No vacuum on my vacuum advance port

[Michael L.]

Guys,

I was just able to get the B28 VAC I need and recently put it on and re-installed the distributor on my 69 L-46. I had tested the VAC before I installed it and it worked fine with total advance in by about 8 in Hg. After I installed the distributor and timed the car again, I re-attached the advance line and I didn't notice any change in the idle. Long story short, after troubleshooting multiple things, I put my vac gauge on the vac advance port and it pulls zero vacuum. The engine still pulls the 12ish in Hg that it has been when I test elsewhere, so it's just my vacuum advance line that isn't pulling vacuum for some reason. Has anyone ever seen this? Why would this be? It has the factory original Q-jet and is running fine otherwise. Thanks for any help you can give.

Mike

[Timothy B.]

Re: No vacuum on my vacuum advance port

Mike,

Put the vacuum gauge back on the V/A carburetor port and rev the engine a bit to see if the vacuum becomes present, you probably have what's called a ported vacuum port on the carburetor.

[Michael L.]

Re: No vacuum on my vacuum advance port

I just tried that and I get the gauge to move a little when I rev the engine considerably but it only goes to 1-2 in Hg.

[David H.]

Re: No vacuum on my vacuum advance port

... I put my vac gauge on the vac advance port and it pulls zero vacuum. ...

Mike

Do you mean vacuum can no longer holds vacuum?

If vacuum can works, plug line into manifold vacuum source. i.e. a vacuum source that lies below throttle plates.

Link below is to a pdf from our "Database of Restoration Documents" in Tech Board's Stickies section - pdf might help.

http://corvette-restoration.com/wp-content/uploads/technical_papers/vacuum_explained.pdf

Dave

[Duke W.]

Re: No vacuum on my vacuum advance port

That's because your '69 L-46 is an emission controlled engine and has ported vacuum advance. There is no vacuum advance at idle speed because the port for the VAC is above the throttle plate at idle. It's part of the emission control strategy. You need to convert it to full time, assuming your car doesn't require field emission tests, and you have apparently found a full time manifold vacuum source.

Also, the B28 is more aggressive than necessary assuming you have an OE equivalent cam. It should pull about 14" at 750 with full time vacuum advance, so you only need a 12" B26 to pass the Two-Inch Rule. B28s are getting rare. Save it for someone with a mechanical lifter small block. All OE mechanical lifter SB cams need a 8" B28.

Copy and paste the follow title, including the quotes, into a Web search engine:

"tuning vintage corvette engines for maximum performance and fuel economy"

Study it carefully and pay particular attention to the page on emission controlled engines.

Duke

[Michael L.]

Re: No vacuum on my vacuum advance port

Thanks Duke. Realize we all got a million things going on but we've been through this already. It has a HR 280H Comp cams camshaft and only pulls about 11-12 in Hg and I needed the B28 per your previous recommendation. I realize it's ported vacuum, but I still don't get why revving the engine didn't give me more vacuum than the 1-2 I got when I tried that. I've plugged it into the side port on the carb and it works fine there. Not sure why that port, which is also above the butterflies, pulls vacuum no problem but the dedicated vac advance port doesn't. I will leave it as is until I get it judged then I'll swap it.

Mike

[Mark E.]

Re: No vacuum on my vacuum advance port

Michael,

Which port are you using?

You mention the vacuum port on the passenger side that's near the fast idle cam. This is full time (manifold) vacuum typically used for cars with an AIR system or thermally controlled flap door in the air cleaner snorkel. Cars without these systems typically don't have this port.

The vacuum port at the front of the fuel bowl above the fuel inlet nut is also full time vacuum. It's typically used as a vacuum source for the VAC but with some cars is routed through the TCS or CEC relay which limits the vacuum signal. This is a good port to use if you want manifold vacuum for VAC as Duke suggests. If this port does not have manifold vacuum, it may be plugged (often because an incorrect gasket is installed).

Some Q-Jets have a vacuum port at the front of the fuel bowl above the driver side idle mixture screw. This one is ported vacuum and is often used for VAC in late 1960s cars. This port is also used for the charcoal canister for cars with one of these.

[Michael L.]

[Jeffrey S.]

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

[Michael L.]

Re: No vacuum on my vacuum advance port

Thanks Jeff. What I don't understand is why there was still nearly no vacuum when I revved the engine? I was going up on the throttle pretty significantly and still almost no vacuum, only 1-2" Hg. 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?

[Jeffrey S.]

Re: No vacuum on my vacuum advance port

Sorry, that's above my pay grade.
Jeff

[Leif A.]

Re: No vacuum on my vacuum advance port

Thanks Jeff. What I don't understand is why there was still nearly no vacuum when I revved the engine? I was going up on the throttle pretty significantly and still almost no vacuum, only 1-2" Hg. 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?

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.

[Timothy B.]

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

Jeffery, doing this modification would create a small vacuum leak, yes? Manifold vacuum at the new drilled hole and pulling air from the open slot above the throttle blade.

[Michael L.]

Re: No vacuum on my vacuum advance port

Yeah I must not be testing it correctly. It seems unlikely that there just isn't vacuum there. I'll have to check it again.

[Timothy B.]

Re: No vacuum on my vacuum advance port

Michael,

The key is to raise the throttle at a steady speed until the ported vacuum feed slot/ hole gets exposed to the manifold vacuum present under the throttle blade.