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Iron Butt
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 Post subject: The Keihin Type VE3B CV carb/airbox/filter
PostPosted: Mon Jun 03, 2013 3:43 am 
A TR member called me with a question about adapting a pod filter to a Keihin carb.
I told him it could be done but there were many challenges in getting it to run correctly and that although the common solution of just jetting it fatter and fatter until it ran does work,
it defeats part of the design of the CV carb. I tried to explain it but later realized that pictures work better than words in cases like this.

Here is a basic image of how a carb works at rest, at idle and at WOT.....


Image

That is correct.....
that picture is essentially how a CV works, BUT a Keihin CV is actually a carb within a carb (really 2 carbs within a carb!) :dubious:

The CV carb is one of the most effective fuel metering methods ever devised. Of all carb designs, it alone can compensate for atmospheric changes due to humidity, heat and altitude. It has been called the Poor Man's fuel injector and it comes close....when calibrated properly, it can give the highest mileage with the leanest safe burn of any carburetor....only fuel injection can beat it and then only because it is using dynamic feedback and constantly adjusting the mix.

The CV does it all mechanically.
The name CV originally stood for Constant Vacuum (and vacuum is key to it's operation) but CV has come to also mean Constant Velocity. The end result is the same....by changing the venturi in the throat with a vacuum controlled slide, it maintains a constant air velocity over the needle jet. The CV's internal injection control is regulated by the difference in outside air pressure (@ 14.7psi) and the airbox/carb running pressure (@ ~13.7psi.) The CVs primary "fuel injector", the emulsion tube, operates on ~1psi.

Here's an image showing the parts involved in the CV "fuel injection" process......
the first image has a legend to identify parts for all the images to follow.
I borrowed and editted it to show how the CVs internal carbs work. :wasted:

Image


On the following images,
dark blue is atmospheric air,
light blue is internal air at reduced pressure due to intake suction and airbox restriction,
yellow is liquid fuel (gasoline)
green is emusified fuel mixed with oxygen.

Why emulsify??
a couple of reasons...
first, by creating little bubbles of air wrapped in gasoline, it evaporates FAST!
bubbles of gasoline are better than drops of gasoline

second, an emulsion doesn't fall back down the needle jet like a liquid would......
the air/fuel emulsion provides a uniform flow from the pulsing action created by 30-150 short intake sucks per second

third, the emulsion chamber can react to changing throttle loads by adjusting the fuel ratio on the fly
and it doesn't even use a processor.
What it does use is vacuum!

Below is our idling scoot happily chugging along......
at idle with the butterfly closed, the slide is down and the needle is fully inserted into the needle jet.



Image


Everything looks normal, but how is that fuel in the emulsion tube and it's well defying gravity?
It's about an inch higher than the fuel in the bowl....the needle jet is sucking a little but with the needle down it's not enough.

What's holding the fuel up in the emulsion chamber is 1 psi.....
the difference between inside air on the top of the emulsion chamber and outside air pushing down on fuel in the bowl.
that's our CV fuel injector at work.....all 1 psi, lifting a few drops of fuel about 1 inch.


Now we twist the throttle open and the fuel being held in the emulsion chamber get sucked out in a quick rich shot and now at Wide Open throttle,
the chamber empties out and the needle jet us pulling fuel directly from the bowl with all ports in the emulsion chamber furiously bubbling air into the fuel stream, emusifying it.


Image


so what could possibly go wrong, right?

it's that vacuum thing.....
everything about a CV is cool until you change anything about it's air box.....
even a free flowing K&N filter will affect this fine balance.

so let's say we want to toss the airbox and run a pod type filter......
the first thing that happens is this:

Image

so we're idling and everything looks ok, until we try to roll on throttle....
without the airbox, we've lost our 1psi.....with the reduction of vacuum, the emulsion chamber now is empty.
it's air bleed cannot provide enough suction to lift the fuel out of the bowl and flood the emulsion chamber.
the result is the mix is being over-oxygenated...at idle the emulsion tube is providing the sort of mix it needs at top end and it is LEAN.
Now when we roll on throttle, we get a lean spike of mix as soon as the needle opens and the mix is lean until we reach topend and the main jet is finally providing enough fuel to even it out.

