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You have an engine problem with the difference in compression if the numbers are true. The exhaust valves commonly leak when they burn.

Compression is engine draw and that CV carb will not be opening as much as it should and the pods make it even worse.


The earlier paper filtered 350 sirbox and look at the part #2 grommet lower down on the inside. It ties both air filters to each other with a tube under battery and that allows the CVs to share the vacuum impulses to make 2X more to induce vacuum that opens the slides. The pods killed that. When you kill CV vacuum the slides at the least open SLOWER so jetting up is often backwards and why so many get lost when jetting up makes bike run even worse, the slides when carbs are rich die like a dog. If the slides have tears they are garbage.

Velocity stacks aren't spit, I've dumped them way more than once to get engines running at higher than original rpm. Nice if you can keep them but not engine killers. The shared cylinder intake impulses the OEM airboxes create are what open the CV slides up, that and the slighted restricted airboxes let the intake become rarefied even more to make it easier to build vacuum in the slides.

A CB350 better hit 90 mph or it is a dog. I've built several of them.

Someday people will figure out that ALL CV carbs like running lean far more than rich.

I wouldn't give that kind of money for that one.
 

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'Don't tell people you can make CV carburetor motorcycles with a cheap pod stuck on the open mouth, you can't.'

Maybe you can't.

I've done it and got them to run faster as well, velocity stack gone. But then I've gone quite a bit deeper into them than most could dream of. Go back and look at DOHC 750 Honda tests for '79-'83 and usually around 12.40-12.50 1/4 mile. I used to bump into the 11s and 12.20s all day on one. 4 foam Unifilters on it. Knowing what I do now I could go even faster. And that's on the labyrinth type CV with no rubber which is even harder to get slides to open.

If you are fuel soaking foam filters you likely have a mechanical issue with reversion, I never once had the issue and I ran them a lot.

Actually the Honda early DOHC had the tuned stack length built into the airbox rubbers, it is not a velocity stack per se but serves the critical lengths that engine head desires, or about 50 mm. length.
 

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Actually the Unifilters shown there cannot possibly block off the port on carbs, impossible to do so as the flanges are stiff rubber.

Play at it? The length given there was used by Honda RSC when racing the bikes in Superbike racing in the days of Freddie Spencer. The RCB1000 factory racer used it too.
 

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'The length of what? the intake stack the bike no longer has?'

The 2 inch length difference is close to nominal, you usually incur problems at 6 inches or so. Do some math on sound waves. 2 inches will easily be in the gas temperature down length of pipe variable, it affects the actual gas speed.

Talking of turbulence is counterproductive, the carb works CV by creating much faster gas speeds. AND by likely creating a boundary layer doing it (which increases turbulence), since the carb no longer has a fully even open throat. The suction point at the slide hole is a maximum turbulence point, directional flow is crashing into right angle flow right there. The gas speed can be 3X faster under slide to wildly increase on both sides of slide when the passage opens up, and we are wasting time talking about turbulence.
 

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Post #18 where to start.......Never ever had the filters block any port if the filter was correctly chosen. I check for it. 30 seconds with a dremel fixes it too.

'The increased air velocity will also lift the CV pistons more effectively, therefore providing a richer mixture at high throttle settings than without it.'

Dead backwards, CV open slower and lower when richer, put an O2 sensor on the exhaust and prove it. Why Honda uses them, the engines run best slightly lean for emissions and look up the term 'lean best horsepower' which is the max amount that can be made, you intentionally richen past it a bit as engines tend to melt at that spec because they are making so much power. Run an engine at stoichiometric or the perfect 14.7/1 A/F ratio and race it to see how fast it burns up, and why all car engines and others go open loop at max power to go closer to high 12-13 A/F ratiowith a fixed fuel map, it is to protect the engine. It's why you no longer can add hi-perf parts to cars without a tune (software rewrite). Oh you can, but you WILL burn the engine up unless it has wideband O2s in it.

Another reason CVs get used is the male testosterone problem, or bragging about who has the biggest carbs and CVs can be much bigger than direct lift carbs. A major legal reason is because modern bikes make so much power they tend to kill idiots who simply jump on one and turn the throttle all the way open, the CVs will not open fully thus saving some d-mn fool's life.and the resultant legal wranglings over it.

'Without variable venturis, the air reaches sonic speeds, which is bad for the atomization function.'

Laughing at that one, your lawn mower will get carb gas speeds faster than sonic waves, ALL engines can do it. Do the math. You actually use sonic speeds in figuring out the intake and exhaust tract lengths. AND engines with no CV are SLOWER in overall gas speeds, again, backward.

And what to do with the CV type carbs that cars use? BTDT too.


On a 440 GTX at full weight nearly 4000lbs, and ran well into the 11s.
 

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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.
 

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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.
 

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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.
 

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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.
 

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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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'...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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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 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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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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The answer stands well by itself. Surprised you didn't go there. And I'm not touching the software thing other than to say you are way behind if that is your line officially. Every computer vehicle I have has a shortcoming in it due to incorrect software, if you can't pick up on them that's on you. Many of them are even intentional to force you to go to dealers more.

'Holley what a pile of dribble. Why would you even suggest that is a response to what I posted'

Uh, NOT to you and clearly marked as such but one has to read.....post FIFTY-FIVE.

No insult intended at all but I do stand my ground.

Happy Thanksgiving everybody...........yes, even you.
 

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'So is stand off present in an EFI system then?'

Not nearly so much when a true timed EFI is used, but batch fired ones can. Batch firing being TB single or double injectors for several cylinders and true timed being MPFI, one injector per cylinder and past TB. Batch firing by necessity is not timed exactly to a port opening event and similar to normal carb except in more pulsing. They can fire more than once too where MPFI usually looks to fire once per intake event per cylinder.

If you have variable cam timing and somehow the intake cam gets stuck it can stand off some.

Most bikes use true MPFI of course. I had two Ford Tempo cars, one batch and the other true MPFI although both used identical 4 injector manifold for 4 cylinders. The true MPFI was smoother and had a peppiness down low the other did not have with identical everything else but for MAP on one and MAF on the other besides the injection difference.
 
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