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Basic, Simple, Jetting Question?

husabutt

husabutt

Joined Feb 2003
3K Posts | 384+
Escondido, Ca. USA
If when raising the air speed or pressure through the venturi (venturi signal?), by means of increasing displacement, a leaner setting is required (everything else remaining equal). Then why is it necessary to sometimes richen the A/F mixture when the displacement remains the same but a free flowing exhaust is installed? Or in the instance of a two stroke a higher flowing reed valve assembly. Doesn't a free flowing exhaust all by itself also increase the venturi signal? And therefore a leaner setting would be required?

This part of jetting process seems quite contradictory to me.
 
Ok let's tackle this one thing at a time, starting with your second example the new modified exhaust.

In the classic case of installing an after market exhaust system that has less resistance to exhaust flow then the stock exhasut, in that case the SAME motor sees less "back pressure" and can pump more combustion product( exhaust) out the pipe. The Motor is like a dumb air pump if you will. It functions within it's surroundings, so less resistance to flow means more flow therefore you need richer needles to allow more fuel too. the richer needles are slightly larger orifices with allows more fluid flow into the carb....OK that is the easy example.

In the example about the Larger displacement bike needing smaller jets then a smaller displacement bike........recognize you have changed the fundamentals of your question now you are comparing two different size motors not two different exhausts on the same motor as in the first example. Answer to your first example of the two different motors is simple: Two different Motors and Carburetors and exhausts need differnt stock set ups - but in the BERG 450 vs 650 example. the 650 is a BIGGER pump if you will and it wants to suck the world in and pump it out so you have to restrict that baby differently then the 450 given the carb and displacement.

does that help?

I could refine that all to being better said, but if you get it from above then I said it well enough.

any one else want to add to this? Dale?
 
bigbob said:
.

does that help?
Click to expand...

Hi Bob:

Sort of but not really :) . I was studying up on this topic and the web site I was on stated that on some highly tuned bikes (also talking about a two stroke) the jetting will need to be leaned from the standard setting.

It seems that perhaps with a pipe only you are somehow getting a increased volume of cylinder filling, volumetric efficiency, at the sacrifice of venturi signal and that may be why a richer jetting configuration may be needed. Just my guess. It still puzzles me :?
 
you are bluring examples again.

Take any "highly Tuned" bike. get to the web site carb tune references. One of them has a A/F mixture chart and graphs that show at different throttle positions/rpms the mixture is changing and generally it moves from slightly leaner to richer back and forth thru the powerband. The game of jetting is getting excat theoretical A/F stochiometric proportions in the motor at all times. ( with refinements of more detail)Typically More highly tuned bike is set up to run more lean and less rich thru the powerband.............it gets more power and performance that ways, but you run the ragged edge....a too lean motor get hotter and can cause damage.so the engineers at the factory tend to ship there bikes out a little rich for safety except in the case of some mfrs who ship the bikes way lean for EPA.....the bottom line is most home garage tuners will settle in on a slightly rich tune condition.......only the professional tuners can dial it in to that peak performance spot and it's usually more lean then the avg garage bike is set up for. hence the guys say a "Highly Tuned" bike is lean, get it?

your second sentance is confusing parts of the machine with others in the air flow stream. I need more time to sort thru what you said,
 
Husabutt, there are so many variables at work here that blanket statements are ineffective. For instance, an engine that is really prepped for optimum performance can endure leaner jetting because it's set up to eliminate some of the production line variables found on non-prepped bikes. An example of this would be the shape, fit, and finish of the combustion chamber. Race bikes run very good fuel of consistant quality and octane levels. A performance pipe on a two=stroke will reduce losses in scavenging and can therefore require a leaner mixture, or a 4 stroke pipe can reduce or increase back pressure and affect the amount of unburned fuel exiting the pipe. Any fuel that goes out the tailpipe is lost from the total jetting and must be compensated in the form of increased jet size. I don't know if this will help you, but I'm getting dizzy now so I'll close. My saying is " never argue with a carburetor". Also, " a carb is like having a beautiful girlfriend: give her what she wants or she'll make your life a living h##$ll. DC
 
First, I only read the question and not the replies.

"Gulp" induction is what you often deal with when tuning via exhaust finite waves and valve duration.

When a sudden vacuum is induced (ie scavenging wave, etc.) and air being lighter than fuel, said mixture will indeed go lean.

In addition, increased valve overlap, big ports and an out of phase exhaust will reduce the Venturi signal when said engine is operating below the tuned range. (ie off the cam)

Dale
 
LINEAWEAVER said:
First, I only read the question and not the replies.

"Gulp" induction is what you often deal with when tuning via exhaust finite waves and valve duration.

When a sudden vacuum is induced (ie scavenging wave, etc.) and air being lighter than fuel, said mixture will indeed go lean.

In addition, increased valve overlap, big ports and an out of phase exhaust will reduce the Venturi signal when said engine is operating below the tuned range. (ie off the cam)

Dale
Click to expand...

So its the increased scavenging wave of the free flowing exhaust causing higher than previous intake vacumn and consequent lean condition as the venturi fuel uptake cannot keep up with the airflow. Does this mean then that the cylinder filling (gulp) duration is shorter and consequently if it were longer the A/F mix would be leaner. Then at some point bigger gulps would be less effecient than slower longer stronger sips at the (venturi) straw ? Especially at low RPM I presume.
 
