Automotive How do the accelerator pedal and rpm interact in an internal combustion engine?

[jack action]

I might repeat stuff already said, but for clarity purposes, here I go:

To change the rpm you need to accelerate (or decelerate) the crankshaft assembly. To get an accelation, you rely on good old F = ma; or in rotation:

\tau_{in} - \tau_{out} = I\alpha (reference)

\tau_{out} is the torque needed to maintain the load put on the engine and \tau_{in} is the torque due to the pressure exerted on the piston (minus the losses).

If they are both equal, then \alpha = 0. Otherwise, the rotational acceleration \alpha becomes either negative or positive, leading to a reduction or an increase of the rpm.

What we can control and vary is \tau_{in}.

Here is a mathematical definition of \tau_{in}:

\tau_{in} = BMEP\frac{V_d}{\theta_c} (reference)

BMEP = Brake Mean Effective Pressure;
V_d = Volume of air displaced for one cycle (displacement);
\theta_c = crankshaft angular duration of one cycle.

Knowing that the Brake Specific Fuel Consumption (BSFC) can be defined by:

BSFC = \frac{\rho_{atm}VE}{AFR \times BMEP} (reference)

\rho_{atm} = atmospheric air density;
VE = Volumetric Efficiency;
AFR = Air-Fuel Ratio.

Replacing BMEP, we get for \tau_{in}:

\tau_{in} = \frac{\rho_{atm}}{BSFC}\frac{VE}{AFR}\frac{V_d}{\theta_c}

This equation gives us all the parameters that affect the torque of the engine:

\rho_{atm}: It is a given so it cannot be controlled by the operator;

BSFC: It depends on the design and construction of the engine (thermodynamic cycle, friction losses, combustion efficiency, etc) so it cannot be controlled by the operator;

That leaves us with 4 different ways to control the input torque of the engine (in other words, how to change its rpm):

VE: We can achieve that by restricting the airflow. Less air means less air-fuel mixture, hence less energy released during combustion. This is what happens in a gasoline engine when we control the throttle with the gas pedal while keeping the AFR constant;

AFR: We can achieve that by controlling the fuel input, while keeping the air inlet (or VE) constant. This is what happens in a diesel engine when we control the fuel pump with the gas pedal.

\theta_c: We can achieve that by varying the length of a cycle. This is the control used in a hit-and-miss engine. The intake valve stays close as long as needed to prevent the air-fuel mixture to enter the cylinder, hence lengthening the cycle (in number of revolutions of the crankshaft).

V_d: We could achieved that method by varying the displacement of the engine. For example, by cancelling intake valve overture for certain cylinders when we want to decrease the torque. Although, I never heard of any engine working this way (Variable-displacement engine are close, but the objective is not to control the torque of the engine).

Other parameters can slightly vary (ignition advance, compression ratio, stoiechiometric AFR (in gasoline engine), etc.) but their purpose is not to control the torque input, but to achieve optimization of the combustion under a given circumstance.

For example, a rich AFR will give more power and a poor AFR will give better fuel economy, and this at any rpm. So it is typical to set a poor mixture at idle (who needs power at that rpm?) and to get the richest mixture at high rpm (why would you go all the way to the max rpm if you didn't want all the power you can get?). But all of this has nothing to do with controlling the torque input of your engine.

 

[Baluncore]

… however, so far we have concluded that out of the four factors I have posted earlier, one of them is constant and that is the AFR, …

That can not be concluded. You are assuming the engine has a simple carburettor.

Carburettors restrict variation of AFR, while direct fuel injection makes any AFR possible.

There is rarely an advantage in running on the rich side of the stoichiometric mix, but there can be significant advantages of running on the lean side. Many engines are designed to operate with excess air when idling cool.

