I have always ridden 2 strokes for my woods, offroad, CC,HS. Recently purchased a 08 crf450r and have performed most of the mods to make it more suitable for woods riding. I like tight technical single track trails. My first outing on the 450 was not what I had hoped. I stalled the bike numerous times while riding my usual style. The motor was recently rebuilt (5 hrs ago) by a very competent and well respected mechanic. The bike flat out hauls the mail... I have a standard clutch (not auto) on the bike. I have been reading a lot of topics on the forum and keep hearing about flame out and stalling. Could this be my issue and if so what can I do to limit this very annoying condition. I really don't want to go with a auto clutch. So any help or info would be greatly appreciated.....
What is flame out?
Started by marley25, Jan 30 2012 09:22 PM
8 replies to this topic
Posted 30 January 2012 - 09:22 PM
I have always ridden 2 strokes for my woods, offroad, CC,HS. Recently purchased a 08 crf450r and have performed most of the mods to make it more suitable for woods riding. I like tight technical single track trails. My first outing on the 450 was not what I had hoped. I stalled the bike numerous times while riding my usual style. The motor was recently rebuilt (5 hrs ago) by a very competent and well respected mechanic. The bike flat out hauls the mail... I have a standard clutch (not auto) on the bike. I have been reading a lot of topics on the forum and keep hearing about flame out and stalling. Could this be my issue and if so what can I do to limit this very annoying condition. I really don't want to go with a auto clutch. So any help or info would be greatly appreciated.....
Posted 31 January 2012 - 02:29 AM
Flame out is a slang term for a too lean fuel mixture hesitation. You can recognize this by the poping/backfiring though the carburetor. You need to determine if it is hesitating when you're twisting the throttle, or at a steady throttle. If it hesitates when you twist the throttle, it's probably the accelerator pump not squirting enough fuel each time you twist the throttle. If you're going into a tight section with steady throttle and the rpm's are low and it just stalls, then it may be lean on the pilot circuit (pilot jet or fuel mixture adjustment screw).
I noticed in your post that you rode 2S previously. Don't know your riding style, but 2S are more forgiving than 4S . If you tend to get a little frisky with the rear brake, the 2S will recover easier when the brake is released. Whereas the 4S stroke won't likely recover if the rear brake is released, it will just slide to stop. As far a fuel mixture goes, the 2S engine due to it's design has excess hydrocarbons (fuel) left over after each stroke, that lean hesitation is not likely. But 4S are very efficient/clean, and a lean condtion will show up at the most inopportune time.
If it stalls/heistaes when you twist the throttle, then it sounds like it's the accelerator pump not squirting enough fuel. This can be due to the accelerator pump linkage adjustment on the side of the carburetor. You can pull the black plastic cover off of the right side of the carb. Slowly twist you accelerator and make sure the linkage is pushing down on the accelerator pump plunger at the sametime. Another possibility is the accelerator pump diaphragm is bagged/worn out, or it may be ruptured. Yet another possibility is the leak jet is too big. When you twist the throttle and the carb linkage pushes down on the plunger, the accelerator pump diaphragm pushes fuel through the tiny passage to squirt fuel out of the nozzle into the throat of the carb. Since you can't compress liquid, a certain amount of fuel is bled off through the leak jet back into the bowl. This is done to keep from rupturing the accelerator pump diaphragm. If the leak jet is too big, too much fuel is bled off, and you will have a lean hesitation when you twist the throttle. One quick test you can do to see if your accelerator pump is squirting at all, is to pull the carb off of the engine with the cables in place. Twist you throttle and see of fuel squirts out of the carb bore. You should twist it at different speeds. Even a defficient pump can squirt if you hit it quickly. Make sure it squirts at slow twisting motion too.
One thing to be careful of is putting too big of a pilot jet to mask a poor accelerator pump system. You can do this, but your compensating for another problem. And too rich of a fuel mixture can lower the hp numbers. It's an art to get the pilot, acclerator pump squirt, metering rod and main to all come together like a symphony. Then when you do get it right, the dogone wheather changes.
This is one reason I like 2 strokes or fuel injection. Sorry about the long write, but there is so much more you could get into. This should be enough to make you hate that darn thing.
