deceleration knock with 2 strokes

georgieboy

Member
Jan 2, 2001
416
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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.
 

hallsy

Member
Dec 19, 2005
16
0
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.
 

georgieboy

Member
Jan 2, 2001
416
0
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?
 

76GMC1500

Uhhh...
Oct 19, 2006
2,142
1
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?
 

georgieboy

Member
Jan 2, 2001
416
0
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
 
Jun 28, 2006
94
0
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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76GMC1500

Uhhh...
Oct 19, 2006
2,142
1
Detonation is caused by high pressures/high temperatures in the cylinder, I don't see that being the case with the throttle shut.
 

georgieboy

Member
Jan 2, 2001
416
0
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.
 
Jun 28, 2006
94
0
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.
 

76GMC1500

Uhhh...
Oct 19, 2006
2,142
1
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.
 
Jun 28, 2006
94
0
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.
 

76GMC1500

Uhhh...
Oct 19, 2006
2,142
1
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.
 
Jun 28, 2006
94
0
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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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.
 
Jun 28, 2006
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0
Rich Rohrich said:
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.

Anybody can copy and paste. Yes your in depth scenario is correct but we must hammer out the basic's first.
 
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76GMC1500

Uhhh...
Oct 19, 2006
2,142
1
Quick, delete your post before Rick sees it.

My take on the situation is that the knocking/bucking during decerlation has a lot to do with the way the engine is ported. KTM 200's have a very strong idle, they'll idle for an hour if you let them. I have ridden a KTM 200 or 2 and they do buck a little going into turns (RM250's have always done it the most, though). None of these bikes were my own so I didn't want to adjust anything on them. My CR250 does not knock, buck, or make any weird noises at all going into turns. My bike has almost no idle. When you shut the throttle, it does not fire. The KTM 200's on the other hand do fire randomly/periodically during deceleration. Since it's something that is easily reversed and doesn't take any time or money to try, I suggested the poster simply back his idle down and see if the problem goes away and maybe find a happy medium between a useable idle and no bucking going into turns. Running a step colder spark plug may also have an effect but that could have other negative effects and isn't free to try.
 

Rich Rohrich

Moderator / BioHazard
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Jul 27, 1999
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Chicago
Big Bore Stroker said:
Anybody can copy and paste.

I cut and pasted this from an article (Fuel for Thought) I originally wrote for Dirt Rider magazine in 1985 and rewrote in 1999 for Eric Gorr's website. It's the most referenced fuel article on the internet over the last 5 years. I'm not some clown incorrectly parroting some BS I read somewhere. If you are going to act condescending out here, you really might want to pick your spots a little better. ;)

As the original poster describes the situation, the basics that you are so intent are hammering out are fairly simple. The conditions for spark knock (or ping if you like) are not normally present in a closed throttle situation. Without hearing it I can't say for sure what it is, but I can tell what it ISN'T. ;)


For anyone who is interested in more specifics on knock and some combustion basics in general here is some additional info.

Original fuel article referenced above
http://www.polariswatercraft.com.au/fuelforthought.htm

Additional References I have found very useful.

Harold H. Schobert - The Chemistry of Hydrocarbon Fuels - Butterworth-Heinemann Ltd.

Keith Owen, Trevor Coley - Automotive Fuels Reference Book - SAE - R151

H.P. Lenz - Mixture Formation in Spark-Ignition Engines - Springer-Verlag

Germane, Wood, Hess - Lean Combustion in Spark-Ignited Internal Combustion Engines - A Review - SAE paper 831694

Z. Warhaft - An Introduction to Thermal Fluid Engineering - Cambridge University Press
 
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georgieboy

Member
Jan 2, 2001
416
0
Thx guys, for the info about pre-ignition and flamefronts etc, but still i feel these are not the answers to my, and others, problem.
It seems that there cld be a issue with the way some cilinders are shaped in accordance to the pipe design(?), that are very senitive on lean spots in a/f mixture. That it occures with shutting down the throttle, when there is no load on the piston, is strange though. So, i was thinking of backpressure, scavenging with the a lean a/f from the cilinder, that plays a role here.
What makes me think it more strongly is that there is a spot just at 1/8 of throttle opening that makes the engine surge and accelerate while coasting. Normally a 2stroke will load up at that point ;)
Yr thoughts pls?
 

dirt bike dave

Sponsoring Member
May 3, 2000
5,349
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georgieboy said:
What makes me think it more strongly is that there is a spot just at 1/8 of throttle opening that makes the engine surge and accelerate while coasting. Normally a 2stroke will load up at that point ;)
Yr thoughts pls?

