Help Why does my flathead backfire?
- Thread starter Alien
- Start date
fastbraden11
Last of the originals
if i remember correct its unburnt fuel in the pipe
fastbraden11
Last of the originals
ours seem to have ran better when they did have that pop to em,if your fast id leave it
atwoodracing20
Dew Dawg
I hear them do that all the time..normal, due to the amount of fuel we run thru these engines..
jack burroughs
member
Has to do with the ign. firing at TDC area on 2 piston strokes, called, "Lost Spark"?
This probably applies:
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”.
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”.
Trackchampion
Member
A backfire is back through the carb, afterfire is out the exhaust. Not that it makes a difference though, lol
Check your exhaust gasket for a leak.
X2 on that, leaking exhaust gasket can do it also, and a broken pipe will do the same thing if the pipe is broken close to the flange and you cant see it because of header wrap on the pipe
CarlsonMotorsports
Site Supporter
Barring an exhaust leak, popping through the exhaust after backing out of the throttle is quite common on the flathead. It is caused by the carb mixture going real rich (throttle plate has closed off the majority of the air, yet the piston velocity (rpm) continues to create a low pressure inside the cylinder, thus pulling excess fuel through the carb.) The popping is from unburnt (raw) fuel igniting inside the hot exhaust pipe. You would see this very clearly with an egt sensor (which would yield ridiculously high temp reading - ie 1600* keeping in mind that cast aluminum's melting point is around 1300 -- it's easy to determine that the egt reading is of fuel igniting in the pipe rather than true combustion chamber exhaust gas temps.)
The fellow that expanded on this earlier in the thread did a fine job of explaining, as did those that suggested that this "popping" from the exhaust is normal and should consider leaving it alone.
-----
Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
www.CarlsonMotorsports.com
26 years of service to the karting industry
765-339-4407
bcarlson@CarlsonMotorsports.com
The fellow that expanded on this earlier in the thread did a fine job of explaining, as did those that suggested that this "popping" from the exhaust is normal and should consider leaving it alone.
-----
Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
www.CarlsonMotorsports.com
26 years of service to the karting industry
765-339-4407
bcarlson@CarlsonMotorsports.com
alvin l nunley
Site Supporter
this would suggest that uncompressed fuel in the exhaust pipe, unlike the fuel and air in the combustion chamber, burns at a higher temperature.
paulkish
old fart
IMHO, It does not suggest it.
CarlsonMotorsports
Site Supporter
I would not think it suggest any such thing -- only that the temp probe is IN the combustion when it ignites in the pipe. I think that the proximity to the actual "ignition" is the reason you see a super high egt reading is all.
added info: Also the reason that you need to be consistent in placement of your egt to have valuable data.
added info: Also the reason that you need to be consistent in placement of your egt to have valuable data.
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