deceleration knock with 2 strokes

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#1
What is deceleration knock?
Is it addressed to squish heights?
Pilot jets?
Pipes?

I have read about it, but nobody seems to have the answer.
Maybe someone can give his experience about it.
Someone who has cured a decel knock succesfully.
 
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#2
I'm curious also, I didn't have a knock but do have an issue with deceleration roughness or surge. I can adjust the air screw in (richer) and eliminate the roughness/surge but then I lose some of the crisp throttle response. I tried a larger pilot but that didn't work.
 
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#4
Perfect. No problem there.
It is after closing the throttle letting it coast into a corner it starts to bang in the pipe.
Very annoying as it accelerates everytime it bangs.
I know there is a lot of knowhow on this board in the names of Rich, Marcus and offcourse Eric, but this banging is never clearly been discussed as far as i know or can find.
Is it jetting(needle) related, squish height related, pipe related, Engine modification related????
Is it a combustion of eddy's in the compress-chamber or is it occuring in the pipe itself?
 
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#5
I asked if your bike idles because it seems that bikes set up to idle are more likely to do this. Try adjusting the idle so that you only get a few seconds after you close the throttle when the bike is hot. This may stop the bucking/banging.

I think the actual bang is caused by unburned fuel/air burning in the pipe. The cylinder doesn't fire when you shut the throttle, therefore, some fuel/air gets pushed into the pipe. Periodically, the cylinder will light and that will ignite the fuel/air in the pipe.

You don't ride an RM, do you?
 
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#6
76Gmc1500,
Ooo i see.
Will adjust the idle to die, first thing tomorrow as it is midnight overhere.
I ride a very new bike....Ktm 200exc. It is an enduro bike, so normally you want it to idle slowly.
Very lovely bike, but not jetted correctly. They come ex factory with a low hp package and you have to change all the jets etc to get it working to its potential.
Still i feel i have to address the lean jetting issue due to the banging.
Thx for thinking with me, though. Much appreciated.
George
 
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#7
georgieboy said:
What is deceleration knock?
Is it addressed to squish heights?
Pilot jets?
Pipes?

I have read about it, but nobody seems to have the answer.
Maybe someone can give his experience about it.
Someone who has cured a decel knock succesfully.

Deceleration knock is the same as detonation it's can accur under deceleration and acceleration. Knock occurs because the pressure and temperature in the cylinder are high enough when combustion is occurring that the unburned fuel/air mixture spontaneously ignites. Some things that can cause this are combustin temps, ignition timing, fuel grade, engine load , compression etc. If the bikes runs great everywhere else it could be just some jetting issues that would need to be addressed. Could start by going one size richer on the pilot jet or maybe the needle, but we don't what your current jetting spec are. Any engine mods?
 
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#8
Detonation is caused by high pressures/high temperatures in the cylinder, I don't see that being the case with the throttle shut.
 
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#9
Strange thing is, when i coast to a stop with the clutch in the knocking is just a few moments and then it stops knocking. With clutch out, so really using the back pressure of the engine to coast it is at its worse.
So downhill on the motor it knocks badly.
I quess a lean needle spot.
 
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#10
Deceleration knock is the same as detonation it's can accur under deceleration and acceleration. Knock occurs because the pressure and temperature in the cylinder are high enough when combustion is occurring that the unburned fuel/air mixture spontaneously ignites. Some things that can cause this are combustin temps, ignition timing, fuel grade, engine load , compression etc. If the bikes runs great everywhere else it could be just some jetting issues that would need to be addressed. Could start by going one size richer on the pilot jet or maybe the needle, but we don't what your current jetting spec are.

What if any engine mods (porting,head work etc)? Have you changed any jets in the carb? What pipe set up? Are you over reving the engine while coasting downhill? Try to coast in a higher gear and then down shift after in the corner.
 
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#11
Yes, detonation occurs only during periods of high pressure/high temperature in the cylinder. With a closed throttle, there is no high pressure. If you had high cylinder pressures, your bike would still be making power and still be accelerating. The knocking during deceleration is not detonation. It's periodic firing of the cylinder and unburned fuel igniting in the pipe.
 
