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Ironhead Sportster Motorcycle Talk (1957-1985) For all those that wanna talk about Ironhead Sportster Motorcycles

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  #231  
Old 3 Days Ago
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Serious Blac Serious Blac is offline
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Quote:
Originally Posted by mikethebike61 View Post
The beauty of a constant vacuum carb is you only get what the engine needs no matter the throttle opening....

Holley has a spring kit for vacuum secondary carbs that allows you to tune the opening of the secondaries to suit your particular engine requirements....

After you have idle and pump settings correct you start with the weakest spring which usually lets your engine fall flat on it's ass when the secondaries open much too soon and it goes lean....

You move up the scale of spring strength until your engine just PULLS with zero hesitation....and a lot of times you end up with the stock spring because, after all, Holley enginers are pretty damned good...

In 1967 Holley and Chevy cured the 3x2's problem of too much crb too soon by making the second and third carbs vacuum.

All the same you can still put too big a carb even if it is a CV. The CV just masks the problem better. Only experience I have with Holleys was setting up a set of 2" SUs on a 3.5 litre Rover after the owner removed the Holleys and slung them. He was much happier with the results. Man, THAT was a story! Did the final adjustments using a cup of coffee on the rocker box. Adjusted the mixture and syncro until there was a standing wave in the coffee cup on idle and just off idle. Puuuurfect.
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  #232  
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There is no doubt the CV (Constant Velocity) carb is a good carb. As long as you can get one that flows enough for your engine at max RPM.
The clue is in the velocity part. With a constant velocity in a port you get a constant signal to the carb to feed in fuel. Less 'errors" in what the carb needs.

A lot of people don't like them as much as they don't have the same "snap" when they jam the throttle wide open as they get with other carbs that have accelerator pumps.

ANY carb will work as long as you can get the A/F graph to be a flat line and it flows enough to feed your engine. That's as good as it gets even with EFI.

Just throwing cams, carbs and pipes at an engine isn't a good idea. You need to work towards a goal with a plan. All of these things work together along with capacity to get you where you want to go. The research has already been done making it easy for us now. Just do it by the numbers.

One good thing about all the S&S's is that it will flow enough for your Ironhead no matter which one you build.
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  #233  
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Quote:
Originally Posted by Ferrous Head View Post
There is no doubt the CV (Constant Velocity) carb is a good carb.
Are we going to start the constant VACUUM vs VELOCITY debate?

Because without VACUUM......there is no VELOCITY.....
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  #234  
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Well, it's been a very long time indeed since I looked at CV carbs. In throry they should be very good for racing motorcycles. In practice we dumped them in favor of Mikuni smooth-bores.
But my memory is that they sense the pressure drop in the venturi to raise or lower the slide. It's not so much a vacuum as a pressure differential.
In any case, there's no "direct" link to the throttle position and it becomes a little like "drive by wire".
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Quote:
Originally Posted by mikethebike61 View Post
Are we going to start the constant VACUUM vs VELOCITY debate?

Because without VACUUM......there is no VELOCITY.....
These will tell you all you need to know:

https://en.wikipedia.org/wiki/Boyle%27s_law
ttps://en.wikipedia.org/wiki/Bernoulli's_principle
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  #236  
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Got my "E" today.

Impressed with the fit and finish. And I own a machine shop so I reckon I'm slitting my own throat LOL

I started a thread asking for help because I am carb dummy
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