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To post #39

The needle/needle jet has nothing to do with that; it only adjusts the fuel ramp itself. You can dump it and go to adding more emulsion bleed holes to do the same. Either or works and depends on what the designer wants.

A velocity stack is only the front half of a venturi, albeit with less angle to it.

You need to drop any idea of compression right there making the air denser, any compression occurs at the very start of the stack as the molecules all fight a clog to get into the smaller space, once they are free of the pack they speed up and are further away from each other to be less dense, and before they ever arrive under the slide. In actual use, they could speed up less dense in stack, hit carb body and then settle in to be denser at slower speeds to then have to readjust to faster less dense as the very low slide pulls them off main carb inlet area. Think about that relationship in light of the positive pressure that goes through the back of carb port to push up on the main bigger slide OD.

Look at any flowbench localized probe readings and all that is verified instantly. You can get 10 different readings based on exactly where you are reading and the points can be literally next to each other.
 

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No inlet of any carb is a constant size. Dunno how somebody says that, measure them.

And the pod fail has absolutely NOTHING to do with turbulence. at ALL. I've removed enough velocity stacks to put pods on to run MUCH better.

Not saying that pods will work on everything but they work on a lot of things.
 

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When he cracks the throttle open at 36 seconds into the video, watch the fuel spray come Out of the open throat of the carburetor, that right there is how your pod filter is going to get gas soaked, and that right there is why you want the funnel shaped air intake, and that right there is why everybody is adding more fuel when they install pods on the open mouth of a carburetor.
 

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'...that right there is why everybody is adding more fuel when they install pods on the open mouth of a carburetor.'

Maybe on yours but not mine. I have driven a good probably 20 years combined on different bikes with them and never had any fuel soaking in the filters. If anything the filters get the oil pulled off to be bone dry after a very long time and I simply squirt some oil on them.

Seen that problem way more than once but in every case pretty convinced something correctible is wrong there, beginning with driver. Look at the same guy with camera on engine under load and it doesn't do it at all. Late timing, late intake closing, messed up overlap, messed up exhaust to mess again with overlap will all do that. Says FIX ME to me, and again start with the driver. I for one was taught to look in disgust at those who quick rev engines up with no load thinking they are 'doing something', you clearly see what it does. Fuel stand-off is common on out of tune vehicles......and I will ask you why somebody would jet for that to add fuel when the carb will simply re-suck that back in to be double rich and now you would add fuel a third time? Carbs add fuel with every pressure pulse there backwards or forward.

No WONDER people have trouble.
 

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We have a guy reving an engine that is not under load, to demonstrate what happens when an engine is under load.
And a quote that says everyone is adding fuel with pods. BUT, I said above, I am running stock jet sizes on my XS400 with foam filters. I guess I am outside of everyone. AND, my question went unanswered when I asked, if folks are talking theory, or if they run a bike of this type. I do. I was on my XS1100 today. The XS400 is resting until next spring. But I have offered to record the sound of it running WOT up a steep hill.

The 2 stroke road race Yamahas belched out the make of the carbs as well, under the same circumstances. Under load they ran fine. I raced one, against about twenty others, so I should be a bit familiar with them. Raced against Steve Baker, the first US world champion, and others. But none of that counts, if you have theory that differs.

The CB350, is 325cc. I sold quite a few of them. Nice little bike.
On the sail boat, we reduce sail when the air is more dense. It has more power to it. Honda did many tests back in the seventies, to determine if the air could be sped up using a venturi. If the same volume of air, that flowed through a big pipe, went through a smaller pipe, it had to speed up. Which is what happens with wings and other foils. Air or water going over foils also has drag. The smart guys work on lift / going over the bump or curve, and recaptured drag. That happens at the tail of a plane, or the trailing edge of foils, like keels and wings. UK
 

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The issue with pods is many that are sold as 'fits', and they do actually fit, is that they partially block the air jet ports of the old Keihin design carbs, which are very near the perimeter of the intake end. This keeps the air from flowing at the designed speed into those air jets, and the emulsion tubes from working the way they should. Note that this will have little effect on the carb shown in that video, as its air jet ports stand very proud of the edge, while the ports on the old Keihin design are actually embedded into the sides, where the step in the boot of many pods, that form the locator stop can, and in many cases does, block them from the direct air flow. There are pods with a smaller step, but you won't know until you have them in hand if they will actually work. This is a known issue among vintage Hondas. With skill, you can reduce the size of the step, but you have to know to check.
Also, with these carbs, the stack is not part of the venturi, which starts some distance inside the intake.
Art Font Engineering Parallel Auto part
 

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Old Honda 450 carb, almost identical to the VB series used all through the early '80s. If we are talking direct line of sight on those air entries it is NOT needed, they wind all around and the VB are even worse, you can claim they are 'blocked off' too and they work perfect. Same with main system air bleed, it just has to be open to air and not direct line of sight as most of the circuit is not anyway. You don't have a restriction anyway until it becomes smaller in cross-sectional area than your bleed hole is. You guys would have fits with Holley carb metering blocks, I drilled holes all over the place in them to change things when needed. I do the same on bikes.

