How a ZXR400 CV Carb Works.

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banner001
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How a ZXR400 CV Carb Works.

Post by banner001 » Sun Jan 24, 2016 11:57 am

Ive been meaning to put something like this on here for a while now, and now that i have the time ive decided to unleash it on you, the unsuspecting public. If you ever wanted to know how a CV carb like the one fitted to the zxr400 works then here it is:

First up, always have it in your head that the carb is reacting to the vacuum produced from the piston moving down the cylinder bore, as that vacuum is produced pressures change inside the carb and fuel is sucked from the bowls as a result. Where that fuel comes from, and how much is delivered is determined by the jets/needles/screw settings etc.

The first diagram is a CV carb, two points to note, the starter enrichment circuit (its not a true choke by the way) is not shown, but if people want to know how it works i can make a separate diagram for it, and secondly this picture shows the pilot and main air jets in the body of the venturi, whereas in our bikes its parallel to the venturi, positioning aside this makes no difference to the functioning of the carb.

Image



When you hit the starter button you create a vacuum on the engine side of the carb that is at a lower pressure than the atmosphere on the other side of the carb. This causes air to be sucked through the main body of the carb. This low pressure on the engine side causes fuel (RED) to be sucked from the float bowl through the pilot jet, mixed with air from the pilot jet (BLUE) and the fuel air mixture (PURLPE) sent through the transfer ports and the pilot opening via the mixture screw...see the diagram below:

Image

The purpose of the transfer ports is to ensure that when you slam the throttle closed after being at WOT there is some fuel that can always be found by the engine to prevent you stalling.

You can see therefore that the purpose of the mixture screw is to vary the fuel/air mixture going into the engine, if you turn the screw so it closes fully the only mixture going to the engine is from the transfer ports, by opening the mixture screw fully you allow fuel to pass through the transfer ports and a large amount to go via the pilot opening.



This next picture shows the engine running at max revs with a WOT:

Image

Three things have happened between idle and WOT:
1. The butterfly valve has moved into its least restrictive position.
2. The increased vacuum present in the carb has created a vacuum (GREEN) in the slide chamber, the rubber diaphragm has been sucked up onto the roof of the slide chamber as air has been sucked via the slide port, this has the effect of raising the needle jet and in doing so creates a larger "hole" in the end of the main emulsion jet.
3. The lower pressure in the carb is causing fuel and air to be sucked from the main jet and from the main air jet respectively, these are mixed in the main emulsion tube, flow past the needle jet and out into the carb.

A diagram of the fuel flows is shown below:

Image

This rpm % on the x-axis, and fuel required by the engine (as a percentage of the max available on WOT) on the y-axis, you can see that the pilot jet meters a near constant amount of fuel that is good for tickover, as engine revs rise more fuel is required by the engine, and this is delivered from the main jet.
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fergy150
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Re: How a ZXR400 CV Carb Works.

Post by fergy150 » Tue Jan 26, 2016 1:19 pm

Cracking informative write up there, cheers :smt001

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Re: How a ZXR400 CV Carb Works.

Post by Zaphod » Thu Feb 04, 2016 7:23 am

good job bammer !!

just one point for the fuelflow..
it is according to %-opening of the slide not revs !!
look at the effectiveness of the different parts of the carb
jetting chart.JPG

©keihin

brilliant post of yours !
cheers
zpahod
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banner001
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Re: How a ZXR400 CV Carb Works.

Post by banner001 » Mon Sep 12, 2016 9:23 am

Zaphod wrote:good job bammer !!

just one point for the fuelflow..
it is according to %-opening of the slide not revs !!
look at the effectiveness of the different parts of the carb
jetting chart.JPG
©keihin

brilliant post of yours !
cheers
zpahod
you are correct, however with a CV carb the slide position is determined by the vacuum on the engine side, and as this is dictated by the revs the two are interchangeable for a CV carb
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Cadorna
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Re: How a ZXR400 CV Carb Works.

Post by Cadorna » Tue Oct 04, 2016 3:06 am

Great post, thanks !

This is my little aport to it: https://www.youtube.com/watch?v=wyspAHrMbb8

In conclussion, my bike smell to gas when idle, so I've two ways to go, increase air on the fuel/gas mixture or lower gas in the same fuel/gas mixture. PAJ (Pilot Air Jet) cannot be regulated/modified (not easily I mean) so I have to play with the mixture screw (float bowl levels are right) and here comes the eternal question: When I'm turnin in this screw, I mean close it, I'm reducing the intake of fuel, right? So the relation air/gas will be better for the first one, insn't it ? In my situation if I have 3 turns out from zero and I want to lower the smell of fuel from I should start screwing in the mixture screw, for example to 2 1/2 tuns , right ?

There is a lot of confussion about it 'cause some people interprete the exact opposite to that...

Apologize for my english.

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Re: How a ZXR400 CV Carb Works.

Post by banner001 » Tue Oct 04, 2016 9:15 am

Cadorna wrote:Great post, thanks !

This is my little aport to it: https://www.youtube.com/watch?v=wyspAHrMbb8

In conclussion, my bike smell to gas when idle, so I've two ways to go, increase air on the fuel/gas mixture or lower gas in the same fuel/gas mixture. PAJ (Pilot Air Jet) cannot be regulated/modified (not easily I mean) so I have to play with the mixture screw (float bowl levels are right) and here comes the eternal question: When I'm turnin in this screw, I mean close it, I'm reducing the intake of fuel, right? So the relation air/gas will be better for the first one, insn't it ? In my situation if I have 3 turns out from zero and I want to lower the smell of fuel from I should start screwing in the mixture screw, for example to 2 1/2 tuns , right ?

There is a lot of confussion about it 'cause some people interprete the exact opposite to that...

Apologize for my english.
yes you are correct.

by screwing the pilot adjuster screw clockwise, you reduce the amount of fuel/air mixture that enters the carb, as the vacuum of the engine always pulls in a given volume of air through the carb that large charge of air will now contain less fuel, and so will be leaner.
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Re: How a ZXR400 CV Carb Works.

Post by Dominik » Wed Apr 19, 2017 9:07 am

Hi,
Does anyone know if the main air jets are interchangeable in the cv carbs? I'll fit a ram air system to my bike and want to try some different sizes there instead of blocking it completely

Thanks!

Edit: Just seen that in the FAQ section...
E. Main air jet and Pilot air jet
It is possible to remove these (they are a press-fit) and tap a hole to allow you to adjust the size of the air jet...but in practice the gains are minimal and if you are doing this a set of fully-adjustable FCR's is likely the best way to go.
What does "tap a hole to adjust the size" mean? If they're press fit can't it just get different sized jets fitted?!
Zxr 400 SL - 130kg

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banner001
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Re: How a ZXR400 CV Carb Works.

Post by banner001 » Wed Apr 19, 2017 10:10 am

The brass tubes are pressed into the carb, i.e. not threaded for a jet. What you can do is get a load of heat on the brass tube and use an easyout to remove the brass tube. You can then drill and use an M5 0.8mm thread pitch tap to get a threaded hole...Into this you can now try different "main" jets of small sizes and they will act as the brass tubes did before. It's been done before, I'll try and find the link.

Link: viewtopic.php?t=11502
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Dominik
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Re: How a ZXR400 CV Carb Works.

Post by Dominik » Wed Apr 19, 2017 1:34 pm

Carbs and loads of heat sounds good, what could possibly go wrong :D
Thanks man that was what I was looking for ;)
Zxr 400 SL - 130kg

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