In this scenario, the emulsion chamber is always empty and the only time your mix will be right is at topend when fuel flow finally catches up with air flow.

There is another scenario for emulsion chamber failure from the exact opposite....
let's say we add some restriction of some type or let's say the air jet is the type located ni the carb throat instead of under the diaphragm.
you can have a situation where you have too much bleed from the air jet and it keeps fuel too high in the chamber.
it would look like this:


Image


In this case, the low pressure air is competing with the needle jet and winning!
with the throttle wide open and slide fully up, the needle is out of the jet, but the emulsion chamber is FULL.
now at WOT, your mix goes way too rich, since the high fuel level is covering all of the ports that should be open at this stage.
Now the midrange is strong, but the mix goes too rich at topend.

in both of these case, with a larger CV carb like those on my Shadow or Ural or those used on many Harleys, the answer would be simple....
install a smaller or larger air jet. Air jets are as common as fuel jets on standard sized carbs.

In one case it would pull the fuel level higher at idle and in the other case it would allow the fuel level to drop at WOT as it should.

We don't have that option....our air jets are solid bushings pressed into place.
We can over jet it or under jet with main jets to try to compensate, but we cannot correct the air bleeds.

We can do some tricks with the emulsion tube.....
when you look at an emulsion tube the needle jet is on top, the bleed section with holes houses the needle and the main jet is on the bottom.

The air holes closest to the needle jet at the top are used at idle and rollon, the holes closest to the bottom are used at WOT.
If we know by observations (an AFR reading or plug read) where the mix is lean or rich, you can plug holes in one location and drill more holes in another location to compensate...it won't be perfect but it's better than running with the emulsion system totally out of whack.
If you know the emulsion chamber level is low, plug the top four holes and drill 4 more towards the bottom of the tube.
If you know the level is high, drill two more air bleeds near the top and plug the bottom two (and downsize the main jet while you're at it!)

think of it this way.....
the needle jet at idle is sucking an even amount from both the main air jet and the main fuel jet
as revs increase and the needle opens the jet, it should overpower the air jet and eventually empty the emulsion chamber and at low revs with the needle down, it should refill.

One way of dealing with this is to find a larger needle jet and matching needle....
this works the same way as drilling the vacuum port in the slide.
with reduced vacuum, we need a larger opening....by increasing the size of the needle jet, you can make it have more suction on the emulsion tube.
If you can't make the air jet smaller, make the needle jet larger.
This will also allow you to use larger main jets without flooding the emulsion chamber.


The easiest way to avoid this issue is to try to use a carb designed for the amount of air you want to feed it....
each CV carb is tuned to operate on a fixed inlet size supplying a fixed volume of air.
Try to find a carb with an airbox inlet the size you want to use.

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Junior Mint
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 Post subject: Re: The Keihin Type VE3B CV carb/airbox/filter
PostPosted: Sat Aug 24, 2013 8:05 pm 
Excellent reading, Ed. Thanks a whole bunch for taking the time to put all this together. Great explanation!

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Junior Mint
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 Post subject: Re: The Keihin Type VE3B CV carb/airbox/filter
PostPosted: Sat Aug 24, 2013 9:50 pm 
I have a question for you, bud. When you say : "We don't have that option....our air jets are solid bushings pressed into place. We can over jet it or under jet with main jets to try to compensate, but we cannot correct the air bleeds." Can't you just move the Needle up or down in the Slider?

For example, my old Yamaha Venture had a circlip and a bunch of notches just under the head of the Needle, so that it could be moved up (to drop the needle down deeper into the Needle Jet) or down (moving the Needle up). Since the Needle was slightly tapered, moving the needle up or down in the Needle Jet would be like changing the diameter of the Needle at any given position of the Slide.

I also vaguely recall the Needle adjustment done with tiny washers, but part of that memory must have been killed by beer. Hmmmm...beer.