 

[Sailor]

Compression ratio is determined by the geometry of the engine only! The volume at BDC and the Volume at TDC.
CR = V1/V2

V1 = Cylinder Volume at bottom of the stroke 10
V2 = Cylinder Volume at bottom of the stroke 1

CR = 10:1

Let's put terminology aside for a second ...

what you have stated here is that in a given condition a certain volume of air and let it be V1 will fill the cylinder and therefor is compressed into a new volume V2 as the cylinder reaches TDC, now this compressed amount WILL have different resultant properties (e.g temperature, pressure) than a volume V3 that equals 0.8V1 and enters the same cylinder to be compressed into V2 while keeping CONSTANT elementary pressure and equal densities

Now aside from the normal "compression ratio" you referred to which indicates the physical boundaries of the metal cylinder, let's make up a NEW terminology in this discussion and call it "air compression ratio"

Now it's clearly that V1/V2 is different than V3/V2, hence the variation in "air compression ratio" which in essence implies a variation in the volumetric efficiency
And that's exactly what I meant

So now the question is how much could V3 differ from V1 and still be combustible ?

 

[Sailor]

Why you think the AFR is constant.
The previous posts have made it clear that it is a dependent variable

No, we actually had an evident that an engine could (and I'm thinking this is the ideal case) accelerate with an absolute CONSTANT AFR

The variation of the AFR (in gasoline engines) should be meant to modify the consumption/economy of the vehicle rather than to accelerate it

And this is exactly what I have observed from other real world readings of the AFR value ...

This clearly states that being richer WILL affect power output, and therefore the speed at which the engine will accelerate.

in a gasoline engine the difference between a richer and a leaner state has a very narrow band that would makes the acceleration effect almost redundant

 

[Sailor]

I might repeat stuff already said, but for clarity purposes, here I go:

To change the rpm you need to accelerate (or decelerate) the crankshaft assembly. To get an accelation, you rely on good old F = ma; or in rotation:

\tau_{in} - \tau_{out} = I\alpha (reference)

\tau_{out} is the torque needed to maintain the load put on the engine and \tau_{in} is the torque due to the pressure exerted on the piston (minus the losses).

If they are both equal, then \alpha = 0. Otherwise, the rotational acceleration \alpha becomes either negative or positive, leading to a reduction or an increase of the rpm.

What we can control and vary is \tau_{in}.

Here is a mathematical definition of \tau_{in}:

\tau_{in} = BMEP\frac{V_d}{\theta_c} (reference)

BMEP = Brake Mean Effective Pressure;
V_d = Volume of air displaced for one cycle (displacement);
\theta_c = crankshaft angular duration of one cycle.

Knowing that the Brake Specific Fuel Consumption (BSFC) can be defined by:

BSFC = \frac{\rho_{atm}VE}{AFR \times BMEP} (reference)

\rho_{atm} = atmospheric air density;
VE = Volumetric Efficiency;
AFR = Air-Fuel Ratio.

Replacing BMEP, we get for \tau_{in}:

\tau_{in} = \frac{\rho_{atm}}{BSFC}\frac{VE}{AFR}\frac{V_d}{\theta_c}

This equation gives us all the parameters that affect the torque of the engine:

\rho_{atm}: It is a given so it cannot be controlled by the operator;

BSFC: It depends on the design and construction of the engine (thermodynamic cycle, friction losses, combustion efficiency, etc) so it cannot be controlled by the operator;

That leaves us with 4 different ways to control the input torque of the engine (in other words, how to change its rpm):

VE: We can achieve that by restricting the airflow. Less air means less air-fuel mixture, hence less energy released during combustion. This is what happens in a gasoline engine when we control the throttle with the gas pedal while keeping the AFR constant;

AFR: We can achieve that by controlling the fuel input, while keeping the air inlet (or VE) constant. This is what happens in a diesel engine when we control the fuel pump with the gas pedal.

\theta_c: We can achieve that by varying the length of a cycle. This is the control used in a hit-and-miss engine. The intake valve stays close as long as needed to prevent the air-fuel mixture to enter the cylinder, hence lengthening the cycle (in number of revolutions of the crankshaft).