I noticed in your post that you rode 2S previously. Don't know your riding style, but 2S are more forgiving than 4S . If you tend to get a little frisky with the rear brake, the 2S will recover easier when the brake is released. Whereas the 4S stroke won't likely recover if the rear brake is released, it will just slide to stop. As far a fuel mixture goes, the 2S engine due to it's design has excess hydrocarbons (fuel) left over after each stroke, that lean hesitation is not likely. But 4S are very efficient/clean, and a lean condtion will show up at the most inopportune time.
If it stalls/heistaes when you twist the throttle, then it sounds like it's the accelerator pump not squirting enough fuel. This can be due to the accelerator pump linkage adjustment on the side of the carburetor. You can pull the black plastic cover off of the right side of the carb. Slowly twist you accelerator and make sure the linkage is pushing down on the accelerator pump plunger at the sametime. Another possibility is the accelerator pump diaphragm is bagged/worn out, or it may be ruptured. Yet another possibility is the leak jet is too big. When you twist the throttle and the carb linkage pushes down on the plunger, the accelerator pump diaphragm pushes fuel through the tiny passage to squirt fuel out of the nozzle into the throat of the carb. Since you can't compress liquid, a certain amount of fuel is bled off through the leak jet back into the bowl. This is done to keep from rupturing the accelerator pump diaphragm. If the leak jet is too big, too much fuel is bled off, and you will have a lean hesitation when you twist the throttle. One quick test you can do to see if your accelerator pump is squirting at all, is to pull the carb off of the engine with the cables in place. Twist you throttle and see of fuel squirts out of the carb bore. You should twist it at different speeds. Even a defficient pump can squirt if you hit it quickly. Make sure it squirts at slow twisting motion too.
One thing to be careful of is putting too big of a pilot jet to mask a poor accelerator pump system. You can do this, but your compensating for another problem. And too rich of a fuel mixture can lower the hp numbers. It's an art to get the pilot, acclerator pump squirt, metering rod and main to all come together like a symphony. Then when you do get it right, the dogone wheather changes.
This is one reason I like 2 strokes or fuel injection. Sorry about the long write, but there is so much more you could get into. This should be enough to make you hate that darn thing.
Posted 31 January 2012 - 07:38 AM
^WALKINGWOUNDED^ I am not the OP but I thank you for the great explanation. I have gained even more knowledge about my bike.
Posted 31 January 2012 - 08:52 AM
You're welcome. I woke up with a migraine and heartburn, so I figured I might as well take advantage of the sleeplessness.
Posted 31 January 2012 - 07:26 PM
After the bike is jetted properly a +17oz fliywheel weight will reduce stalling and make the bike easier to ride.
Posted 31 January 2012 - 08:05 PM
Thanks WW that gives me some good points to check and yes I do have a FWW ready to be installed......
Posted 01 February 2012 - 10:19 AM
@walkingwounded,
WOW!!! That was great explanation! Thanks for the knowledge! I didn't' quite understand the hole leak jet purpose, but I can certainly understand it now.
WOW!!! That was great explanation! Thanks for the knowledge! I didn't' quite understand the hole leak jet purpose, but I can certainly understand it now.
Posted 02 February 2012 - 07:55 AM
WALKINGWOUNDED, on 31 January 2012 - 02:29 AM, said:
Flame out is a slang term for a too lean fuel mixture hesitation. You can recognize this by the poping/backfiring though the carburetor. You need to determine if it is hesitating when you're twisting the throttle, or at a steady throttle. If it hesitates when you twist the throttle, it's probably the accelerator pump not squirting enough fuel each time you twist the throttle. If you're going into a tight section with steady throttle and the rpm's are low and it just stalls, then it may be lean on the pilot circuit (pilot jet or fuel mixture adjustment screw).
I noticed in your post that you rode 2S previously. Don't know your riding style, but 2S are more forgiving than 4S . If you tend to get a little frisky with the rear brake, the 2S will recover easier when the brake is released. Whereas the 4S stroke won't likely recover if the rear brake is released, it will just slide to stop. As far a fuel mixture goes, the 2S engine due to it's design has excess hydrocarbons (fuel) left over after each stroke, that lean hesitation is not likely. But 4S are very efficient/clean, and a lean condtion will show up at the most inopportune time.