I haven't looked inside the CV side of the BR carb yet 'cause I'm too lazy. I'm going to wait for that *&%# ethanol to separate form the gas turn to sludge over the winter, and then I'll have to take the carb off. I have no idea if the needle is adjustable, but this view of the carb seems to suggest that there is some sort of adjustment possible : http://www.cheapcycleparts.com/oem-part ... carburetor

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Iron Butt
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 Post subject: Re: The Keihin Type VE3B CV carb/airbox/filter
PostPosted: Sat Aug 24, 2013 11:12 pm 
Shimmng the needle with washers is a common step in fine tuning response after up jetting, but it will not correct the air bleeds for the emulsion tube.
In fact with larger CV carbs, shimming the needle is often done even after installing the correct air jets.

There is a reason that Honda uses a specific carburetor model for each bike's airbox configuration, resulting in dozens of versions of the same carb.....
if it were just a matter of shimming the needle height, the Honda engineers would just use a different needle jet/needle for each airbox type and use a common carburetor model for all versions.

So yes, there are many things you can do to make the engine run better after modifying the airbox, but all of them are just work arounds for the problem you create when you change the inlet pressure by feeding the engine more air. None of the work arounds will correct the fine balance between incoming air flow and fuel flow needed for the emulsions tube to work as designed. With the VEB3 type carb, my adivice is to match the carb model to size of the air inlet it was designed for.

You'll probably still end up shimming the needle, if you get very far into tuning it.

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Junior Mint
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 Post subject: Re: The Keihin Type VE3B CV carb/airbox/filter
PostPosted: Sun Aug 25, 2013 1:23 pm 
Yessir, I agree that changing the airbox is a big parameter in the proper functioning of the carb. Correct me if I'm wrong but, aren't airboxes also tuned for certain resonances, something like a two stroke exhaust pipe, in order to get the desired torque or HP?

Anyway, back to the needle adjusting - I take it that shimming the needle would only be good for minor changes, like installing a K & N filter?

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Iron Butt
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 Post subject: Re: The Keihin Type VE3B CV carb/airbox/filter
PostPosted: Mon Aug 26, 2013 4:18 am 
kakeyed wrote:
aren't airboxes also tuned for certain resonances, something like a two stroke exhaust pipe, in order to get the desired torque or HP


Yes, that's correct and it's definitely a major factor with most cable slide carbs like Mikunis, but not as much on the vacuum controlled slide carbs, like the Keihin CV.
The internal configuration of the airbox is also part of the EPA Noise Emission controls.....the running engine sound levels.
The inlet diameter and the length of the air passageway leading to the carb inlet is very important to a CV, as is the fact that the crankcase ventilator is connected to the airbox....it is part of the "scavenging" you mentioned.

The main differences between tuning a carb for a HD or VT1100 Shadow and tuning a scooter carb is that scooters operate at much higher throttle settings than do the bigger bikes at road speeds.....at 80mph on a scooter, the throttle is WFO, while on the VT1100 the throttle is probably just over 1/4 open.
It's all about available HP.

On the air box configuration for different Honda 250 scooters, the Reflex air inlet is roughly twice the size (maybe a little more) than the Big Ruckus inlet....
the Foresight air inlet is even larger than the Reflex. Each one uses a different carburetor with fixed air jets sized relative to the airbox inlet diameter (restrictor).

On each of these carbs/airbox setups, shimming the needle and resizing the needle/needle jet can have a huge impact on midrange performance.

By modifying the airbox (like opening up the inlet or changing to a pod type filter) the tuner can create a condition inside the 2 emulsion chambers that's impossible to retune for....the pilot jet and main jet will let more fuel into the carb throat, but that fuel is not being properly premixed and emusified with air in the emulsion tubes.

I'm not saying the engine won't run...it will, you just won't get optimal HP from it.

The effects from modifying the airbox will always be at the extreme ends of the throttle scale.....@0 - 1/4 throttle and @3/4 - WOT.
Once you modify the airbox, you can tune for bottom end or for topend, but not for both.

kakeyed wrote:
I take it that shimming the needle would only be good for minor changes, like installing a K & N filter


All of the needle mods primarily effect 1/4-3/4 throttle or midrange....you would still need to up jet to compensate for higher air flow.