That is probably your powervalve just starting to open. When you don't give it more throttle, the bike slows down again. If you hold the throttle steady, the bike just surges back and forth. This is most noticeable when trying to cruise with steady throttle on a road. I've had several dual sported two strokes that did that, most noticeably a KDX 250 (at about 1/4 throttle). You never notice it in the dirt, as you are never cruising at a sustained low throttle opening.

As to the knocking you originally posted, my guess it is small amounts of unburned fuel igniting in the pipe (probably close to the cylinder). Many bikes do this on decel. But as Rich noted, without hearing it there is no way for us to be certain.
 

georgieboy

Member
Jan 2, 2001
416
0
Yes, i adjusted it down, but not to the point of dead ;) I am waiting for some needles with richer diameters and single tapers, iso the ktm-nozh needles which are tripple tapers. I feel it is a needle problem, but will surely go for the idle screw aswell when i am further testing the needles.
Thx for reminding me, Gmc.

Dave, the accelaration/surge of the engine started way before the powervalve opened, so it is not that. It really was with a barely opened throttle, say 1/8, in 4th gear, and low rpm's. With that throttle opening i want a little fourstroking, and not a surge.
Good that you mentioned it though.

The deceleration sound is in no way a surge or ping, but a real kadang kadang in the pipe, and not in the cilinder. I understand that there are lots more who experience this fenomenon, and live with it. But i know that a engine does not have to do it, although race-engines have a habit of not wanting to slow down. :)
 
Jun 28, 2006
94
0
:worship:
Rich Rohrich said:
I cut and pasted this from an article (Fuel for Thought) I originally wrote for Dirt Rider magazine in 1985 and rewrote in 1999 for Eric Gorr's website. It's the most referenced fuel article on the internet over the last 5 years. I'm not some clown incorrectly parroting some BS I read somewhere. If you are going to act condescending out here, you really might want to pick your spots a little better. ;)

As the original poster describes the situation, the basics that you are so intent are hammering out are fairly simple. The conditions for spark knock (or ping if you like) are not normally present in a closed throttle situation. Without hearing it I can't say for sure what it is, but I can tell what it ISN'T. ;)


For anyone who is interested in more specifics on knock and some combustion basics in general here is some additional info.

Original fuel article referenced above
http://www.polariswatercraft.com.au/fuelforthought.htm

Additional References I have found very useful.

Harold H. Schobert - The Chemistry of Hydrocarbon Fuels - Butterworth-Heinemann Ltd.

Keith Owen, Trevor Coley - Automotive Fuels Reference Book - SAE - R151

H.P. Lenz - Mixture Formation in Spark-Ignition Engines - Springer-Verlag

Germane, Wood, Hess - Lean Combustion in Spark-Ignited Internal Combustion Engines - A Review - SAE paper 831694

Z. Warhaft - An Introduction to Thermal Fluid Engineering - Cambridge University Press

:worship:
 

Pushin50

Member
Dec 18, 2006
136
0
I picked up a new 08 XCW. It has the deceleration knock or ping. The dealer had fueled the bike with regular gas so for the first couple heat cycles the ping was pronounced. I then mixed some premium and that reduced the intensity on the ping. I am now running 110 race gas and the ping has mellowed but it is still there. I dialed down the idle to almost nothing but that had no effect. The ping happens at idle as well as on deceleration. Watching the exhaust as the ping happens it seems obvious that the cause is fuel burn in the pipe. Exhaust gasses are jetted out the pipe when the pinging occurs. On the KTMtalk site others have had the same problem (if it is even a problem). I have not owned a 2t since the Seventies. I am clueless as to a solution for this problem (if it is even a problem). You guys seem to know allot about this stuff so any help will be appreciated.

thanks Pushin 50
 
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