2 Questions about Carburetors

Frank Castle · 04-02-2014, 11:51 PM

1. Whats the size of a stock Carburetor on a GY6 (150cc) scoot?
Is it 24 mm?

2. What does 'CVK' mean with Carburetors?

cheapeto · 04-03-2014, 04:55 PM

Hey FC, I found this over on Modern Vespa, in the archives.

[You would expect that when you pull on the throttle a cable opens a slider on the carb. It may. The problem is then for a bit you're running quite lean as the air speed is decreased because of the larger volume opening. Then as the engine speeds up, gradually the air speed increases and Bernoulli starts sucking gasoline again. This give poor throttle response. This is compensated for on some carbs with a pump connected to the cable which will throw some raw gas into the chamber. Very primitive, dirty, and not common on small carbs.

On a CV (constant vacuum) carb, the slider is connected to a vacuum chamber. When you yank the throttle, you're opening a butterfly. The slider stays closed until the motor starts speeding up. This maintains good airflow speed. As the motor speeds up, the vacuum increases, pulling the slider out of the way.

Better, smoother throttle response, no lurching and consistent fuel/air ratio.

Simple mechanical solution.

Looking at your carb, if the cable goes right into it, usually at the top, you're not CV.

If the cable goes to an arm on the side, and there is a sort of bump at the top, you're CV.

P.]

CVK, is constant vacume Keihin

Not that I knew, but I was curious also.
One of the mods I did to my scooter, along with rejetting, was to drill out the slide vacume port.

Frank Castle · 04-04-2014, 12:42 AM

Quote:

Originally Posted by cheapeto

<snip>

CVK, is constant vacume Keihin

Not that I knew, but I was curious also.
One of the mods I did to my scooter, along with rejetting, was to drill out the slide vacume port.

thanks cheap, great info

epyon96r · 04-04-2014, 08:13 PM

I think 24 sounds right, I read that going to a 30 improves performance, but I dunno

cheapeto · 04-05-2014, 09:00 AM

Hey Frank, I found some great information about carb's over on another site I belong to.
It's an artical that really open up my eyes to how my Keihin works. I know not everyone is running that carb, but the thread really helped me.
http://totalruckus.com/phpBB3/viewto...p?f=28&t=82639
BRed and his posts are gold.
Well thought out, great insight to mechanical issues and his write ups, have saved me hundreds of dollars. Heck, my scooter runs like a different machine with just following his threads.
I hope it helps you understand these carb's a bit anyway.

blueboy5000 · 04-05-2014, 09:43 AM

CVK stands for Constant Vacuum Kiehin. 24mm is the correct stock size carb. 30mm DO NOT improve the performance of stock unmodified gy6 157/152/1p57 as they are too large. A 30mm FLAT-SLIDE carb is for a gy6 with engine mods, such as a significant BBK.

The cheapest and easiest way to get optimum carb performance is with a an open-element air filter and a larger main jet ( #110 to #125 for UNMODIFIED gy6 152/157/1p57) the 24mm is more than large enough for a stock engine.

04-04-2014, 08:13 PM	#4
epyon96r Join Date: Jan 2013 Location: pasadena, tx Posts: 135	I think 24 sounds right, I read that going to a 30 improves performance, but I dunno __________________ 2009 Vitacci Elite 150cc Custom shortly muffler hi flo air filter minor weight reduction removed unnecessary hoses larger Pirelli rear tire 1 Rockstar sticker.(adds HP) 59 mph GPS verified
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04-05-2014, 09:43 AM	#6
blueboy5000 Join Date: Jul 2011 Location: Connecticut Posts: 493	CVK stands for Constant Vacuum Kiehin. 24mm is the correct stock size carb. 30mm DO NOT improve the performance of stock unmodified gy6 157/152/1p57 as they are too large. A 30mm FLAT-SLIDE carb is for a gy6 with engine mods, such as a significant BBK. The cheapest and easiest way to get optimum carb performance is with a an open-element air filter and a larger main jet ( #110 to #125 for UNMODIFIED gy6 152/157/1p57) the 24mm is more than large enough for a stock engine. __________________ 2011 Roketa MC-23-150 4T 150cc 157 qmj 24mm carb w 115 main jet and paper cone air filter Manual petcock w Tygon fuel lines Scrappy Dog Scooters Retro-slash stainless straight-thru exhaust RED spring clutch Adjustable CDI (brand unknown, it's blue and red and works great!) KOSO high performance variator w 12g sliders Gates Powerlink 835-20-30 belt GPS verified 65mph on flats.
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04-03-2014, 04:55 PM	#2
cheapeto Join Date: Jul 2008 Location: York PA Posts: 339	Hey FC, I found this over on Modern Vespa, in the archives. [You would expect that when you pull on the throttle a cable opens a slider on the carb. It may. The problem is then for a bit you're running quite lean as the air speed is decreased because of the larger volume opening. Then as the engine speeds up, gradually the air speed increases and Bernoulli starts sucking gasoline again. This give poor throttle response. This is compensated for on some carbs with a pump connected to the cable which will throw some raw gas into the chamber. Very primitive, dirty, and not common on small carbs. On a CV (constant vacuum) carb, the slider is connected to a vacuum chamber. When you yank the throttle, you're opening a butterfly. The slider stays closed until the motor starts speeding up. This maintains good airflow speed. As the motor speeds up, the vacuum increases, pulling the slider out of the way. Better, smoother throttle response, no lurching and consistent fuel/air ratio. Simple mechanical solution. Looking at your carb, if the cable goes right into it, usually at the top, you're not CV. If the cable goes to an arm on the side, and there is a sort of bump at the top, you're CV. P.] CVK, is constant vacume Keihin Not that I knew, but I was curious also. One of the mods I did to my scooter, along with rejetting, was to drill out the slide vacume port.
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