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#12
76GMC1500 said:
Yes, detonation occurs only during periods of high pressure/high temperature in the cylinder. With a closed throttle, there is no high pressure. If you had high cylinder pressures, your bike would still be making power and still be accelerating. The knocking during deceleration is not detonation. It's periodic firing of the cylinder and unburned fuel igniting in the pipe.
Your last sentence describes detonation, some what. Detonation is two flame fronts colliding and it doesn't matter when they colliding acceleration or deceleration.
 
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#13
Pinging is two flame fronts colliding and is caused by hot spots in the combustion chamber. A seperate flame front starts elsewhere in the combustion chamber and collides with the flame front from the spark plug making a high pitched "ping" noise. Your dirt bike is so loud, you'd likely have a hard time hearing any pinging. Detonation is the explosive ignition of fuel/air under very high pressures/temperatures. Detonation's sound is best described as shaking a can full of rocks and is definately audible over the bike if it becomes severe. Pinging can lead to detonation because the 2nd flame front can cause a sharp rise in cylinder pressure similar to running too much spark advance. But, the two are not the same. None of these have anything to do with fuel burning in the pipe.

If you want to hear detonation, try running a hill climb with your fuel petcock off. Your bike will run into severe detonation by the time you hit 3rd gear. If you're lucky, you'll kill the motor before you melt the piston.
 
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#14
76GMC1500 said:
Pinging is two flame fronts colliding and is caused by hot spots in the combustion chamber. A seperate flame front starts elsewhere in the combustion chamber and collides with the flame front from the spark plug making a high pitched "ping" noise. Your dirt bike is so loud, you'd likely have a hard time hearing any pinging. Detonation is the explosive ignition of fuel/air under very high pressures/temperatures. Detonation's sound is best described as shaking a can full of rocks and is definately audible over the bike if it becomes severe. Pinging can lead to detonation because the 2nd flame front can cause a sharp rise in cylinder pressure similar to running too much spark advance. But, the two are not the same. None of these have anything to do with fuel burning in the pipe.

If you want to hear detonation, try running a hill climb with your fuel petcock off. Your bike will run into severe detonation by the time you hit 3rd gear. If you're lucky, you'll kill the motor before you melt the piston.
Pinging is a slang word. Pinging is detonation. Let me back up here pinging or knocking is the after effects of detonation. detonation causes the structure of the engine to ring, or resonate, pinging or knocking is the noise/vibration that is carried through the engine. Ever driven a car or truck with bad plug wires or timing issues and the car/truck makes that pinging/rattling sound while under a load? That means detonation is accuring. There could be more serve cases of detonation that could change the sound but detonation is simple two flame fronts colliding period.
 
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Rich Rohrich

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#15
Big Bore Stroker said:
but detonation is simple two flame fronts colliding period.
No it isn't.

The Knocking sound you hear when an engine is experiencing abnormal combustion is the result of a rapid pressure rise in the cylinder. Unburned mixture and the end gases at the edges of the combustion chamber are being raised to extremely high temperatures as the advancing flame front compresses and heats up the mixture directly in front of it. This activity before the flame front reaches the end gases at the edge of the chamber are sometimes called pre-flame reactions. The longer it takes for the complete burning to take place the greater the chances that these pre-flame reactions will force the end gases to reach the auto ignition point and cause a rapid uncontrolled pressure rise, along with a huge increase in cylinder temperature. If brought to the auto ignition point the end gases of the combustion chamber can cause a pressure and frequency rise that is high enough to be audible. That's the KNOCK or PING that you hear. Ideally, the burning of the mixture will be completed before any of these end gases have an opportunity to reach the point of auto ignition.

As was already pointed out, if you close the throttle you reduce the load and it makes it extremely unlikely that abnormal combustion will occur.

What the original poster is describing sounds like a mechanical issue of some sort not a combustion issue.