Constant Velocity Carburetor Explained

This is the first in a series of videos on general carburetor theory and repair service, with specific emphasis on 1979 thru 1983 Honda DOHC inline carburetors such as CB750 CB900 CB1100 GL1100 CB1000 and CBX.

[@Vhdrumm3r]

This channel was exactly what I've been looking for a long time since I got a carburated bike. Thank you Sir, more videos please!

[@tara3570]

Thank you for making this. I know carbs but these things blew my mind. I have a 4 wheeler that the slide is not moving on its own and now I think I can figure out why!

[@RonHuizer]

Very informative, thanks!

[@2pacalypse9]

Great video!

[@ShashankB94]

I was searching everywhere if velocity decreases as slide goes up thank you

[@Andrewmylife]

Fascinating

[@maxiscootercomua3102]

Super!

[@Lifehardened]

Love the video. But I’m wondering how we can play with this effect. I would like my slide to lift quicker with the throttle. I have clipped a couple turns off the spring. That has seemed to help. But still not getting the response I would prefer. I’ve heard of people drilling the hole in the slide bigger so that it lifts easier. But in my vacuum operating mind that would only slow the lift of the slide. Now with a tiny bit of understanding of this Bernoulli effect. What does drilling this hole bigger actually do. Allows to lift easier? Any info on getting quicker throttle response is appreciated. Cvk40 is my carb. Trying to avoid buying a slide card.

[@ikifkif]

this is great, but i was going to share this to a friend, but the piece of string you've incorporated gives the impression the slide lifts mechanically due to the opening of the butterfly, which might be misleading to someone who doesn't already know how a cv carb works.

[@iantennison6640]

Hi, you mention other video 'segments' that you have created regarding these carbs. Where can I see them please ? I cannot see them in your profile ?

[@137bob3d]

i wish you'd a moved your left hand out of the way and labelled other carb parts. and showed us the path gas takes when the choke is engaged.

[@robertsanders9915]

I need help! My shovelhead has a cv carb and spits gas out the air filter. I replaced the accelerator pump diaphragm and pulled the bowl every thing seems clean. What am i missing?

[@dago_0730]

since the displacement is constant, it has to be frequency that causes vacuum to rise in the upper chamber, right? Regards

[@ronenfe]

I don't think the frequency increases the vacuum. It's all about the opening of the butterfly.

[@ricardocalles140]

Slide goes up, velocity goes down. Slide goes down, velocity goes up. When then is it called a constant velocity carb? If anything, the designers should have called it a Variable Venturi carb or VV since this is what is actually occurring.

[@ronenfe]

I think it's because the air always goes in the same speed. if vacuum increases, it's supposed to increase speed, but the opening increases compensating and maintaining the same speed.

[@RossGoneRogue]

@@ronenfe your comment makes way more sense than the dude's explanation in this video. It sounds like it actually is a constant velocity through the carburetor and it's just changing the volume flow rate which makes sense.

[@ronenfe]

@@RossGoneRogue Now I'm thinking, when the butterfly is closed, the air flow is restricted causing the engine to run slower and creating less suction force so air speed is slow. When butterfly is opened, engine gets more air, creating more suction force which should increase air speed. But if more air is passed then the airspeed decreases. So what's the difference between the butterfly and the slide? They both control diameter of the air passage.

[@RossGoneRogue]

@@ronenfe so you're partly right. The venturi effect is explained by Bernoulli's equation. The suction force you describe is actually called Dynamic Pressure. When Dynamic Pressure increases, static pressure in that area must decrease which creates a vacuum. I think you were right the first time that velocity remains (roughly) constant through the slide. The butterfly valve controls the amount of air that is allowed to come through, but he didnt explain well that when you open the butterfly, it also creates a vacuum over the slide which is what causes the slide to move up allowing more air through the venturi (where velocity remains constant), but the volume flow rate increases going to the engine. More air means more combustion and higher power.

[@ScottMiller-tu3gg]

The slide is controlled by air passing through a moon shaped inlet at the top of the carburetor that in turn raises a vacuum diaphragm and thusly raises the slide and needle dependent on the degree of the butterfly opening. There is no mechanical linkage as your model depicts. I am sure you installed this for demonstration purposes only, but it adds to confusion as to the actual operation of this type of carburetor.

[@quadflopper1012]

why is it that people only talk and make videos about the easy obvious part of CV carb operation, twisting the throttle opens the butterfly causing vacuum to lift the slide which raises the needle allowing more fuel delivery ect ect but no one ever goes into how a CV carb delivers the fuel volume necessary when you are cruising 65 mph down the highway at a constant 5000 rpm but only using 5% or less of the throttle, there's no slide moving so no main jet fuel contribution and your throttle position is positioned just off idle, so where is the fuel to sustain that kind of rpm without going lean coming from? I've asked every self-ordained motorcycle expert on youtube, some more then once, but the lack of response tells me no one knows the answer......

[@ScottMiller-tu3gg]

A manual can explain this better than I can but here goes, Your main jet and low speed jet work in conjunction with each other - they basically overlap. When you are just cruising, the slide / needle are slightly elevated and the taper of the needle and the main jet size determine the amount of fuel that passes through the main jet. Fuel also passes through the low speed jet and combines with the fuel added by the main jet. Less fuel is required at speed and the movement of the slide is much less noticeable. Most all of the motorcycle manuals that I have show diagrams of this overlap between the jets and jet circuits. Hope this helps!@@quadflopper1012

[@quadflopper1012]

thanks it helped quantify some of questions I had concerning the basic operational theory of a CV carb but still doesn't give me any insight into finding a solution or even a cause for the mixture problem I've been chasing for 3 months, I'm talking plugs removed and read multiple times a day, carbs been removed and gone through more times then i can even remember. i've been working on bikes for 40 years and for the first time ever I'm at a lost and out of ideas but i don't have a lot of CV experience so i'm hoping someone can suggest some possible causes or solutions I may have missed...thanks

[@ScottMiller-tu3gg]

What kind of problems are you experiencing? @@quadflopper1012

[@quadflopper1012]

You might be sorry you asked that question…lol
NO air leaks ,,,, No matter what I do when cruising (cruising = a steady 5% or less throttle opening) at 3500 rpm's and lower all 4 cylinders run equally extremely rich, when cruising at 4000 rpm and above (5000 in 5th is 65 mph highway speed) all 4 cyl go equally extremely lean.
Specs: Stock 1980 Yamaha XJ650, stock Hitachi HSC32 carbs that in the last 3 months have been pulled and had every jet, orifice and circuit re-addresses with carb cleaner and compressed air more times than I can remember. New stock size main & pilot jets (110/40), stock non-adjustable needle in factory position, float level set to factory specs and rechecked multiple times, stock airbox and filter, stockish exhaust, exit diameter opened up by ½” but all baffles still in place
In an attempt to just get the bike to go rich when cruising above 4K rpm’s I’ve tried all the normal stuff in different combinations including increasing the mains from 110 to 117, raising the float level by up to 3mm, adding washers to raise the needles.
The end result was the bike running so rich below 3.5k rpm’s is was unridable but still going lean at cruise above 4k rpm’s. I’ve also tried running all the above changes with spark plugs 2 heat ranges colder but still goes lean above 5k with no change to the lean condition when cruising above 5k.
Any idea’s?

[@whollyman]

Not clear enough, sorry!

[@ronenfe]

unfortunately there is no clear explanation anywhere