V_d: We could achieved that method by varying the displacement of the engine. For example, by cancelling intake valve overture for certain cylinders when we want to decrease the torque. Although, I never heard of any engine working this way (Variable-displacement engine are close, but the objective is not to control the torque of the engine).

Other parameters can slightly vary (ignition advance, compression ratio, stoiechiometric AFR (in gasoline engine), etc.) but their purpose is not to control the torque input, but to achieve optimization of the combustion under a given circumstance.

For example, a rich AFR will give more power and a poor AFR will give better fuel economy, and this at any rpm. So it is typical to set a poor mixture at idle (who needs power at that rpm?) and to get the richest mixture at high rpm (why would you go all the way to the max rpm if you didn't want all the power you can get?). But all of this has nothing to do with controlling the torque input of your engine.

:thumbs:

Excellent intricacy jack action, and thank you very much for the highly valuable input, that pretty much nailed almost all of the aspects about this subject

So again we can conform the following :
- the AFR is/ could/ should be CONSTANT, in other words we DO NOT require richer AFR to accelerate the engine.
- the VE here relates to both the change in the compressed volume of air (air compression ratio as I've called it) and the change in air flow rate, so it's clearly that these two factors are VARIABLES.
- Firing rate is totally obsolete, though it is still variable.

Now the definition of \theta_c is very interesting because I think it describes BMW's Valvetronic system, and infact I did think about adding this technology to the discussion later on

varying V_d in order to control the rpm is almost infeasible and doesn't make sense at all

So back to my question about keeping the air compression ratio constant while still being able to change the rpm, I thing this COULD be done by altering the value of \theta_c, although this may not be the case with Vaivetronic, nevertheless, I believe there would not be enough room to have a complete variation in speed which would then makes it similar to the variation of AFR in a gasoline engine

As for diesels, I think their concept is pretty much straightforward and easy to understand


Again thank you jack action very much,

 

[Sailor]

That can not be concluded. You are assuming the engine has a simple carburettor.

Carburettors restrict variation of AFR, while direct fuel injection makes any AFR possible.

There is rarely an advantage in running on the rich side of the stoichiometric mix, but there can be significant advantages of running on the lean side. Many engines are designed to operate with excess air when idling cool.

check post #51 it shows the outputs of a modern ECU controlled engine

This coincides with my findings before starting this thread, and again what you describe could be related to fuel economy and should not be confused with the pure acceleration action of the engine by the throttle pedal inputs

 

[PhysicoRaj]

This is still not getting anywhere. Sailor, clearly specify what other clarifications you need. I think except for A/F ratio, all other three have been seriously dealt with and you don't have any objections regarding that.
Regarding A/F ratio, it is both constant and variable, depending on the construction of the engine (carburettor? Injection? Special carburettors?).

 

[Sailor]

This is still not getting anywhere. Sailor, clearly specify what other clarifications you need. I think except for A/F ratio, all other three have been seriously dealt with and you don't have any objections regarding that.
Regarding A/F ratio, it is both constant and variable, depending on the construction of the engine (carburettor? Injection? Special carburettors?).

Actually I think this thread has progressed nicely and we've come to the conclusion in post #66

The only one question that remains is :

how much could V3 differ from V1 and still be combustible ?

regarding the variability of the AFR in a gasoline engine and by eliminating the deficiencies of carburetors and ECONOMY intentions, we are left with a very narrow range of values to play with, therefore IDEALLY it should be CONSTANT which means the accelerator pedal could (and is) function normally when we hold the AFR constant

 

[Baluncore]

This coincides with my findings before starting this thread, and again what you describe could be related to fuel economy and should not be confused with the pure acceleration action of the engine by the throttle pedal inputs

Many vehicles couple the accelerator pedal directly to the injection pump.
The air flow is kept proportional to RPM. The accelerator pedal causes a change of the AFR alone.
The title of this thread is “Accelerator pedal and rpm”.

