Velocity Stack Vacuum Source

[cebcb]

Hello everybody.

I had a question about how to set up the vacuum lines when installing velocity stacks?

With the standard airbox, the vacuum is created between carb2, intake, and the pump below the battery, correct? When installing stacks and disconnecting the pump, therefore removing the source of vacuum, how does one go about creating the negative pressure on the back side of the carbs?

Rejetted to 120

Is a vacuum rail between all four of the relief valves on the back side of the carbs routed to the original pump a viable option?

I must be missing something very obvious, because I can't find any info regarding this issue.

Any help is greatly appreciated.

BM

[seestheday]

Is this for DOHC carbs?

[cebcb]

NO. 78 K. KEIHIN PD I believe.

[seestheday]

Ok, someone who knows the carbs from that year will have to help, I only know the dohc carbs well.

Sent from my BlackBerry device on the Rogers Wireless Network

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From: "cebcb [via Honda CB750'S]" <[hidden email]>

Date: Fri, 23 Nov 2012 09:48:21 -0800 (PST)

To: seestheday<[hidden email]>

Subject: Re: Velocity Stack Vacuum Source

NO. 78 K. KEIHIN PD I believe.

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NAML

[big-nate]

so how do they work on a 79 DOHC? would it be the same as with pod filters?

[Lucky 1]

I know the 78 carbs backwards and forwards as well and carbs going back to 1969.
I have no idea what he is talking about.

The only thing the vacuum ports should be used for is when you want to sync your carbs.

Maybe he is confusing vacuum lines with float bowl vents.

He should post a photo.

Vacuum/suction is created when the piston goes down -end of story.

[TOOLS1]

Vacuum pump? What kind of bike do you have?
TOOLS

[seestheday]

regarding the 79K question - velocity stack will work in a similar way as pods.

That means that they will be difficult to tune.  There is an RSC mod that blocks off one of the jets.  This is how the cb966 race bikes worked with velocity stacks.  You will have to experiment in order to get the jetting right.  

The DOHC carbs can be made to work with pods and velocity stacks, but the more I read about them, the more I think that it's just better to buy different carbs so you can get them to work easily and get a performance boost at the same time.

[big-nate]

Ive been hearing that allot. that its just better to chuck the old carbs. what are some of my options? i have seen some bikes with just 2 carbs instead of 4. i also heard people but injectors on them. but that sounds complicated and expensive.

[seestheday]

If I had the money I'd just get some cr29's.

They'll be $8-900. Most people change their plans when they find out how expensive they are.

[big-nate]

will SOHC carbs work on a DOHC bike?

[TOOLS1]

There is a thread on here where a guy installed keyhole (77-78) carbs on his DOHC. Worked good for him.
TOOLS

[Lucky 1]

NEVER,NEVER "chuck carbs".

They are valuable to someone and there are no more made.
Please do not throw away any carbs.

[Lucky 1]

The engine does not care what is on it as long as it gets fuel and air.

[Lucky 1]

Yes...very complicated and very expensive.  Like a modern bike. Do you like buying $500.00+ parts???

[Lucky 1]

Difficult does not MEAN impossible.

Experienced mechanics can get almost any combo to work.
IT is the beginner mechanics that see it as impossible.