No vacuum on my vacuum advance port

[Mark E.]

Re: No vacuum on my vacuum advance port

It is very easy to permanently get full time advance to the stock advance port. The modification is totally hidden and is reversible if, for some reason, you want to go back to ported. In the throttle body there is a horizontal slot that is covered by the throttle plate at idle. At off idle the slot is opened to allow vacuum to the advance port. By simply drilling a very small hole below the slot, vacuum will get to the port. The first picture shows the slot off idle. The second shows the slot covered at idle. The third shows the top side of the throttle body A different one removed from the float bowl). The well just above the slot feeds into the matching well in the float bowl. The last picture shows a black dot where a small hole can be drilled right into the well. This hole allows the vacuum into the well and to the advance port. I hope this helps. I did this years ago on my 69 and will never go back. If neded, a bit of JB Weld will fill the small hole.
Jeff

That's a neat mod. But don't most (maybe all?) Q-Jets already have a vacuum port with full-time vacuum?

[Mark E.]

Re: No vacuum on my vacuum advance port

Michael,
When "revving" your engine you will not have vacuum response as the mechanical advances are taking over. You will only have vacuum under a steady RPM. Hook up your vacuum gauge to a known vacuum source and get a reading at normal idle. Then, increase and hold the RPM at 500RPM increments. You should see a steady vacuum number at these stages. Once, and if, you convert over to manifold vacuum vs the current ported vacuum, you will see vacuum at normal idle.

Leif brings up a good point why you may not see a vacuum signal while blipping the throttle. To do this, I've routed a long vacuum hose from the port, through the unlatched hood and into the open window so I could see the vacuum gauge while driving.

[Jeffrey S.]

Re: No vacuum on my vacuum advance port

Timothy,
No. No vacuum leak at all. All the vacuum is directed to the advance port only. Drilling the small hole only opens the vacuum port earlier than off idle. It is the same path as off idle through the slot.
Jeff

[Jeffrey S.]

Re: No vacuum on my vacuum advance port

Mark,
They do have some full vacuum ports but are used for other things. For example, on the 69, there is a port on the passenger side that services the AIR pump. On some there is a port at the rear that is connected to something else. These could be "T"d into but this modification leaves everything bone stock. It only feeds the port that goes to the vacuum advance as it is designed to do.

[Jeffrey S.]

Re: No vacuum on my vacuum advance port

SORRY!!! I made a mistake. Disregard the photo with the black dot. That's not where the hole needs to be drilled. It needs to be on the bottom side of the throttle plate at rest not above the plate s shown. That brings vacuum to the advance port at all times. It's been many years since I did this and I just forgot. Sorry.
Jeff

[Michael L.]

Re: No vacuum on my vacuum advance port

No problem Jeff. I see what you're referring to. Next time I have the carb apart I'll look into that but it may not be for some time. In the meantime I've plugged it into another port that has constant vacuum.

[Owen L.]

Re: No vacuum on my vacuum advance port

Is it possible there is an incorrect gasket somewhere in the carb that is covering a port or pathway that would prevent the vacuum from reaching the VAC port?

I think that's a distinct possibility. There were so many applications and variances in Q-Jets over the carb's history it's certainly possible someone installed an incorrect gasket. Some that I've seen are so subtle in differences it would be easy to overlook a blocked hole.