 

[PhysicoRaj]

Actually I think this thread has progressed nicely and we've come to the conclusion in post #66

You mean this?

This coincides with my findings before starting this thread, and again what you describe could be related to fuel economy and should not be confused with the pure acceleration action of the engine by the throttle pedal inputs

This is not true for all engines.

Edit: As Baluncore said, it depends.

 

[Sailor]

Now if I were to answer this question my self

how much could V3 differ from V1 and still be combustible ?

I would say that it depends on many variables such as air temperature, density, rpm besides the CR and AFR values,

these variables could then be measured and monitored by sensors, now whether this is the case with Valvetronic or not, I'm not sure, from what I've read so far I think there could be a defined value that is set for the minimum amount of air that enters the cylinder at idle

What I'm trying to find is the V3/V2 ratio that should be combustible at minimum save we have a stoichiometric mixture

 

[Sailor]

Many vehicles couple the accelerator pedal directly to the injection pump.
The air flow is kept proportional to RPM. The accelerator pedal causes a change of the AFR alone.
The title of this thread is “Accelerator pedal and rpm”.

when you referred to carburetors in your previous comment I thought you were talking about gasoline engines specifically,

what you describe here is how the accelerator pedal in a diesel engine increases the rpm

However, if you are referring to a gasoline engine that has it's accelerator pedal attached to a fuel pump, then this would be interesting, and I would then ask you for links that entails this

 

[Sailor]

You mean this?

This is not true for all engines.

Edit: As Baluncore said, it depends.

There are other points as wellCan you confirm that there are gasoline engines that utilize AFR solely to increase rpm ?

 

[PhysicoRaj]

Here are some images of the carburettor from my motorcycle. I found the manual and looked for the carburettor, found out the Air to fuel ratio controlling screw and the throttle housing location.

This is the complete carburettor:

This is the A/F ratio controlling screw (I have a screw driver at it):

This is the throttle housing and control cable (look at the screw driver):

I turned on the engine and set out to test:
When I turn the A/F ratio screw, I can change the rpm in any position of the throttle.
When I open the throttle, it also increases the rpm, at any angle of the A/F control screw.
Note that the A/F screw can be turned by 180 degrees on both sides, can have a considerable effect on rpm.

 

[Sailor]

Here are some images of the carburettor from my motorcycle. I found the manual and looked for the carburettor, found out the Air to fuel ratio controlling screw and the throttle housing location.

This is the complete carburettor:


This is the A/F ratio controlling screw (I have a screw driver at it):


This is the throttle housing and control cable (look at the screw driver):


I turned on the engine and set out to test:
When I turn the A/F ratio screw, I can change the rpm in any position of the throttle.
When I open the throttle, it also increases the rpm, at any angle of the A/F control screw.
Note that the A/F screw can be turned by 180 degrees on both sides, can have a considerable effect on rpm.

thanks PhysicoRaj for the nice post,

can you indicate the values of rpm you're having ?, could you vary the rpm from idle to redline ?

because only then we can say that varying AFR in a gasoline engine could substitute air flow rate as an influential factor otherwise it would just be similar to how θ can alter the rpm which isn't sufficient enough to be considered separately in a gasoline engine

 

[PhysicoRaj]

thanks PhysicoRaj for the nice post,

can you indicate the values of rpm you're having ?, could you vary the rpm from idle to redline ?

because only then we can say that varying AFR in a gasoline engine could substitute air flow rate as an influential factor otherwise it would just be similar to how θ can alter the rpm which isn't sufficient enough to be considered separately in a gasoline engine

In the manual it is stated that the idle rpm is about 1400 rpm. The manual doesn't say how much it can be varied by the A/F screw, but I am going to guess, I could alter it by around 1000 rpm. So with the A/F control screw, I can vary the rpm as $1400\pm1000$ rpm. But not to the redline (which is around 10500 I guess).