Here's a chart that shows relative effect of internal carb components......
the most important thing the graph shows (in my opinion) is how little impact the main jet has on overall performance.
It mainly affects wide open throttle.

Image

The needle size, taper and height (shim or clip) has a huge effect on mid-range, everything from 1/4 throttle to 3/4 throttle.

The point I was trying to make in this post is this...if you intend to open up the airbox or switch to a pod type filter, you really want to replace the BR's carb with either a Reflex or a Foresight carb....both were designed to use twice the air volume of the BR's carb and to operate properly with a lower inlet suction (less restriction).

These observations were made while retuning the carb after opening up the airbox, installing a K&N filter and a high flow BEAMS exhaust system.
Each of these incease air flow through the engine and you'll reach a point where the BR's stock carb simply cannot meter fuel accurately, no matter how you rejet or shim the needle.

This is why after months of experimenting and gradually adding more and more Reflex carb parts to the Big Ruckus carb, I finally swapped a complete Reflex carb into #2. The results, as shown in the Veypor graphs posted in this section, were impressive...a second off the 1/4 mile time, 5mph higher topend at lower rpm and 8mph higher speed when topping a steep grade.

After months of searching on eBay, I now have a new Foresight carb that I intend to install and I think it will show even better performance.

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 Post subject: Re: The Keihin Type VE3B CV carb/airbox/filter
PostPosted: Wed Sep 11, 2013 12:54 am 
I just purchased a big ruckus and started reading a lot of post (about modifying a big ruckus). I have one question; why not add a filter pod and reduce the air flow by necking down the connecting air tube to match the air flow of an OEM filter for the ruckus? I was thinking running several base line test and have a small metal sleeve that can be drilled out until it matches the base line test. This way you can add a pod, which is smaller than the OEM air filter box. My idea was just matching the air flow and fool the CV carb.

Is this possible or am I out in left field??

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Iron Butt
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 Post subject: Re: The Keihin Type VE3B CV carb/airbox/filter
PostPosted: Wed Sep 11, 2013 12:25 pm 
You're on the right track but there's another attribute of the airbox that needs to be addressed.....the length of the incoming air tract.

Since the engine is a single cylinder, it's only pulling in air through the intake about 1/4 of the time.

A long air tract allows the incoming air flow to develop some inertia that continues through the compression, power and exhaust stroke.....
when I was experimenting with a pod filter, I found that the longer the pipe (or stack) between the carb and filter pod, the better the performance.
With a short air inlet tract, the engine starves for air at higher rpm....the airflow cannot develop any inertia. It lacks sustainability.

The single cylinder can not suck in enough air at topend, unless the air flow is being sustained by a longer inlet.
Quad riders deal with this by using a long inlet hose wrapped around under the body....usually the longer the inlet tract the better it works (within reason).

Also keep in mind that the crankcase ventilator is assisting in sustaining the air flow.....
as the intake valve opens and the piston moves down, the engine sucks air into the air box (or air tract, in the case of a pod filter).
As the piston moves up to compress the mix in the cylinder (compression stroke) it creates low pressure (vacuum) in the crankcase.....
this vacuum sucks more air into the airbox....this helps to sustain the incoming air flow.

This "scavenging" is very important in small single cylinder engines.
When I removed the crankcase vent line and put it on a separate filter, the Veypor showed an immediate drop in power at topend.
To make sure the filter I used was not restricting the out flow from the crankcase (causing back pressure beneath the piston), I even ran several tests without a filter....
it made no difference.

The best way to know for sure is to try it and see what happens, as long as you have some means of obtaining objective data to compare.

I'm not saying it can't be done, but I am saying this is something that needs to be addressed if you're tuning for maximum performance....
if the pod filter is just for cosmetic reasons, then it doesn't really matter if it reduces performance.

I still believe that for the serious tuner, you need a better carb than the one supplied on the BR.
The tiny OEM 98 main jets are the best proof of that.

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