drilled air box?

[cmpnbd2]

how is this done and what benefit is there other than the obvious?

[GARTHXS]

easy

Take off the bottom of the air box and look at the part that is inside the filter this is prefiltered space simply drill a few holes in this area an viola done

[cmpnbd2]

airbox

thanks for help

[Steptoe]

air box---velocity?

I was always under the impression that our carbs are constant velocity,ergo needing the suction caused by intake stroke and the small air filter throat to supply vacuum to operate,so how do you fella,s get your machines to run efficiently and smoothly with far to much air around the rear of the carbs,either thru drilled boxes or single pod filters?
tried it once eons ago, the thing ran like a hairy goat, went back to original and left it at that

[GARTHXS]

velocity

I think the intake tubes to the airbox are sufficient enough and that and the filter do provide backpressure , I have it both ways with my 78 ,pods and 4/1 exhaust,and my 79e stock with drilled airbox. It did take a lot of time to tune the 78 and major rejetting

[TopCatGr58]

Hey Steptoe,

These are CV carbs, but they do work quite well with drilled boxes or indy filters provided that the filters are either mounted on Velocity stack style mounts, or use the OEM stacks to mount them onto the ends, or just have a properly fitting Indy filter that doesn't obstruct the inlet bell ports.

If you are familiar with YouTube, you can look my username up, and you'll find a few vids, one about the bike being run with the filters totally OFF, and you can see that the vacuum slides work very well without any restriction or vacuum chamber on the inlet side! But as was eluded to, due to more air flow more carb tuning will be needed to get them to behave properly, but it's not that difficult!
T.C.

[bikerphil]

[Crazcnuk]

The airbox does not provide 'backpressure'. It's extra drag.

The vacuum is mechanical, provided by the piston going down the cylinder.

A gas engine is an air compressor. The easier you can get air in, and exhaust out, the more efficient it is.

These bikes will run fine with no air cleaner, if you want to tune it that way.

technically, if everything was perfect, you shouldn't even have to retune for exhaust or air box changes, as the carb should meter fuel the same either way. x air in, y gas added.

There is a whole article around here somewhere where a guru explained that, for the most part, people are rejetting for no reason, when making minor changes.

May explain why so many of these bikes get such bad gas mileage....

[Ivan]

The slides work by bernoullis principle.

If you look at the end of the slide there is a small hole, the one that you drill out with a dyno jet kit. Since there is air flowing passed this hole during engine operation, and the air inside the slide is static (more or less), the moving air has less pressure than static. This causes a vacuum condition on the end of the slide, which pulls air from inside the slide through the small hole. Less air in the slide makes it go up. Everything is balanced, hopefully, to provide optimal fuel air mix at all ranges.

It works the same way as a solvent gun, an air brush, or even just blowing air across the end of a straw to get the fluid to rise. Same thing as the old grade school experiment of blowing across a piece of paper and making it rise.

[webbcraft2150]

[QUOTE=Ivan;198002]The slides work by bernoullis principle.


A here all this time I thought XS carbs just sucked.

[Ivan]

[QUOTE=webbcraft2150;198095]

The slides work by bernoullis principle.


A here all this time I thought XS carbs just sucked.

No, its the engine that sucks, the carbs just get in the way and introduce a bit of sauce that makes the whole thing blow.

[dbeardslee]

An engine is an air pump, and you really need to look at both sides of it. If you have a freer flowing aftermarket exhaust, then you may also need to address the intake side of the motor to keep everything flowing smoothly - like drilling your airbox. You may also need to rejet the carb afterwards as it will affect the mixture.

[Crazcnuk]

But that's the rub, it SHOULDN'T affect the mixture.

The mixture is regulated by the jets. x amount of air flows by, sucking through y ratio of fuel.

Unless you push the intake or exhaust beyond the limits of the carbs, changes like those shouldn't change the mixture.

If I can ever find that article, where the carb expert explained this, I will post it.

You have to understand that most of the 'improvements' made by users are felt in the head, not in reality.

Like those people that can feel thier bikes improve if they buy the expensive oil, or use premium pump fuel....

if putting a loud exhaust on actually improved performance, half the pickups around would be able to wheelie...

[Ivan]

But that's the rub, it SHOULDN'T affect the mixture.

The mixture is regulated by the jets. x amount of air flows by, sucking through y ratio of fuel.

Unless you push the intake or exhaust beyond the limits of the carbs, changes like those shouldn't change the mixture.

That isn't entirely true. Think on the level of boundary layer. The only part of the air that actually effects the draw of fuel is the very thin layer of air that is flowing past the jet. Bernoulli's principle again. The critical factor in determining the amount of suction created to draw fuel is the velocity of the boundary layer of air going past the jet. So, if you have a little bit of air going past the jet at the same speed as a lot of air going past the jet, the amount of suction at the jet will be the same. Since these are Constant velocity carbs, the boundary layer will more or less always be the same speed. A free breathing engine will pass more air at the same speed (slides open further) and since the amount of suction at the jet is the same, the same amount of fuel will flow. Now admittedly this is dampened by the needle being higher and allowing more fuel, but, it is such a fine balance, that the needle should be tuned to fit the new free flowing engine as well (Dynojet kit ) All this needle and slide stuff, of course goes out the window when the engine draws enough air to open the slides fully, then it is all about main jet. If the engine can pull enough air that the slide opens at half throttle, then the velocity of the air past the jet will increase from there up, creating more suction on the jet. This is the way most auto carbs work, by the throttle varying the velocity of the air to control fuel.

TBH, in the automotive world, the CV carb was a bad design. Ask any old hand mechanic about the Ford VV (Variable Venturi) carb. It was supposed to be self tuning and compensate for all the density changes in the air by its self. Great on the drafting board, but not so good in the real world. There was a racing carb in the early 90s, and may still be out, called the Predator which was a CV carb as well. http://www.predatorcarb.com/performance/carburetor.html From what I heard it was great for 1/4 milers that spend all their time at full power, but it had almost no "drivability" meaning it wasn't well suited for the street. I am surprised that motorcycles get them to work so well. I have no complaints about my carbs, but it is still surprising to me that they work as well as they do.

In light of the above, i am not surprised that they require lots of tweaking to get right. But, hey, that's where the fun is, right?

[b.walker5]

... So, if you have a little bit of air going past the jet at the same speed as a lot of air going past the jet, the amount of suction at the jet will be the same. Since these are Constant velocity carbs, the boundary layer will more or less always be the same speed. A free breathing engine will pass more air at the same speed (slides open further) and since the amount of suction at the jet is the same, the same amount of fuel will flow....

I may be picking this up wrong, probably am, I'm an Electrician not a Mechanic, but what I'm getting from this is that irrespective of the amount of air flowing through the venturi the pressure on the fuel metering system will not change..?? If this is the case then Crazcnuk would be correct and no change in jet sizing would be required .... ?

As for the Predator carb, I had one of these on a 400 small block a few years back, and they worked okay if you had well designed and ported manifold and heads. Thier biggest problem was that a lot of guys simply unbolted their Rochester Quadrajet, or the equivalent Ford thingy and fitted the Predator in it's place. The standard Chev manifold design back then was a disaster for this type of carb and there were that many pressure pulses in the manifold that the carb couldnt work, except at WOT. I had a good manifold but I eventually took that and the carb it off in favour an Eidelbrock manifold and twin 600 Hollies. Went much better. The Predator was really only aimed at the racing market.

I think the reason CV's work so well in bikes is because they are generally dedicated to one only cylinder and don't have to deal with pressure pulses from other cylinders caused by the inlet valve opening and closing.