Important note: I recently found out in the manual that the A/F screw is not linear. That is, With different positions of the throttle, different changes in rpm can be made by the same amount of turn of the A/F screw. That's why I managed to give you a value at the idle state.

 

[Sailor]

In the manual it is stated that the idle rpm is about 1400 rpm. The manual doesn't say how much it can be varied by the A/F screw, but I am going to guess, I could alter it by around 1000 rpm. So with the A/F control screw, I can vary the rpm as $1400\pm1000$ rpm. But not to the redline (which is around 10500 I guess).

since the redline is at 10500 then you are only allowed to make less than 10% modification to the rpm, so I think we can say that the change in rpm as a result of varying the AFR is actually a side effect of a more direct function which could be increasing/ lowering the consumption rate or perhaps it's related somehow to NHV or something or it might aid in fine tuning the idle rpm ...

also note that for the gasoline engine to have a reasonable efficient combustion the variation in AFR should be kept that much small, because if these variations were to be open to the extreme without any restrictions then you could either flood the engine or kill it ...

 

[Sailor]

Important note: I recently found out in the manual that the A/F screw is not linear. That is, With different positions of the throttle, different changes in rpm can be made by the same amount of turn of the A/F screw. That's why I managed to give you a value at the idle state.

Maybe they do not want you to touch it in the first place

 

[PhysicoRaj]

Recent update: I opened the throttle slightly more than half, turned the A/F ratio screw to the max extent and the tachometer almost touched the redline!

My example of my carburettor was not to state that my bike has A/F ratio as the sole reason for revving up. I am just asserting that A/F ratio can have a significant effect on rpm. This convinces that A/F ratio is variable, but independent of throttle in my engine. Other engines with computerized fuel injections do have some means of altering A/F ratio as the accelerator pedal is pressed.

Conclusion: A/F ratio affects rpm. It is left to the discretion of the manufacturer of the engine to decide whether he wants it constant or variable through out the rpm range.

 

[PhysicoRaj]

Maybe they do not want you to touch it in the first place

Then they would not provide a screw

since the redline is at 10500 then you are only allowed to make less than 10% modification to the rpm, so I think we can say that the change in rpm as a result of varying the AFR is actually a side effect of a more direct function which could be increasing/ lowering the consumption rate or perhaps it's related somehow to NHV or something or it might aid in fine tuning the idle rpm ...

I am just guessing the redline. I can give you the exact value later.
10% modification can have a huge effect on rpm when the throttle is considerably open, since A/F screw is non-linear. That's why I got the redline with throttle more than half open and A/F to the max. extent.

 

[Sailor]

Recent update: I opened the throttle slightly more than half, turned the A/F ratio screw to the max extent and the tachometer almost touched the redline!

My example of my carburettor was not to state that my bike has A/F ratio as the sole reason for revving up. I am just asserting that A/F ratio can have a significant effect on rpm. This convinces that A/F ratio is variable, but constant in my engine. Other engines with computerized fuel injections do have some means of altering A/F ratio as the accelerator pedal is pressed.

Conclusion: A/F ratio affects rpm. It is left to the discretion of the manufacturer of the engine to decide whether he wants it constant or variable through out the rpm range.

A/F ofcourse does affect rpm like for example if you would have 0 AFR then you will have 0 rpm but more important is you would actually have 0 rpm much earlier than that I'm guessing around 5 or even 6, a little higher ratio and you should now have extremely lean mixure that is not suffecient enough for normal usage, again this means you have a very narrow band in a gasoline engine to deal with which is not useful for varying the rpm on it's own like the other factors

Now back to your experiment, it is very important to mention the rpm values before and after the variation in AFR, only then we can build a solid conclusion about it

 

[Sailor]

Then they would not provide a screw


I am just guessing the redline. I can give you the exact value later.
10% modification can have a huge effect on rpm when the throttle is considerably open, since A/F screw is non-linear. That's why I got the redline with throttle more than half open and A/F to the max. extent.

Awaiting for the exact values ...

10% isn't suffecient enough to make it an effective factor in the processs, this could be equal to the variation in valve timing for example which is a feature utilized for tuning power and effeciency and not to increase rpm, even though the rpm might actally change a little from it

 

[PhysicoRaj]

A/F ofcourse does affect rpm..

This is the answer to post #1.

...again this means you have a very narrow band in a gasoline engine to deal with which is not useful for varying the rpm on it's own like the other factors

That's why there's something called the throttle.
Okay, you are talking about gasoline engines. See my quote, I have added something:

This convinces that A/F ratio is variable, but independent of throttle in my engine. Other engines with computerized fuel injections do have some means of altering A/F ratio as the accelerator pedal is pressed.

Now back to your experiment, it is very important to mention the rpm values before and after the variation in AFR, only then we can build a solid conclusion about it

I searched the net and got the info that my bike gives maximum torque at 6750 rpm. So I am guessing that must be the limit. Manual says idle is 1400 rpm.
At throttle slightly open, variation in rpm due to AFR change was 1000 rpm. With throttle half open, same amount of change in AFR got me to the redline.

 

[Ranger Mike]

pardon me for raining on you parade but that screw on the carb is to adjust the idle speed of the engine. If there are two screws present, one is for idle adjust and the other is low rpm idle richness adjustment. These have no other use than to keep the engine running when the throttle is not in use. If you will read the legend you will find number 8 and number 10 jets. These are jets that are set by the factory. NOTE - MAIN JET controls the Air/ Fuel Ratio. Nothing the rider can do can adjust the A/F ratio. This is to say, there is a metering hole in the jet. The amount of gasoline passing thru the jet is fixed. What is not fixed is the amount of air passing thru the carb and the throttle controls this. Nothing the rider can do can adjust the compression ration. Both are fixed. This engine has a 8 to 1 compression ratio and you can not change it. Also the timing is set and unless the operator has access to a timing light and degree wheel, the timing is set for idle and for the amount of spark advance. The only thing the rider can do it twist the throttle and let more air into the engine to be mixed with the fixed amount of gasoline as dictated by the JET.

Now Chris xx and Jack have both taken a lot of time to explain these things but unless you can grasp the concept that this fixed JET metering out gasoline is constant as is the compression ration , then we can not continue to post. Savvy?

 

[PhysicoRaj]

pardon me for raining on you parade but that screw on the carb is to adjust the idle speed of the engine. If there are two screws present, one is for idle adjust and the other is low rpm idle richness adjustment. These have no other use than to keep the engine running when the throttle is not in use. If you will read the legend you will find number 8 and number 10 jets. These are jets that are set by the factory. NOTE - MAIN JET controls the Air/ Fuel Ratio. Nothing the rider can do can adjust the A/F ratio. This is to say, there is a metering hole in the jet. The amount of gasoline passing thru the jet is fixed. What is not fixed is the amount of air passing thru the carb and the throttle controls this. Nothing the rider can do can adjust the compression ration. Both are fixed. This engine has a 8 to 1 compression ratio and you can not change it. Also the timing is set and unless the operator has access to a timing light and degree wheel, the timing is set for idle and for the amount of spark advance. The only thing the rider can do it twist the throttle and let more air into the engine to be mixed with the fixed amount of gasoline as dictated by the JET.

Now Chris xx and Jack have both taken a lot of time to explain these things but unless you can grasp the concept that this fixed JET metering out gasoline is constant as is the compression ration , then we can not continue to post. Savvy?

Ranger Mike, my manual says that is the main jet screw. And main jet adjusts the A/F ratio. Either the manual is wrong or I'm turning the wrong screw?

 

[Ranger Mike]

correct..it is for the purpose of setting the idle rpm. But note that the adjustment screw only permits gasoline to enter the carb during idle. If you observe, t here is another passage next to the main jet fuel feed that i think is the idle fuel feed passage. Once sufficent rpm is achieved the main carb circuit takes over. Also note that the main jet is fixed. Again the diameter of the main jet dictates the final fuel/air ratio and this can be assumed to be at wide open throttle.
So you set the throttle stop needle that keeps the slide body open enough to keep the engine running and adjust the idle fuel mix to trickle in enough gas so when you twist the throttle there is enough fuel to make transition to the main fuel circuit smooth with no stumbling ( off idle response).
Once there is enough RPM the engine is totally on the main fuel jet.

 

[Sailor]

This is the answer to post #1.

Worng, my question asks about the acceleration pedal and it does not lead to 0 rpm nor to any value that would kill the engine and again we have confirmed in post #62 by jack and #66 what the variations are about and for what reasons

That's why there's something called the throttle.
Okay, you are talking about gasoline engines. See my quote, I have added something:

Throttle range is much wider than the 10% you have stated, there is no relation between them

I searched the net and got the info that my bike gives maximum torque at 6750 rpm. So I am guessing that must be the limit. Manual says idle is 1400 rpm.
At throttle slightly open, variation in rpm due to AFR change was 1000 rpm. With throttle half open, same amount of change in AFR got me to the redline.

Your point isn't clear

What should the maximum torque indicate in here

Again, unless there are solid numbers, this would implies nothing

 

[PhysicoRaj]

correct..it is for the purpose of setting the idle rpm. But note that the adjustment screw only permits gasoline to enter the carb during idle. Once sufficent rpm is achieved the main carb circuit takes over. Also note tbat the main jet is fixed. Again the diameter of the main jet dictates the final fuel/air ratio and this can be assumed to be at wide open throttle.

But even when I 'half' opened the throttle, I could register a significant change in rpm when I turned that screw. That screw can be operated even while riding (I've used my nails to lower it when I am low on gas. Then I open the throttle fully to get more torque.)
So I guess that even if is an idle rpm adjuster, is connected to the main jet and somehow aletrs A/F ratio, resulting in change in rpm.

 

[Ranger Mike]

without having a diagram of the internal passages of your carb i can take a guess. You have two circuits connected to the main jet. The needle screw adjusts the idle like i said. This fuel is still sucked up the carb even at high rpm and is always open so it contributes to the total amount of fuel the carb is using. At high rpm you can shut off the idle circuit but risk making the fuel /air mix too lean and burning an intake valve. Going wide open throttle invites this.

 

[Sailor]

pardon me for raining on you parade but that screw on the carb is to adjust the idle speed of the engine. If there are two screws present, one is for idle adjust and the other is low rpm idle richness adjustment. These have no other use than to keep the engine running when the throttle is not in use. If you will read the legend you will find number 8 and number 10 jets. These are jets that are set by the factory. NOTE - MAIN JET controls the Air/ Fuel Ratio. Nothing the rider can do can adjust the A/F ratio. This is to say, there is a metering hole in the jet. The amount of gasoline passing thru the jet is fixed. What is not fixed is the amount of air passing thru the carb and the throttle controls this. Nothing the rider can do can adjust the compression ration. Both are fixed. This engine has a 8 to 1 compression ratio and you can not change it. Also the timing is set and unless the operator has access to a timing light and degree wheel, the timing is set for idle and for the amount of spark advance. The only thing the rider can do it twist the throttle and let more air into the engine to be mixed with the fixed amount of gasoline as dictated by the JET.

Now Chris xx and Jack have both taken a lot of time to explain these things but unless you can grasp the concept that this fixed JET metering out gasoline is constant as is the compression ration , then we can not continue to post. Savvy?

Thanks sgain Rabger for your input,

As for the compression ratio and the ignition rate I think were discussed thoroughly and I have made a explanation of what I meant about the variation in compression contrary to what Chris has stated

Ignition rate of course has no effect whatsoever regardless of it being constant or vsriable because it's a driven entity not a driving one