If it stalls/heistaes when you twist the throttle, then it sounds like it's the accelerator pump not squirting enough fuel. This can be due to the accelerator pump linkage adjustment on the side of the carburetor. You can pull the black plastic cover off of the right side of the carb. Slowly twist you accelerator and make sure the linkage is pushing down on the accelerator pump plunger at the sametime. Another possibility is the accelerator pump diaphragm is bagged/worn out, or it may be ruptured. Yet another possibility is the leak jet is too big. When you twist the throttle and the carb linkage pushes down on the plunger, the accelerator pump diaphragm pushes fuel through the tiny passage to squirt fuel out of the nozzle into the throat of the carb. Since you can't compress liquid, a certain amount of fuel is bled off through the leak jet back into the bowl. This is done to keep from rupturing the accelerator pump diaphragm. If the leak jet is too big, too much fuel is bled off, and you will have a lean hesitation when you twist the throttle. One quick test you can do to see if your accelerator pump is squirting at all, is to pull the carb off of the engine with the cables in place. Twist you throttle and see of fuel squirts out of the carb bore. You should twist it at different speeds. Even a defficient pump can squirt if you hit it quickly. Make sure it squirts at slow twisting motion too.
One thing to be careful of is putting too big of a pilot jet to mask a poor accelerator pump system. You can do this, but your compensating for another problem. And too rich of a fuel mixture can lower the hp numbers. It's an art to get the pilot, acclerator pump squirt, metering rod and main to all come together like a symphony. Then when you do get it right, the dogone wheather changes.
This is one reason I like 2 strokes or fuel injection. Sorry about the long write, but there is so much more you could get into. This should be enough to make you hate that darn thing.
I noticed in your post that you rode 2S previously. Don't know your riding style, but 2S are more forgiving than 4S . If you tend to get a little frisky with the rear brake, the 2S will recover easier when the brake is released. Whereas the 4S stroke won't likely recover if the rear brake is released, it will just slide to stop. As far a fuel mixture goes, the 2S engine due to it's design has excess hydrocarbons (fuel) left over after each stroke, that lean hesitation is not likely. But 4S are very efficient/clean, and a lean condtion will show up at the most inopportune time.
If it stalls/heistaes when you twist the throttle, then it sounds like it's the accelerator pump not squirting enough fuel. This can be due to the accelerator pump linkage adjustment on the side of the carburetor. You can pull the black plastic cover off of the right side of the carb. Slowly twist you accelerator and make sure the linkage is pushing down on the accelerator pump plunger at the sametime. Another possibility is the accelerator pump diaphragm is bagged/worn out, or it may be ruptured. Yet another possibility is the leak jet is too big. When you twist the throttle and the carb linkage pushes down on the plunger, the accelerator pump diaphragm pushes fuel through the tiny passage to squirt fuel out of the nozzle into the throat of the carb. Since you can't compress liquid, a certain amount of fuel is bled off through the leak jet back into the bowl. This is done to keep from rupturing the accelerator pump diaphragm. If the leak jet is too big, too much fuel is bled off, and you will have a lean hesitation when you twist the throttle. One quick test you can do to see if your accelerator pump is squirting at all, is to pull the carb off of the engine with the cables in place. Twist you throttle and see of fuel squirts out of the carb bore. You should twist it at different speeds. Even a defficient pump can squirt if you hit it quickly. Make sure it squirts at slow twisting motion too.
One thing to be careful of is putting too big of a pilot jet to mask a poor accelerator pump system. You can do this, but your compensating for another problem. And too rich of a fuel mixture can lower the hp numbers. It's an art to get the pilot, acclerator pump squirt, metering rod and main to all come together like a symphony. Then when you do get it right, the dogone wheather changes.
This is one reason I like 2 strokes or fuel injection. Sorry about the long write, but there is so much more you could get into. This should be enough to make you hate that darn thing.
EXCELLENT DUDE!
Posted 02 February 2012 - 04:58 PM
Since there has been a few veiws and replies, I would like to expound on why you need an accelerator pump, and why an engine will "lean pop"/"flame out" when lean.
