Afterfires are from running rich, not lean. Air goes through the motor no matter what because the valves are mechanical and the opening and closing doesn't change, except for vtec but that's beside the point. When there is extra fuel, like bikes are tuned to have, there is only enough air to burn most of the fuel and the unburnt fuel is ejected out of the motor and into the exhaust. The PAIR lets air into the exhaust on decel with the intention of lighting off the unburnt fuel within the confines of the stock exhaust system thereby creating little to no noticable afterfire.

When you swap the exhaust out, you are increasing the flow of exhaust gasses, moving the little explosions further down the pipe and muffling them less. With the PAIR disabled, you are no longer allowing air into the exhaust on deceleration so the ony way for the fuel to burn is to travel all the way through the exhaust until it reaches fresh air. Your popping on decel is the effect of that. More than likely you are shooting flames out of your exhaust as well each time you let off the gas. A custom fuel map will keep the A/F ratio within the correct range and do away with most or all of your problem.

 

I have to disagree and refer back to my "lean" explanation. When running rich "Un burnt fuel vapor" is releasedthrough the exhaust valve and out with exhaust emmisions, Once the exhaust vale open and releases the "unburnt fuel" it will not reignite causeing a back fire or popping.There is no source of ignition to re ignite the "unburn fuel".Explosionsor back fires are results of too much air mixed with not enough fuel.

You ever dropped a match into a gas tank it goes out, but when you hold a match over the vapors they ignite. IF you move a match close tosmall container of fuel it will ignite the air directly above the fueland a falme will burn steady, it will not explode.

Remember this is a 4 cycle engine, one cycle of the pistonis ignition, the next is the opening of the valves, releasing "unburnt fuel" and exhaust gases. once this fuel leaves the ignition chamber it won't reignite. The term rich came from the "rich smell of gas in the exhaust fumes. So therefore if it was "exploding" and popping, i.e burning off...they never would have coined the term, because there wouldn't have been a strong smell of gas.

 

Do you think I just make this stuff up? Who tells you these things?

I assume that you have seen common movies like Fast and Furious or some street racing vids or something. I know that most of it is fake but the flames blowing out the back of the cars is real. In order to do that, you simply turn your amount of fuel up until flames shoot out. It happens in cars that have non-boost-based fuel controllers or when the intake pressure is vented to the atmosphere on deceleration.

The flashpoint of fuel vapor is far lower than the internal temperature of the exhaust, thus the fuel will ignite supposing there is enough oxygen to mix with. If there is not enough fuel (lean), the fuel that is present will burn with the appropriate amount of air and the leftover air simply gets heated up and leaves through the exhaust valves and then out the exhaust with no further drama. There is no fuel left, so the air is incapable of burning with anything.

If there is too much fuel, the available air is used and there is leftover fuel vapor in the cylinder that is past its flashpoint but has no air to mix with, thus preventing combustion. As soon as this heated fuel vapor contacts air, it is ignited. That is the entire purpose of the PAIR system. It introduces air to the exhaust on decel to burn the exiting unburnt fuel vapor so as to remain within the federal guidelines for the exhausting of unburnt fuel vapor. When the exhaust is changed to not have a cat or much muffling capacity, the popping of the igniting fuel vapor within the exhaust is more audible. This always happens, it's just that Honda doesn't engineer the bike for free flowing exhaust.

Anyway, when you let off the throttle at high RPM, or even lower RPM, the fuel injectors are spinning so fast that they cannot stop flowing fuel the exact instant that you let off the throttle. Your throttle plates, however, are connected to the throttle grip via a cable and mechanically close exactly with the turn of your wrist. The resulting lack of air and increase in vacuum pressure leaves more fuel than air in the combustion chamber. The air is fully used, and like I said above, the unburnt fuel vapor that is left over enters the exhaust well past its flashpoint. As the vapor encounters air in the exhaust, from the next cycle of the cylinder or as it leaves the exhaust, it will combust. The combustion can be very small and heard as slight popping, which is normal, or it can be quite volatile and result in flames shooting out and loud popping, which is not normal but indicates a temporary rich condition.

