Winter Project
Senior Member
SuperSport
Joined: Dec 2005
Posts: 963
From: Sunshine Coast, Australia
The FCR at 41mm is the go from what I know. More power across the range compared to a 48CV. This is the opinion of a guy I know that has fitted them, and is experimenting with the tuning. He reckons there are better all round, and are easy and smooth to ride with on the street.
Senior Member
SuperSport
Joined: Dec 2005
Posts: 963
From: Sunshine Coast, Australia
So if you're saying that the freer flowing intake will cause what, a reduction in vacuum, right? That makes sense. Hopefully you'll agree with that fact. If I'm wrong tell me.
So the slides are controlled by vacuum through a small hole in a metal part of the diaphragm, right? So if you were to modify (read: make larger) that hole then the slides would respond more quickly, right?
Alternatively, you could reduce the spring strength on top of the carb slide and it would also respond more quickly, correct?
Have either of you guys looked into these two options?
So the slides are controlled by vacuum through a small hole in a metal part of the diaphragm, right? So if you were to modify (read: make larger) that hole then the slides would respond more quickly, right?
Alternatively, you could reduce the spring strength on top of the carb slide and it would also respond more quickly, correct?
Have either of you guys looked into these two options?
So what my guess was, is that the vaccumm to the slides is constant, and the extra air is too much for that vacuum, not that the vacuum has been decreased by the increased airflow. But of course you may be right.
In regards to the spring, that was one of my options, along with perhaps getting a different size spring. I have DJ springs, so maybe the stockers may be better in my bike, or perhaps I can find something else to substitute them. Spacers are also an option to test.
I reckon you would need to be careful in modifying the carb body, as that could turn out expensive! Us backyarders tend to get stuff wrong when we are stuffing about....
Administrator
World Champion
Joined: Dec 2004
Posts: 105,287
From: Fulton, MO
I was just thinking about this and something doesn't add up. When you are syncing the carbs with mercury sticks the most vacuum occurs at idle. Vacuum goes down as rpm rise. So how are the CV slides controlled then? Vacuum pulls them up but if there's less vacuum...? Okay, I'm now confused.
Administrator
World Champion
Joined: Dec 2004
Posts: 105,287
From: Fulton, MO
Spacers will not work. That will only put additional preload on the spring, not change the rate of the spring. All that will do is delay the time the slide opens. This is similar to fork springs. 
Wow! How in the hell did I just relate jetting to suspension???
Wow! How in the hell did I just relate jetting to suspension???
Senior Member
SuperSport
Joined: Dec 2005
Posts: 963
From: Sunshine Coast, Australia
I was just thinking about this and something doesn't add up. When you are syncing the carbs with mercury sticks the most vacuum occurs at idle. Vacuum goes down as rpm rise. So how are the CV slides controlled then? Vacuum pulls them up but if there's less vacuum...? Okay, I'm now confused.
Surely!
Senior Member
SuperSport
Joined: Dec 2005
Posts: 963
From: Sunshine Coast, Australia
You could be right there.
Senior Member
SuperSport
SuperSport
Joined: Dec 2006
Posts: 880
From: Victoriaville, Quebec, Canada
From what I see, the vacuum port is measuring both side of the slide, not the post throttle vac, in fact it's the vacuum differential before and after the slide that's opening it up to keep the airflow constant improving fuel atomization.
When opening the throttle, the inside of the slide "see" more of the manifold vacuum and pulls the slide open. At idle, manifold vac is way high but past the throttle plates, the vacuum is the same as the airbox. This si really hard to control something as complex, I'm really waiting for microsquirt release to convert the whole fuel/timing to electronic. Next winter hopefully !!
When opening the throttle, the inside of the slide "see" more of the manifold vacuum and pulls the slide open. At idle, manifold vac is way high but past the throttle plates, the vacuum is the same as the airbox. This si really hard to control something as complex, I'm really waiting for microsquirt release to convert the whole fuel/timing to electronic. Next winter hopefully !!
Senior Member
SuperBike
Joined: Mar 2007
Posts: 1,611
From: Belgium, WI
I am fairly new here, and I have only posted one other time. I am a technology education teacher in Wis. hello all and I would like to say that I have enjoyed reading the topics here.
As for the operation of the slides:
If you are talking about the vacuum that draws fuel in, you are talking about the key concept on any carburetor: a venturi and bernoulli's principle. As a mass of fluid(gas is a fluid) is accelerated, the pressure of that fluid drops. the vaccum does not suck gas in, but the pressure differential between the gasoline and the air in the carb causes the gasoline to flow from an area of high concentration to an area of low concentration. If the slides are operating off this pressure differential, then they can indeed have a change negative change in pressure, even as the butterflies open and decrease engine vacuum. I may be wrong here, but this is the best explanation I could come up with.
Bill Kunst
2000 SH
As for the operation of the slides:
If you are talking about the vacuum that draws fuel in, you are talking about the key concept on any carburetor: a venturi and bernoulli's principle. As a mass of fluid(gas is a fluid) is accelerated, the pressure of that fluid drops. the vaccum does not suck gas in, but the pressure differential between the gasoline and the air in the carb causes the gasoline to flow from an area of high concentration to an area of low concentration. If the slides are operating off this pressure differential, then they can indeed have a change negative change in pressure, even as the butterflies open and decrease engine vacuum. I may be wrong here, but this is the best explanation I could come up with.
Bill Kunst
2000 SH
Junior Member
Squid
Joined: Aug 2006
Posts: 17
Hi Seaton
I believe you had a RC 51 and a 929 front end on Ebay recently and the 929 front end did not sell do you still have it?? could you email me at pilot1@tpg.com.au plz and let me know
Many Thanks Keith
I believe you had a RC 51 and a 929 front end on Ebay recently and the 929 front end did not sell do you still have it?? could you email me at pilot1@tpg.com.au plz and let me know
Many Thanks Keith
Long Member
Back Marker
Joined: Mar 2007
Posts: 126
From: Northwood, NH
Not knowing a damn thing about slides I can tell you that the pressure after the throttle butterflies is lower at low power settings. In the case of a piston airplane, the highest possible manifold pressure you can acheive (normally aspirated) is the ambient pressure, let's say 30" Hg, while running at full throttle. Any throttle setting less yields a lower pressure. All of this does not answer the initial question of how the slides move. Wikipedia lists to types of CV carburetors -- one in which the slide is controlled by vacuum, the other it is directly controlled by the throttle. I've never seen the guts so I dunno.
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