Accelerator Pump:
In theory a good fuel mixture ratio is 14.7 to 1. That being 14.7 parts of air and 1 part of fuel. When you bike is setting there idling with this good air/fuel mixture, and you twist the throttle, you are lifting the slide in the carburetor. This gives the engine a momentary gulp of air (lean condition) before the fuel comes through the jets, intake port and into the combustion chamber. The accelerator pump makes up for the momentary lack of fuel and delivers a squirt of fuel to go along with the gulp of air to try and keep the mixture correct, and not have a "lean pop" or "flame out". Once the accelerator pump gets the engine past the hesitation, the correct jetting keeps the engine going. The accelerator pump only squirts when you twist the throttle. On a related subject they make a carburetor bowl that has an adjustable leak jet screw on the bowl. Instead of pulling the bowl off of the carburetor to change the leak jet, you simply turn a screw to change the amount of fuel bleeding back into the bowl, therefore changing the amount of fuel squirting when you twist the throttle. This bowl also comes with a "moat" surrounding the main jet to help eliminate fuel sloshing away from the main jet on hard landings. I don't know that the 450's had that problem, but there were some of the 250f's that did. Here is an article about the R&D bowl. http://motocrossacti...N-FCR-2124.aspx
Lean Pop/Flame Out:
A correct air/fuel ratio is easier for the spark plug to ignite it. It's good for a fast, even and complete combustion of the air/fuel. When the mixture is lean as in when you hit the throttle with a poor/no acclerator pump, the lean mixture can burn slow enough that the fuel is still burning when the intake valve opens again. That will kill any incoming air/fuel and give you the "pop" through the intake and carburetor, and the dreaded stall. You can also get a backfire through the exhaust with a lean mixture too, and with the right conditions and backfire/pop out of both. Lean conditions can some strange problems, and can do damge to an engine as well.
Hope this gives a little better understanding of these two items. Sometimes it's hard to put things on paper. I see things in my mind like I'm part of it. I'm the fuel in the carb or the electrons flowing through the wiring. It's weird I know. Thanks for listening.
Accelerator Pump:
In theory a good fuel mixture ratio is 14.7 to 1. That being 14.7 parts of air and 1 part of fuel. When you bike is setting there idling with this good air/fuel mixture, and you twist the throttle, you are lifting the slide in the carburetor. This gives the engine a momentary gulp of air (lean condition) before the fuel comes through the jets, intake port and into the combustion chamber. The accelerator pump makes up for the momentary lack of fuel and delivers a squirt of fuel to go along with the gulp of air to try and keep the mixture correct, and not have a "lean pop" or "flame out". Once the accelerator pump gets the engine past the hesitation, the correct jetting keeps the engine going. The accelerator pump only squirts when you twist the throttle. On a related subject they make a carburetor bowl that has an adjustable leak jet screw on the bowl. Instead of pulling the bowl off of the carburetor to change the leak jet, you simply turn a screw to change the amount of fuel bleeding back into the bowl, therefore changing the amount of fuel squirting when you twist the throttle. This bowl also comes with a "moat" surrounding the main jet to help eliminate fuel sloshing away from the main jet on hard landings. I don't know that the 450's had that problem, but there were some of the 250f's that did. Here is an article about the R&D bowl. http://motocrossacti...N-FCR-2124.aspx
Lean Pop/Flame Out:
A correct air/fuel ratio is easier for the spark plug to ignite it. It's good for a fast, even and complete combustion of the air/fuel. When the mixture is lean as in when you hit the throttle with a poor/no acclerator pump, the lean mixture can burn slow enough that the fuel is still burning when the intake valve opens again. That will kill any incoming air/fuel and give you the "pop" through the intake and carburetor, and the dreaded stall. You can also get a backfire through the exhaust with a lean mixture too, and with the right conditions and backfire/pop out of both. Lean conditions can some strange problems, and can do damge to an engine as well.
Hope this gives a little better understanding of these two items. Sometimes it's hard to put things on paper. I see things in my mind like I'm part of it. I'm the fuel in the carb or the electrons flowing through the wiring. It's weird I know. Thanks for listening.