An adjustment lowering the amount of fuel entering the system on decel is needed.

Please research this on your own prior to arguing with me. Your assumptions about AF ratios could cost somebody serious amounts of money. I'm not trying to call you out, but the kid asked for help and you are telling him the opposite of what is going on. And yes, as a racer and an engineer, I am aware that this is a four cycle engine jackass.

 

First of all the forum screwed up and the message didn't get through before you could repost this long unnecessary post but i will retype it and hope it works this time,

I never once gave any advice saying what to do or not to do that would cost any one money. Second i too aman engineerand (jet engines to be exact)i dealwith suck, bang, blow, theory all day. and air fuel mixtures.

What i tried to say before you got all huffy and puffy and resorted to name calling, Was that bothof of our theorys are correct, aboutlean and rich (ness) can cause back fireing, when you closea throttle after a had pull it createsan increase in vaccum pressure (we both agree on) however when that valve closes from the throttle it is not air tight it has to allow some air in for idle reasons, but the increase vaccum is still sucking some air in on top of theincrease amoutn from wide open throttleamounts(even more with mods)the only part i disagree with you is the injectors. I say they stop before air flow, you say they stop after airflow. They are controlled by electronic pump , the air entering into an engine is controlled by "fluid dynamics"i.e. vaccum and the flow of airpressures (not instant), anyways the increase air is going to cause popping within the engine as the engine continues to make revolutions (until vaccum pressures decreas) popping occursbecause it is sucking more air, but injectors are reducing fuel causeing a lean mixture which more oxy means bigger more powerful burns.,

Yes you are rigth with increased fuel it will cause a backfire and flames. NOW back to your claim about fast and the furious cars... i agree with you to some extent, but most of those cars blowing flames are due to boost, because when you boost a car your adding more pressure into a cylinder with air causeing a bigger more powerful burn. or these cars are running with NOS which all it does it increase a high mixture of oxygen into the mix again causeing bigger more powerful burn. Fuel doesn't burn without oxygen the more oxygen you get the bigger the burn or explosion. Flames occur because no car is 100% efficiant and there will always be unburnt fuel but because ofbeing a lean running engine, it is also a hotter running engine andthis means hotter exhaust to burn unspent fuel causeing flames.

We are both rigth so back to myoriginal solution "TAKE IT TO HAVE IT RE-MAPPED"that advice is not goign to cause him $$$$ because it solves your theory and mine. so unless there is something else you want to bring up let me know.

 

Man you are missing some information.

First, a BACKFIRE is combustion within the intake tractof the engine usually associated with a lean condition on carburated vehicles. The slow burn due to the lean condition can "chase" the flame back out of the intake valves because the flame gets behind the timing, so to speak. That's why carbs need to be cleaned and set. Maybe that's what your thinking of.

An AFTERFIRE is what we are talking about, caused by a rich condition. An afterfire takes place in the exhaust tract, and cannot be caused by a lean condition. The flames on boosted cars are from running purposefully rich without compensatory boost-based AF controllers. If I make 15 pounds of boost at WOT at 5000 RPM, then I tune my fuel system to deliver that amount of fuel. Obviously, I can only create that amount of boost in high gears and do not always need that much fuel. In lower gears or while sitting still, much more fuel is delivered to the combustion chamber than is required and is burned when it reaches a source of air, which in most cases is the end of the exhaust pipe. The flames are from not having enough boost to support the amount of fuel entering the chamber, otherwise known as rich. Air cannot simply burn on its own, otherwise the whole world would explode every time I light a cigarette.