I ran pods on CB200 (ex CB175) and the bike ran all day to 10,000 rpm but not CV type, simple direct 20 mm. carbs. On that one I had to cut the slides to change the cutaway slightly, it was too rich at barely opening throttle, the cutting fixed it. Otherwise used OEM jetting. That carb type used the faired in bleed entries, no problems. The exhaust was custom, the high scrambler pipes but with two megs one on each pipe that gave a good burst of torque when properly straight thru muffled. That little 200 would bust 80 mph+ with my then 200+ lbs. weight on it. I drove it several years until the cam chain literally cut the head in two. Or cylinder, can't remember which.

Yes Krusty, some use the OEM jet but so many have it drilled in their heads it just isn't done. You 'gotta jet 'em up' and then the problems begin. On a couple I actually had to jet DOWN, the engine tells you if you pay attention.

There is no difference in theory and real world to me, I expect them to be pretty much the same and they pretty much are.
 

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If as Wintr says in post 47, the filters partially block any jet or air intake, then I can see the need for changes, and they will not work correctly. On my more modern CV carbs, I have run the stock filter, paper filters, foam / oil filters, extensions and anything else I could think off. Not much change with all of the above, except it runs better the more I lean it.
That was mentioned above, and well known to road race guys. The old 2 strokes ran great just before they seized. If we sensed they were starting to scream, we shut them off, checked the plugs, and added more fuel.
Setting the carb on a chain saw, or small OBM, both with a high and low mix adjustment, is a good learning tool.
As an aside, I cautioned all owners of a Honda, to not mess with them. They ran fine stock. It was easy to make more noise, and go slower. On my XS400, the rubber boots connection from the carb to the air box, had deteriorated, and was leaking air. UK
 

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If you study a VB carb carefully as to the choke plate location you will see another CV slide opening enhancement device...........the choke plate when fully open severs the intake nozzle into essentially two barrels more or less. It allows most active air to flow in the lower section to prompt the slide to lift faster and it prevents a great walking distance of air in top of carb entry from moving down to interfere with smooth laminar flow to the slide at lower throttles. The top stays relatively slower in flow to let the top main air bleed get more accurate pressure. Then as the slide gets even with and passes the chokeplate level, the top section of air gets much faster to equal the bottom and it is why you often lose slide lift simply removing the chokes but most never grasp that.
 

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None so blind as those who do not wish to see.

"We have a guy reving an engine that is not under load, to demonstrate what happens when an engine is under load."
No, we have the only Short video I could find quick that shows what I've seen happen in real life.
 

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You don't get that blow-back of fuel from the fuel injected engine version for the fairly obvious reasons, the fuel mist is introduced at an angle away from the intake and well after the throttle body.


... next time buy a fuel injected motorcycle.
 

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None so blind as those who do not wish to see.

"We have a guy reving an engine that is not under load, to demonstrate what happens when an engine is under load."
No, we have the only Short video I could find quick that shows what I've seen happen in real life.
Fair enough, I agree. It is often most difficult to find video to back up any position. We do not know the running condition of the XS1100 in the video, and it was running without the rubber inlet tubes, that you have been suggesting are required. That or stacks. I think the velocity stacks go back to old British 650cc bikes of about 45 hp. Some of the clever guys got them to work, and might have made 50hp. Norton smart guys got 71hp from a 750. Axtell was running 85hp, a lot more than anyone else, with no stacks.

Back to the OP. Trying to go a bit faster on a CB350 is a waste of time, IMO. UK
 

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The CB450 carb does not have air inlets that 'wind around' the intake, it has three small holes centered about 3mm from the outside edge, two near the top to feed the underside of the CV piston, one at the bottom with a brass jet pressed in to feed the main jet emulisifier; If the step in the mounting boot for the filter is more than 2mm high, the air mostly flows past them at higher airflow.
The needle/needle jet has nothing to do with that; it only adjusts the fuel ramp itself. You can dump it and go to adding more emulsion bleed holes to do the same. Either or works and depends on what the designer wants
Really? The volume of fuel drawn through the main jet is determined primarily by two things: the air velocity passing over it and the area of the jet. Since the air velocity doesn't change much in a CV carb, the jet size must increase, which is the function of the jet needle and needle jet, until the piston rises near the top of its motion. If you want a proper air/fuel ratio throughout the full range, you have to adjust the fuel volume; adding air bleed holes to the emulsion doesn't provide this control. The air velocity through a direct-lift carb may change more, but doesn't change enough to pull enough more fuel through a fixed jet, which is why they also have needles.

