Trying to understand
Outstanding HDF Member
Joined: Jun 2011
Posts: 2,023
Likes: 6
From: New Jersey
I have an 08 Road King Classic, no mods to engine or exhaust. the bike runs well and I'm getting good mileage, 41-48 mpg. But, I have a lot of popping when I decelerate. Without spending a fortune, what is the best fix for this?
Am I correct in my thinking that the cause of the problem is that the engine is running lean?
Or shouldn't I worry about this and I can leave it as is?
I appreciate your help, I'm pretty ignorant about this.
Am I correct in my thinking that the cause of the problem is that the engine is running lean?
Or shouldn't I worry about this and I can leave it as is?
I appreciate your help, I'm pretty ignorant about this.
Here's what Mikuni has to say on the subject:
"Backfiring in exhaust system when the throttle is closed, especially noticeable from high RPM, is not caused by lean mixtures, although lean mixtures can contribute to its intensity. This is a normal condition when using high performance exhaust systems."
Here's an excerpt I kept from a post on the VTXOA forum:
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, thats what causes it. But the bigger question is how does gas get there in the first place, and thats 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 theres 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, doesnt mean theres anything wrong. And consequently:
Getting rid of the noise means de-tuning your motor.
Yup. If youve jut got to eliminate that popping, youll have to accept the fact that your motor is going to be forced to run rich to do it, and that isnt necessarily a good thing. So lets talk about what causes the problem.
Ok, so youre 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, its also intentionally amplified by Honda! Of course, normally, that massive bazooka pipe Honda hangs on your bike hides most of the noise, but its there, even when you cant hear it.
So the bottom line, is: That backfiring is perfectly normal and expected. If youve just got get rid of it, thats up to you. Youre entitled to set your motor up the way you want, and your goals are your goals. But dont 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.
I hope this helps.
Tourer
Joined: Jul 2011
Posts: 354
Likes: 0
From: Indianapolis, IN
Another way to look for exhaust leaks is to run it somewhere dark and look for the "fire". My friend had a loose exhaust at the rear cylinder and when I saw the flash under his *** when we were riding. Torqued the bolts down and eliminated backfire. I personally like a little popping when I'm idling down and the post above explains why it's not necessarily a bad thing. You can also check your exhausts. When cool stick a finger in and barely touch the inside of the pipe, and see if there's a lot of residue.
Outstanding HDF Member
Joined: Jan 2010
Posts: 2,916
Likes: 31
From: earth
FenderGuy53, Thanks for sharing the external sources, Iv'e known about the normal popping for a long time but it's hard to convince others that a little bit of popping is actually normal.
another way to check is to hold a cigarette below the connection and let the smoke float up and around the seal, it will be real obvious if you got a leak.
another way to check is to hold a cigarette below the connection and let the smoke float up and around the seal, it will be real obvious if you got a leak.
Cruiser
Joined: Nov 2010
Posts: 135
Likes: 1
From: Ottertail, MN
Good article above on what causes "normal" popping. I get this too sometimes and what I've found on mine is that the exhaust flange nuts at the head work themselves loose over time. Every couple of weeks I snug them up and the popping goes away. Using a search on this topic here in the forum will give you losts of good info. I found links for self-locking nuts to help with my problem, just haven't got to it yet.
Thanks for all the replys - the information you all have shared is very helpful. I check the exhaust when I get home tonight.
Regarding my pipes - I was under the impression they were stock. I'll also check to see if they are in fact rush slash/cut slipons.
Thanks again!
Regarding my pipes - I was under the impression they were stock. I'll also check to see if they are in fact rush slash/cut slipons.
Thanks again!
From the pic they look like Rush mounting bosses. Knowing what size baffle will help to identify the issue. On a stock 08, you should be running the 2.00" or smaller baffle. The 1.50" and 1.75" can be run with no tuning, and usually the 2.00." Anything larger will require a tuner.
Our slipon mufflers use thicker ga steel than stock and most manufacturers. This means you should tighten the exhaust clamp to it's max of 65 ft/lbs of torque to ensure a good seal. An easy way to check is place the bike outside on a sunny day, look down the exhaust from the outlet. If you see any light there is a leak.
I have also seen the y on the rear exhaust crack, it can get glowing hot on a stock bike and the metal can fatigue.
As some other members stated, there is a huge difference between a backfire and normal exhaust sounds on decel. There should be what I call an exhaust flutter on decel, no guns going off.
Feel free to PM me or call our techs.
-Ryan
Our slipon mufflers use thicker ga steel than stock and most manufacturers. This means you should tighten the exhaust clamp to it's max of 65 ft/lbs of torque to ensure a good seal. An easy way to check is place the bike outside on a sunny day, look down the exhaust from the outlet. If you see any light there is a leak.
I have also seen the y on the rear exhaust crack, it can get glowing hot on a stock bike and the metal can fatigue.
As some other members stated, there is a huge difference between a backfire and normal exhaust sounds on decel. There should be what I call an exhaust flutter on decel, no guns going off.
Feel free to PM me or call our techs.
-Ryan
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