Motorcycle Forum banner
21 - 40 of 91 Posts

·
Registered
Joined
·
380 Posts
Another thought...........CVs work better with pods on even firing event engines than on lopsided ones like the CB350 has. The 350 is half a 750 with two firings then a long interval of lag that lets air die off in speed and too much energy needed to speed it back up again.

Why the double Mikuni set up on a four is such cr-p, it divides the motor up into TWO lopsided ones.
 

·
Registered
Joined
·
1,089 Posts
Buying a vintage bike for your first bike is a really bad idea, imo.

The CB350 has crummy brakes. The last thing a new rider needs is brakes that barely work. Plus, at $2500, it must be pretty nice. Regardless of actual value, you're going to feel really bad when you wreck a piece of history.
 

·
Registered
Joined
·
4,182 Posts
Speed of sound is 343 meters per second. If the air is dry and at room temperature.
Not hard to believe air can move at or exceed that speed when it's only moving a few centimetres and it's happening in less then a second, but unless you are designing an air intake that operates on the Helmholtz Resonator principal it's kind of irrelevant to making a carburetor efficiently atomize fuel into an engine.

... predator carb on a 454 Chevy, lol amc you are such a car guy.
 

·
Premium Member
Joined
·
5,797 Posts
First, when the air velocity exceeds sonic speeds in the venturi, the shock wave after the venturi causes a greater drop in temperature. It chills the vapor to the point that it turns into something more like rain, which will cling to the intake tubes and doesn't burn well. My FIL was an engineer at Carter Carburetor, and explained this to me. It is also one reason many intake manifolds are heated. No one cares if this happens in lawn mowers; they aren't that efficient to begin with, and are governor controlled and rarely reach this condition. Performance car engines are a different matter.

Direct-lift carbs do go lean when the throttle is lifted quickly, because the air velocity over the jet briefly drops, so doesn't draw as much fuel as when it reaches the steady state, when the jet/needle combo takes full control. If you snap the throttle too quickly, it can be very lean, making the engine feel fuel-starved.

CV carbs are briefly rich under the same conditions, because of the opposite effect. When the piston lifts from the vacuum above the piston, which increases from the increasing air velocity, the opening of the venturi is larger, the velocity drops, and mixture control returns to the jet/needle. Velocity stacks, by decreasing turbulence and increasing air velocity ahead of the venturi, increases air density and pressure entering the venturi, so the total velocity through the venture is greater, and air velocity is what causes the piston to rise and open the jet/needle, until you approach WOT. The increased density of the air leans the mixture some, but also compensates for the reduction created by the air filters. The increased velocity at the entrance of the venturi will increase the total velocity through it, making the piston rise a bit higher, countering the leaning effect. It's all a careful balance, which is why taking the filters off, or using low-resistance filters, can make it so hard to re-tune a CV carb. You can't just throw in a bigger main jet and lift the needle some with most of them, you have to adjust the whole airflow vs. fueling curve. Unless you are only going for WOT, as in dragging the 1/4 mile.
 

·
Premium Member
Joined
·
10,054 Posts
Interesting reading above.
Has anyone played with CV carbs, to back up the info. I have run the stock air box on my XS400. The left hand carb ran one size smaller main jet stock, due to the unequal air from the box. The battery takes up a bunch of space. The air box was removed years ago, by me. I read about using velocity stacks, not running with paper or oil filters, or none at all.
Plus using larger jets for oil filters. None of it worked. I run the stock jet sizes, foam / oil filters, and it runs fine. Now and then the idle jets get a bit clogged so it idles rough. The left hand head gasket leaks a little, and DP is down 5 pounds on that side. DP nudged 100 when new. Now 92 is tops with 45hp originally. A CB350 might make 80 or 85 on a good day, with only about 35hp. I will check. UK
 

·
Registered
Joined
·
380 Posts
Mostly right except that air in increasing velocity is LESS dense, the molecules spread out. What happens when pressure is lowered on one end of a tube. The restriction meters the air molecules out to spread them out. And thoroughly missing the point of why a CV OEM airbox makes the entire engine spectrum perform better. It has absolutely nothing to do with turbulence, only density.

Fuel fallout has little to do with most bike engines which have much straighter paths to cylinders, at least most of them do. Several other reasons there too. I don't get worked up over A/F distribution issues nearly so much on a bike as a car.
 

