? for ported vacuum advance gurus

[Jetboy]

I drilled a port in my aisan carb and I get a spike of vacuum just off idle all the way up to 14 or more, then it drops back to a normal gradual increase. is this bad? it hesitates and this may be the cause. Does this mean my port is too far down. Is it too close to the butterfly?

Thanks
Jetboy

 

[honk]

The amount of vacuum is governed by the airspeed through the venturi, so my best guess would be a port placement as close to the narrowest part of the carb throat so that vacuum would rise with engine speed. BUT this is only a guess as I don't have a full knowledge of the physics of this subject. You really need an answer from Jim, or someone who's walked the path.

 

[Jetboy]

the port hole is just above the butterfly when it is closed, so I think when I open it a little it may be getting manifold vacuum. I guess that might make sense because it is then below the butterfly, but I'm not sure.

 

[Coolerman]

Ported Vac

I drilled a port in my aisan carb and I get a spike of vacuum just off idle all the way up to 14 or more, then it drops back to a normal gradual increase. is this bad? it hesitates and this may be the cause. Does this mean my port is too far down. Is it too close to the butterfly?

Thanks
Jetboy

I'm wanting to do the same thing as I have a 1987 Dizzy I need to install. It requires ported vac. I have a 1974 carb base that appears to have TWO ports that are already drilled through to the venturi ABOVE the throttle plate they currently have soft metal plugs in them. The top plug is almost one inch from the base of the carb. Will that port give me the ported vac I need if I drill out the plug and press in a brass nipple?

http://tinyurl.com/qfl4q

 

[65swb45]

Coolerman, considering that you are looking at a plug on the SECONDARY side of the carb, which is at a point that will not get vacuum until you are almost at WOT, I would say that THAT is a BAD idea!

Port needs to be on the primary side of the carb, prolly close to where Jetboy has it already.

 

[Coolerman]

Darn, thought I had an easy fix... Didn't even look at the fact they are on secondary side. Thanks for 'schooling' me! Any idea why they are there in the first place? Might be an easy way to get a WOT indicator. Hook a vac switch up there to switch on when WOT...