Such is pure physics as related to mass.
Fuel is heavy and shall always lag behind air.

Given enough distance (ie time) a one ton truck and a 400 pound motorcycle will both likely be capable of 50 mph. However, it will take the truck more time and distance. Should the distance run out before the time said motorcycle shall be in the lead.


For a given power level:
Big displacement = slow and steady one ton.
Smaller displacement tuned = 400 pound motorcycle.

Dale
 

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For clarification:
"Free flowing exhaust" is not simply about reducing exhaust gas impedance.

Harnessing finite waves via proper exhaust tuned length, valve timing and overlap is the key.

Take the expansion chamber for example. Timed back pressure boosts top end performance. 8O

Intake tract tuned length is also an important consideration.

FYI:
The OE low overlap camshafts of the late model KTM / Husaberg engines do not favor finite wave tuning.

Back to back testing with a stock Husaberg muffler and that of an Akrapovic slip on proved no measurable difference. However, the later indeed sounded faster.

Ones perceived reality often becomes their reality.

Dale
 
LINEAWEAVER said:
For a given power level:
Big displacement = slow and steady one ton.
Smaller displacement tuned = 400 pound motorcycle.
Click to expand...

Would a small increase in displacement have the same effect on the carburetor as larger valves and/or less restrictive exhaust.
 
husabutt said:
LINEAWEAVER said:
For a given power level:
Big displacement = slow and steady one ton.
Smaller displacement tuned = 400 pound motorcycle.
Click to expand...

Would a small increase in displacement have the same effect on the carburetor as larger valves and/or less restrictive exhaust.
Click to expand...

In a word.... No.
 
doctorcorey said:
Husabutt, there are so many variables at work here that blanket statements are ineffective. For instance, an engine that is really prepped for optimum performance can endure leaner jetting because it's set up to eliminate some of the production line variables found on non-prepped bikes. An example of this would be the shape, fit, and finish of the combustion chamber. Race bikes run very good fuel of consistant quality and octane levels. A performance pipe on a two=stroke will reduce losses in scavenging and can therefore require a leaner mixture, or a 4 stroke pipe can reduce or increase back pressure and affect the amount of unburned fuel exiting the pipe. Any fuel that goes out the tailpipe is lost from the total jetting and must be compensated in the form of increased jet size. I don't know if this will help you, but I'm getting dizzy now so I'll close. My saying is " never argue with a carburetor". Also, " a carb is like having a beautiful girlfriend: give her what she wants or she'll make your life a living h##$ll. DC
Click to expand...
Husabutt, as to your simple, basic, jetting question, when you change one thing (pipe, cam, carb, timing, etc.) you change everything to a degree. Just remember, no two engines are alike, and no two will respond the same to any given change. Jet it to where it runs good and leave the voodoo to the witchdoctors. Recall the last two lines of my post. :D :?: DC
 
doctorcorey said:
doctorcorey said:
Husabutt, there are so many variables at work here that blanket statements are ineffective. For instance, an engine that is really prepped for optimum performance can endure leaner jetting because it's set up to eliminate some of the production line variables found on non-prepped bikes. An example of this would be the shape, fit, and finish of the combustion chamber. Race bikes run very good fuel of consistant quality and octane levels. A performance pipe on a two=stroke will reduce losses in scavenging and can therefore require a leaner mixture, or a 4 stroke pipe can reduce or increase back pressure and affect the amount of unburned fuel exiting the pipe. Any fuel that goes out the tailpipe is lost from the total jetting and must be compensated in the form of increased jet size. I don't know if this will help you, but I'm getting dizzy now so I'll close. My saying is " never argue with a carburetor". Also, " a carb is like having a beautiful girlfriend: give her what she wants or she'll make your life a living h##$ll. DC
Click to expand...
Husabutt, as to your simple, basic, jetting question, when you change one thing (pipe, cam, carb, timing, etc.) you change everything to a degree. Just remember, no two engines are alike, and no two will respond the same to any given change. Jet it to where it runs good and leave the voodoo to the witchdoctors. Recall the last two lines of my post. :D :?: DC
Click to expand...

Thanks for your input Doc!

I am able to get the things jetted well enough most of the time.
It would just be nice to know why I am doing a particular procedure (jetting leaner or richer) rather than it runs better when I change this to that. I still can't understand completely why increasing airflow is not always just increasing airflow :? I guess its the quality part of the airflow I don't get. Maybe :?
 
I still can't understand completely why increasing airflow is not always just increasing airflow
Click to expand...

Hi Eric,
It helps to keep in mind air to be compressible, elastic if you will.

Dale
 
In regards to increasing displacement causing one to rejet the carb leaner, if the air filter system is only adequate, the larger displacement can pull a higher vaccum on the airboot side of the carb helping pull the fuel into the throat. On the same note, change the air cleaner restriction leaving displacement alone, and rejetting will also be in order. A Ducati is very sensitive to airbox changes and taught me a lot. Changes would be mostly noticible at higer RPM's where the restriction increase or decrease would come into effect. The pipe tuning issue is complicated and best left to Dale.
dan
 
Very informative. So thats why high performance engine builders are called tuners.
 

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