance?