Carburetor Thread

[Old Fox]

<s>Ok, so why not a bigger jets to take more volume of fuel</s>


Teacher? Who? Me? I can NEVER become one

Just seen your post sir , I'm going through it. Tks for the help, really appreciate along with PSR ji.

Edit
OF Sir - Fantastic. All doubts cleared in just one post. Brilliantly explained on the finger & hose example. Also my comparison on venturi size versus CRDi is miles apart & really don't have anything in common which I understand clearly.

Also I've understood that making the fuel reach farthest (reducing the size of venturi) does not help, but only harms on top end like PSR ji said earlier, which I've thoroughly understood by your explanation.

Thank you very much both PSR ji & OF sir

PS - This is not all, I'll come up with more questions as I keep chewing these thoughts

Thanks for the appreciation.

As an end-note, I'll give you an analogy that you'll probably remember for long. Its to do with 'pumping efficiency' and its something you survive on. The human heart. Thats a pump whose output keeps you alive. The heart's pumping efficiency lies within a certain 'pulse' range - somewhere between 50 BPM (beats per munite) to 160 BPM depending upon your age. Below 50 BPM the suction and pumping strokes are not strong enough to push blood through to the farthest reaches of the body. Above 160 BPM, the suction and pumping happens so quickly that the heart can just pump a very small amount of blood during that short time. So a black-out through lack of blood to the brain lies at either end.

It is the same with all pumps and the engine is one too. Air has inertia just like blood. And so needs time to start moving. If the suction stroke ends even before the air has reached the peak velocity of travel through the venturi, then the engine will starve for air. Which is what happens with narrow venturis at high RPM's.

And it is precisely this 'pumping efficiency' that makes peak torque available at max intake manifold vacuum.

[aargee]

Yes with a smaller venturi your air speed is higher but not enough will flow at WOT...poor high end.

<S>Ok, so why not a bigger jets to take more volume of fuel</S>

You are a student of the twilight zone Aargee, just bordering between being a student and teacher

Teacher? Who? Me? I can NEVER become one

Just seen your post sir , I'm going through it. Tks for the help, really appreciate along with PSR ji.

Edit
OF Sir - Fantastic. All doubts cleared in just one post. Brilliantly explained on the finger & hose example. Also my comparison on venturi size versus CRDi is miles apart & really don't have anything in common which I understand clearly.

Also I've understood that making the fuel reach farthest (reducing the size of venturi) does not help, but only harms on top end like PSR ji said earlier, which I've thoroughly understood by your explanation.

Thank you very much both PSR ji & OF sir

PS - This is not all, I'll come up with more questions as I keep chewing these thoughts

[Old Fox]

Yes, I understood the basics very strongly, no doubt on it. But I'm getting into next level (confuse & learn hehe), to think, what happens when I use even smaller dia. That would gush up AF mixture isn't it?

Just trying to relate it with the CRDi Diesel technology (I know its wrong, but still), where the fuel is sent under high pressure, which is also atomized, where as, here it is not; so the Diesel burns more effeciently & quickly & doesn't act up like lazy

Trying to compare & learn that's all.

You are a student of the twilight zone Aargee, just bordering between being a student and teacher .

I guess I'll give Psr a helping hand here...not that he needs one though.

First, your 'feeling' that reducing venturi diameter (or using a narrower venturi) will lead to increased 'pressure'. Take the already cited analogy of water flowing through a pipe. If you reduce the diameter of the outlet by putting your finger partially across the opening, the water seems to flow with greater 'pressure'. it is not pressure actually. Pressure is a force (measured across a certain unit area of application) and it is your finger that feels that force. The water simply comes out faster and so goes further.

A simple equation relates these things.

Q=AV

Q - flow rate (CFM as psr says or can be any other combination of units)
A - Area of cross-section (thats where the venturi diameter is hidden )
V - Velocity of the flow.

Seeing this equation it is apparent that for a certain given flow rate, reducing the cross section (by reducing the dia) should result in a proportional increase in flow velocity. Vice-versa, an increase in cross-section will result in a reduction of flow velocity for the same flow rate.

Now since an engine has a certain limitation in respect of its suction ability, it follows that it can handle air-fuel mixtures when fed within certain fixed 'flow rate' limits. (Remember peak RPM's. No engine runs with an unlimited red-line eh!). So it follows that there is a certain range of venturi diameters that allow air to flow in within the flow rate range needed by that engine. Go above or below those diameters and you change the way the engine breathes. Too small a dia and it cannot supply enough air at higher RPM's because the short time duration of the 'intake' or 'suction' stroke is not enough to accelerate enough air through it. So the engine 'chokes' or suffocates at high RPM's. Too large a dia and you overwhelm the carb jetting at low RPM's with too much air.

Your comparison with a CRDi engine is pretty off the mark actually as there's nothing like feed-pressure involved in a carbed engine. Why the CRDi is better than non-common rail engines is the realm of stratified charges, extreme injection pressures etc etc which are nowhere close to petrol IC engines running with carbs.

[psr]

Yes, I understood the basics very strongly, no doubt on it. But I'm getting into next level (confuse & learn hehe), to think, what happens when I use even smaller dia. That would gush up AF mixture isn't it?

Just trying to relate it with the CRDi Diesel technology (I know its wrong, but still), where the fuel is sent under high pressure, which is also atomized, where as, here it is not; so the Diesel burns more effeciently & quickly & doesn't act up like lazy

Trying to compare & learn that's all.

Yes with a smaller venturi your air speed is higher but not enough will flow at WOT...poor high end.
All diesel engines require diesel to be compressed and sprayed inside the cylinder...the higher the pressure and smaller the injector holes...the finer the spray..resulting in a smoother engine and pickup...
CRDi is one of the technology used in this..

[aargee]

I thought you understood it when you compared it to tap water flowing through a small pipe and a big pipe....yes we can discuss in person.

Yes, I understood the basics very strongly, no doubt on it. But I'm getting into next level (confuse & learn hehe), to think, what happens when I use even smaller dia. That would gush up AF mixture isn't it?

Just trying to relate it with the CRDi Diesel technology (I know its wrong, but still), where the fuel is sent under high pressure, which is also atomized, where as, here it is not; so the Diesel burns more effeciently & quickly & doesn't act up like lazy

Trying to compare & learn that's all.

[psr]

Thanks PSR sir for the education. But I'm still not convinced on the small venturi stuff...may be we should discuss that in person probably.

I thought you understood it when you compared it to tap water flowing through a small pipe and a big pipe....yes we can discuss in person.

[aargee]

Thanks PSR sir for the education. But I'm still not convinced on the small venturi stuff...may be we should discuss that in person probably.

[psr]

Completely agreed, completely agreed, no debate on that.

But the point is, when the venturi size is reduced, I'd expect a more pressurized AF mixture to be thrown for performance. But again, I'm not changing ONE parameter here, put it this way...

Small venturi, large jets, after market air filters (for better air suction), so, shouldn't the carb do the job even effectively?


What's CFM stand for? Constant Fuel Mixture

CFM is Cubic Feet per Minute ( a measurement of total air flow).
Choosing The Right Carburetor - Tech Articles - Classic Trucks Magazine

Carb CFM Calculator

[aargee]

Adjust any one parameter and you will stir a hornet's nest..

Completely agreed, completely agreed, no debate on that.

But the point is, when the venturi size is reduced, I'd expect a more pressurized AF mixture to be thrown for performance. But again, I'm not changing ONE parameter here, put it this way...

Small venturi, large jets, after market air filters (for better air suction), so, shouldn't the carb do the job even effectively?

but the CFM

What's CFM stand for? Constant Fuel Mixture

[psr]

Beautiful explained sir; Tks. I'm able to visualize & all the more, able to realize my mistakes too

Now, if venturi velocity is all that matters, then, I'll put it this way. Suppose the manufacturer says a certain engine requires VM24 that is an ideal one. So, if I use VM22, wouldn't I be actually sending more pressurized AF mixture? Does that mean a performance improvement?

Yes, I understand jets plays a major role in determining the fuel quantity. Lets say I upjet on the VM22 to match the throw of AF mixture from venturi, so would that be considered as a benefit? Pls let me know, if I need to elaborate more. Tks.

The engine is a complex vacuum pump ,cum explosion chamber,cum exhaust gas scavenger etc.,in a nutshell from a stock size of carb if you go to a smaller size carb..your venturi velocity will be high and the low and mid will be good...but the CFM will reduce with the reduction of venturi and the top end will suffer.So when the bike is designed the specific carb is selected taking into consideration the variuos Vacuum,venturi velocity and the AFR required for the particular design,throughout the RPMs of the engine...Adjust any one parameter and you will stir a hornet's nest..

[aargee]

No reducing jet size will reduce the...with the bore vacuum.

Beautiful explained sir; Tks. I'm able to visualize & all the more, able to realize my mistakes too

Now, if venturi velocity is all that matters, then, I'll put it this way. Suppose the manufacturer says a certain engine requires VM24 that is an ideal one. So, if I use VM22, wouldn't I be actually sending more pressurized AF mixture? Does that mean a performance improvement?

Yes, I understand jets plays a major role in determining the fuel quantity. Lets say I upjet on the VM22 to match the throw of AF mixture from venturi, so would that be considered as a benefit? Pls let me know, if I need to elaborate more. Tks.

[psr]

@Psr sir, I was going through the second link & was able to make things much better in terms of pressure & venturi. So that made me think, how come a 4 mm of difference (VM22 & TM28) make such a huge variation? Just can't imagine it.

Also, do you think if I lower the jets further, wouldn't that be sufficient to generate more pressure on the AFR mixture from the venturi to cylinder?

No reducing jet size will reduce the petrol metered out...jets determine the max amount of fuel that can be sucked through them..it is the venturi velocity which decides on the amount of suction in combination with the bore vacuum.

[aargee]

@Psr sir, I was going through the second link & was able to make things much better in terms of pressure & venturi. So that made me think, how come a 4 mm of difference (VM22 & TM28) make such a huge variation? Just can't imagine it.

Also, do you think if I lower the jets further, wouldn't that be sufficient to generate more pressure on the AFR mixture from the venturi to cylinder?

[aargee]

The float level matters a lot and makes a big time contribution to leaning and richening up the mixture

Pls, some explanation is solicited. Tks.

[dafc]

The float level matters a lot and makes a big time contribution to leaning and richening up the mixture