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Taylorman
07-02-2005, 11:09 AM
Just out of curiosity, i checked my manifold vaccum today while running my boat. Its a 455 with a single plane intake and 850 Demon. It shows about 13 in hg of vaccum. Is that about right? How does on use vaccum to tune a carb?
Beer-30
07-02-2005, 11:11 AM
Sounds about right.
Tuning carb = adjust idle mixture screws until highest vacuum is displayed. :wink:
Edit: I would adjust a jet in the water, warmed up.
Taylorman
07-02-2005, 01:41 PM
Does it matter which vaccum port i connect the gauge to, pcv or the two small vaccum ports?
Beer-30
07-02-2005, 05:55 PM
Does it matter which vaccum port i connect the gauge to, pcv or the two small vaccum ports?
On the front or rear of the BASE of the Holley, there is a large (usually capped off) vacuum fitting. It is normally for vac brakes. That is manifold vac. It's been too long since I have even touched a carburetor to remember which of the two or three smaller ones are venturi and manfold. Your intake will have one or two ports near the carb also. Venturi will basically be zero at idle, since it is above the throttle blades.
AZKC
07-02-2005, 06:02 PM
Anything below the throttle plates is manifold vacuum. :coffeycup I think :D