·
Registered
Joined
·
380 Posts
Yeah, I know that you can't use a fuel pump to push fuel through an injector on either one......love ya.

Look at post #21 for why the airbox can't correct uneven jetting on a 400 Yammie.

I've done a good 20 years more on bikes than cars, never even had a car until my late 20's.
 

·
Registered
Joined
·
4,182 Posts
Still going to have a rough time pushing fuel through an injector without that fuel pump, a fuel injector is a nozzle. We are looking for a fine mist spray not a dribble.

& I'd rather look at my own motorcycles they all run like a jet :LOL: but 2 of them need some cleaning and one fork seal needs replacing. Might do the seal replace after supper or maybe tomorrow, I don't think anybody is coming to ride tomorrow.
 

·
Premium Member
Joined
·
5,797 Posts
Yes, air at higher velocity is less dense, but the velocity stack compresses the air somewhat, increasing the density before it enters the venturi.
 

·
Registered
Joined
·
24 Posts
Yes, air at higher velocity is less dense, but the velocity stack compresses the air somewhat, increasing the density before it enters the venturi.
Junk Science...

There is no way the density is going to increase unless the air is forced into the stack by some other source, such as turbocharging or supercharging, or ram.
 

·
Registered
Joined
·
4,182 Posts
The force is called vacuum and it comes from the movement of the piston drawing air into the combustion chamber. It's the Earth's Atmosphere that provides the air and powers the pressure differential.
You can pull or draw air into a venturi just as easy as you can push it, unless you are in outer space at the time.
 

·
Registered
Joined
·
24 Posts
The point I was trying to make is that the density of the air will not increase because it is going through a velocity stack when being drawn in by the engine.

You are not going to increase the density of the air going into the stack without some outside force.

Vacuum will not do it.

The earths atmosphere provides 14.7 PSI, it can only provide that pressure going into the intake stack, there is no physical way to increase the pressure above atmospheric unless you supercharge the intake air.

In fact, as the air passes the opening of the stack (which is wider than the inlet of the carburetor), the pressure will decrease and the velocity will increase, this is known as Bernoulli`s Principle.
 

·
Registered
Joined
·
380 Posts
Yep.

There is NOTHING in a vacuum so it cannot be a force at all. Only positive pressure which quickly stacks up mass in a glut at the squeezing down of the velocity stack to have only partial amounts fill into the empty vacuumed spaces. Why the lower density happens. If the density got higher then no carb on earth could possibly work. give it some thought.

In a velocity stack you have most density at the entry and less going through it and even less at the exit. The faster the speed of molecules through it the less density there is. They are able to speed faster due to lessening friction of other molecules close to them. The straightening of the path can help with unity of motion too.

The mass speeding under a slide is the fastest and least dense there is and what fills the slide interior to help lift it.

Why direct lift carbs make more power, because the slide is controlled by hand you can open it 'too far' but it allows denser air to enter and as long as it is not so much to bog the engine it being denser makes more power.
 

·
Premium Member
Joined
·
5,797 Posts
The stack gets smaller at the carb end, so the air entering has to fit through a smaller opening. The engine vacuum mostly applies to the other side of the venturi, so even though the stack increases the velocity of the air, it also compresses it to fit into a smaller area. The big velocity increase is in the venturi, where it reduce the air pressure the most, and enables the fuel to be drawn through the jets. In both the direct lift and CV carbs, the air velocity doesn't change as much as with a fixed venturi, which is why the opening above the main jet has to change by withdrawing the needle from the needle jet.
 

·
Registered
Joined
·
241 Posts
Man. OK. The inlet of the carb is a constant size. It makes no difference if you have a stock air box, velocity stacks, or the dankest eBay pods.

What does matter is how the air enters that carb inlet. The bell shape of the velocity stack allows the air drawn in to flow smoothly maximizing the amount of air in the space. Like the difference between stacking 2x4sin the back of a pickup compared to throwing them in from 20 feet. One way is gonna get a lot more stuff in the same space.

When you run straight pods it causes a lot of turbulence tumbling around the sharp corner into the intake boot. Now the air is tumbling and moving slower. Slower is less over time.

Yup you can run pods. Yup you can tune it. Yup, I have a set of K&N pods for an 1227cc GSXR engine if anyone wants them, I will never use them.
 
21 - 40 of 91 Posts
Top