in the first pic you can see the vacuum port at the base of the carb on the passenger side of the primary bowl. according to Holley that vac line should lead to the PCV.
In the second pic, you can see the line that comes off of the back of the carb (off of the secondary side) that goes into a T, leading to the brake booster and the PCV.
here is the issue. i almost am almost done tuning, and am connecting the vac gauge to the port under the primary side (pic #1), to adjust the needle screws to maximim vacuum and whenever i hook it up it bogs terribly and sometimes dies. i have tried having the needle screws at pretty much every possible position, from 1/2 thread to 2 1/2 threads out, and it still bogs on me.
i did have a backfire through the carb, but i used holley's procedure to test the power valve, and it was fine (screw both needle screws in all of the way in and the engine died... which means that the PV is fine).
so where should that vac port be leading to? and why does it die when i hook up the gauge?
according to the tech guy on the phone, that port is for the PCV (i described it to him in detail... at the base of the carb underneath the passenger side of the primary fuel bowl).
do you know what else it could be for? and why would hooking up the vac gauge to it cause it to bog?
From the sound of it, that's a full-time vacuum port. It needs to be capped if you're not using it. When you put the vacuum gauge on it you're effectively plugging it, changing the air flow through the engine which causes it to bog. Retune your idle once that port is plugged (or connected to your distributor if you want to use full-time vacuum advance).
The PCV port is the 3/8" one on the center rear of the carb. Your brake booster needs to be connected to a port on the manifold. Using a T in the PCV line isn't a good idea.
That is a "full" or "manifold" vacuum port. As Aperion says, it's for the distributor vacuum advance, or something else that needs full-time vacuum. It is too small for PCV or power brake booster.
The larger 3/8" port is on the back of the throttle plate. It is best to use it for the PCV, and hook the power brake booster to a port on the intake manifold. I've argued this in the past, but the PCV and power brake should not be tee'd together - the power brake will get robbed in the process.
i have the 20-95 and the 20-121. and everything is fine with that.
it will prodominately be strip, with the occasional cruise on the weekends.
i figured out that i need to tap it right after i posted. i have a couple of issues with that though. the manifold is aluminum, as far as the heating and cooling of the manifold would the hole compramise the structural integrity of the manifold? or cause it to crack?