There is no possible way that lean conditions can create afterfire. Nor does the amount of air in the combustion chamber have any capability to light off in the exhaust without fuel. I thought it was common knowledge that a temporary rich condition is created on immediate deceleration. There will always be fuel spilled because one of the cylinders is always on the intake stroke when the throttle plates are closed. The computer has already calculated the fuel and then the cylinder gets half the air that it was supposed to get due to the intake vacuum upon throttle release. Depending on the speed of the processor in the fuel computer, this can continue for several cycles depending on the RPM at the time.

12,000 RPM means 6000 RPM injector speed, times four cylinders means that theinjectorsare firing 24,000 times per minute. That's every 2.5 milliseconds. Can your fuel controller make adjustments to intake pressures and throttle setting in less than 2.5 milliseconds? If your computer has a response rate of 10 milliseconds (which is extremely fast), the engine will have one stroke on each cylinder that recieves fuel without the appropriate amount of air during a 12,000 RPM throttle-off because the throttle plates are closed and the intake will havevacuum pressure after only one stroke by one cylinder. More realistic would be about 100 milliseconds for a factory controller and 50 for an aftermarket controller. The reason the aftermarket controllers are faster is that they use less information to determine the amount of fuel needed as well as that they do not share processor space with other systems as a vehicle would.

I do not doubt that you have abilities as a jet mechanic. The correlation between the two is non-existant, with the exception that they both turn air and fuel into propulsion. I would encourage you to research vehicle engine operation due to the vast differences.

And I only called you a jackass because you asked if I knew that this was a 4-stroke engine, lol. I take it back.

 

just for the record i am not disagreeing with what you say. or trying to prove you wrong, i beleive you and am not saying i am right by any means.i am at this point only debating the possiblilty of lean cauing an "after fire" the only part i don't think is quite right...You say the flames only appear because they touch the air at the exhaust tip. I think the flames are coming from deeper in. I don't think you have the sources needed for ignition (fuel,oxy,source)at the exhaust tip. The gas exiting is not hot enoughnor' themuffler to ignite when it touches the air. You would need an ignition source. (thats why show cars install electrobes to create the ignition source.) they rev the car up a few time then hit the button and the flames shoot.

My theory is, a lean condition creates more oxy, mixing with the alreadyunburnt fuel from the "rich" cylinders (within the exhaust chambers). But because there is more air for (1) revolution it is enough to make a "high air" to "normal unburnt fuel"ratio. this give you 2 of 3 parts needed for ignition, and the ignition source would be the hot gases and exhaust chambers directly exiting the cylinders. causeing the flame. just a thought.

A side note?? we know this happens on modded cars because most likely they won't have cats. but if you modd a car with stock cats would you still get as big of flames??

 

My bike is on the rich side. My friend told me to hook the pairmod back up.

 

Don't waste your time with Ty. He's a proven bull****ter and enjoys posting lengthy replies filled with speculation and technobabble. I'm not sure he understands anything about how motorcycles work. I wonder how he defines the word engineer. I find it hilarious that he of all people has the nerve to ask, "Do you think I just make this stuff up? Who tells you these things?" Yes, Ty makes things up, and when asked where he gets his crazy theories, he's remarkably silent. For example, he pitched this idea that the intake flapper (in stock form) has a sort of variable position function and will partially open at low RPM with heavy throttle. This is complete BS, but it didn't stop him from taking it to the next level by describing it's role in an impossible sequence of events that he called a "rigged dyno run". I asked him at least thee times where he got this idea of the variable position flapper, but he offered absolutely nothing. No names or sources, not a shred of evidence. He just made it up.

 

1-how do you know it's rich?2-why did he say to do this?

 

I don't think it's too much to worry about it. I get it on most of my bikes after I do something with the exhaust. It's lounder the higher the rpms that you come down from and yeah cool lookin flames shoot out, but on normal deceleration braking, cornering it burbles I dont' think its anything to worry about.

Now it you are not decelerating and your just reving the throttle and letting off and it backfires then I think that would be something to worry about.