CV carb question

[sk8norcal]

New to carbs here,
I was watching this video and was wondering what is going on at 1:19

engine is idling, so should be on the idle/pilot circuit.
He manually raise the slide piston/jet needle with his finger.
Butterfly is closed (since it's on idle)

Why did the rpm drop when he raise the piston?
Why did the rpm go back up when he lowered the piston?

[YOUTUBE]A95SZs6eqHI[/YOUTUBE]

 

[ScottRNelson]

CV stands for constant velocity. The air goes through at a specific rate and the carburetor relies on that to get the mixture right. Lift a slide and the air slows way down causing a lean condition. The one cylinder is getting a mixture that it can't fire.

 

[Pushrod]

Loss of intake velocity vs. gain of velocity.

 

[ST Guy]

CV stands for constant velocity. The air goes through at a specific rate and the carburetor relies on that to get the mixture right. Lift a slide and the air slows way down causing a lean condition. The one cylinder is getting a mixture that it can't fire.

More specifically, since letting more air in by lifting the slide doesn't also increase gas into the engine, you get a lean mix. (The butterfly is still closed.) The carb works when all parts are running correctly.

 

[sk8norcal]

thanks for the response,

what I still don't understand is that if the butterfly is closed, isnt the air just getting through via the pilot air hole?

maybe there is leakage around the butterfly?

 

[Pushrod]

Opening the throttle makes the butterflies pivot. Causes a vacuum to lift the diaphrams and the slides and the needles in the jets.

More air flow and fuel flow until a balance on the throttle position is achieved. Rinse, repeat as the wrist moves the cables.

You will now be able to awe your grandchildren with tales of archaic technology in the coming age of electric sport bikes.

 

[ST Guy]

The butterfly is not completely closed, even at idle.

 

[sk8norcal]

Opening the throttle makes the butterflies pivot. Causes a vacuum to lift the diaphrams and the slides and the needles in the jets.

More air flow and fuel flow until a balance on the throttle position is achieved. Rinse, repeat as the wrist moves the cables.

Yes, I understand all that. If you watch the video, the butterfly didn't move, (on idle).

 

[sk8norcal]

The butterfly is not completely closed, even at idle.

ah, ok, I see.

 

[AdamFiveFive]

This is a fairly reasonable animation explaining the way a constant velocity carburetor operates.

https://www.youtube.com/watch?v=IBFq9nHVmks

 

[AdamFiveFive]

I took the air filter off for cleaning. Noticed that the breather bolts brought a little oil into the carburetor. It's a little dirty in there. No big deal. I'll spray some carb cleaner.

Then I got the idea that I should just rebuild the carb, since I was already taking off the float bowl to clean the jets and passages, and pulling off the top to remove the vacuum slide. No big deal. These things don't have that many parts.

What I found, after taking things off, was that my diaphragm on the vacuum slide, the diaphragm in the accelerator pump, and the float bowl gasket were fine. Now I'm wondering if I should just clean the carburetor body, and reassemble it with the existing parts. Or should I install new parts, even though the parts that came off the bike are perfectly fine.

From previous experience, the cheap carb parts kits worked just like the identical kit which cost 5 times as much. And for the price of an expensive carb rebuild kit, I could buy a cheap carburetor and use all of those parts. For what it's worth, those cheap carbs being sold on eBay & Amazon are identical to the OEM carburetor, and work just fine if you jet it properly, set the idle mixture correctly, and the float is properly adjusted.

 

[motomania2007]

Clean and reassemble

 

[afm199]

If you have carbs open, you can easily check the main jets by a look through them, and check the needles for wear. The pilot jets aren't like that. It's not necessarily a bad thing to just replace them with new.

 

[islemann]

Do beware the use of any chemical carb cleaner on the CV diaphragm. Denatured alcohol or (of all things) clean/new brake fluid will not harm the diaphragm.

 

[afm199]

Do beware the use of any chemical carb cleaner on the CV diaphragm. Denatured alcohol or (of all things) clean/new brake fluid will not harm the diaphragm.

Absolutely this. Brake cleaner can rot carb o rings, making them swell and deteriorate.

 

[HadesOmega]

If you raise the slide you are raising the needle and also increasing the amount of fuel. So the mixture was probably overly rich in that one cylinder is why it was lowering the RPM.

 

[sk8norcal]

If you raise the slide you are raising the needle and also increasing the amount of fuel. So the mixture was probably overly rich in that one cylinder is why it was lowering the RPM.

Seems like some people are saying lean, and some are saying rich.

One of my friend believes that the butterfly is fully closed, so raising the CV piston would not have fuel coming out the main jet.

Merlin, have you physically try this with your thumpers?

maybe my friend can let me mess around with his Hornet 599, so I can test this out for myself.

 

[somesuch]

If the idle mixture is off, you will have to compensate (open the throttle valve more) with idle screw to get the desired idle speed, this will then increase the percentage that the intermediate and main circuits play.....by manually lifting the slide/needle you decrease the depression that those circuits see and therefore decrease the amount of fuel that they contribute..... so you are leaning out the mixture..... the amount of air consumed by the motor would have stayed the same had the idle not dropped

Probably not a very clear explanation

 

[somesuch]

So if your idle mixture was right to begin with, the intermediate and main circuits would have played a lesser role in supplying fuel, and the whole effect of lifting the slide would have been less.

I have run carbs without slides when testing...