As to changing air filter restriction not affecting fuel/air ratio, consider that the ratio is between the fuel and air mass, not volume, so if the air pressure changes at the carb inlet, the ratio also changes. This is why engines with simple carbs run rich at high altitude, if you don't make changes in the jetting system; less air density, less mass flowing through the carb, and less mass means higher fuel/air ratio. A high restriction air filter also reduces air pressure at the carb inlet at higher throttle settings, and reducing the restriction increases that pressure, so more air mass enters the engine; more mass requires more fuel to maintain the fuel/air ratio, which is why CV carbs run lean when they have high-flow filters.
 
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There was probably more work done on the CB450 to get more speed, than just about any other bike.
I do not like the CV carbs very much, but the ones on my XS1100 work fine. I have lifted the needle one notch as the pipes prefer that setting. Otherwise i have never had them apart. Not so the XS400. I would prefer the old style Mikuni.
The 1100 like many twin overhead cam early motors, runs a bit flat under 3500 revs, due to the cam profiles. It might cough a bit if the throttle is cracked open. Why squeezing works better. UK
 

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The carbs on my VS800 were worse for changing mixture with low-restriction intakes than the old Hondas. Whenever someone complained about running problems after taking the stock filters out, we told him to just put them back.
 
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Good advice. Most do not have a clue how the carbs work. And not out to insult anybody at all. I simply believe one has to work on the drivers often as much as the machines themselves.

Post #55.......

'If you want a proper air/fuel ratio throughout the full range, you have to adjust the fuel volume; adding air bleed holes to the emulsion doesn't provide this control. The air velocity through a direct-lift carb may change more, but doesn't change enough to pull enough more fuel through a fixed jet, which is why they also have needles.'

Uh, no, mostly. I can add fuel by gallons using air only in holes to do it. And shape any fuel ramp I want. I certainly did it enough. In your last sentence there simply making the main air jet slightly smaller will instantly richen up like changing the main fuel jet to bigger.

Ever hear of a Holley jet passage referred to as a well? For a reason, the main jet gets picked to be slightly too big and then like an idle pilot jet the used mixture is made up of it. Pure idle in a bike engine comes from the idle main jet or pilot which oversupplies to allow the mixture screws to have adjustability. The screws either add pure air to dilute the pilot output or simply choke back on it if they are forward true mixture screws instead of the pure air type. Back to the Holley, the main jet is too big and the channel it is in becomes a 'well', it fills up above the main to act something like a mini-fuel bowl and then you add air holes in the sides of the well to get various results, the fuel level in the well above the jet then drops at a certain rate based on how much air is added and where in the height of the tube. The air emulsion holes actually in effect begin to perform main fuel jet function. The main jet ceases to be the absolute predictor of top end WOT richness alone, the main air jets do some of it and did it all along and why there is air there to begin with. You don't have to have needles at all. My thinking is they are used because it's the cheapest way to get good repetitive very close tolerance fittings of restrictions, the number of holes might be cheaper but hard to get that many as exactly precise as they need to be on small engines, talking holes in the few thousandths there, Typical Honda #38 pilot is a .013" hole, you break lots of tooling going that small and making the parts gets too expensive.

The back half of a Holley vacuum secondary carb is CV basically and uses no needles at all to get very good fuel metering. Other carbs like Carter, Rochester have used needles, depends on design vs. money. The Motorcraft CV carb used just before EFI used needles that had to be carefully set to within .002" to be right and multiple dial indicator settings on top of that, a nightmare.

To the fuel injection post #53 a more not obvious reason if the software is correctly written, the injectors cut off when TPS recognizes what goofball owner is doing. All injectors cut off on quick decel now to save fuel and popping in the pipes.
 

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To post # 59 :ROFLMAO:
" a more not obvious reason if the software is correctly written, the injectors cut off when TPS recognizes what goofball owner is doing. All injectors cut off on quick decel now to save fuel and popping in the pipes."
Holley what a pile of dribble. Why would you even suggest that is a response to what I posted :ROFLMAO: software is correctly written, hang on I gotta pull over to Ctrl+Alt+Del after that one.
 
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