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Hey guys! I am doing the final button up on my 292 and have run into a snag. I have a Holley 390 and I only have one large vacuum port on the back of the carb. I am assuming this would be fore the pcv line correct? If so the problem is I need another large port for my brake booster. I guess my question is can I "T" off this port and run both lines? Holley sez you should not hook booster and pcv up to the same port. If this in fact true, can I use this plugged hole in the Clifford manifold as a vacuum port? Sorry if these questions seem elementary but, i don't remember ever graduating any elementary classes. LOL!  [/URL]  [/URL]
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Stan z:
I'm sure no expert but I believe at least some Holley 4bbl carbs have the PVC port located on the side of the carb mounting flange under the vacuum secondary device. If I'm right, from your top photo, it looks like your carb may have a plug in that port. There are typically other ports (one is full vacuum and the other "ported" vacuum [for distributor advance]) located on the opposite side of the carb from where you're holding it. As far as the power brake booster is concerned, I believe it needs a continuous (not ported) vacuum source like where you're pointing in the second photo.
Like I said, not an expert by any means. I'll be interested to see what some of the other guys (who have forgotten more than I'll ever know) have to say on the subject.
Russ
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PS:
I've also read that you don't want to use the same source (via a "T" fitting) for both the power brakes and the PVC system. And most say the PVC source should be the one provided on your carb base or, if into the intake manifold, very close to the carb base. Apparently, the thinking is that you want the PVC gases to be spread as evenly as possible to all the cylinders. Locating the PVC vacuum source at one end (or the other) of the manifold presumably causes all those gases to impact just a couple of cylinders.
Russ
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I have the 2 ports at the front of the carb. The upper one I was going to use as the vacuum advance. The problem with using the lower port for the pcv is, the nipple is much smaller then the nipple on the breather I got from tom Lowe. Looks like the carb nipple is 3/16 and the breather nipple is more like 5/16. It is my understanding that you really don''t want any leakage around these vacuum ports. I am just not sure as to ho to go about using this port. hopefully tom Lowe will chime in on my stupidity.  [/URL]
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Stan:
The upper port on your most recent photo is a "timed" port, which would connect to your distributor vacuum advance.
If you look at your first (top) photo, from where you're holding the carb, look to the right across the fuel bowl and then down to the side of the carb mounting flange where the mounting bolt hole is. It looks to me like there is a threaded plug in what would normally be the PVC port. That port normally has a hose nipple install for connection to the PVC valve. But I sure could be wrong …
Russ
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I have a 390 carb. I've rebuilt it twice. I know it inside and out. The top small port is for vac advance. Bottom small port (on the base) is for the trans vac line. The large port on the backside in your first pic is for the PCV. For your brake booster use a threaded port on the intake manifold. You'll have to buy a nipple to attach it.  On this pic you can see the vacuum line for the brake booster coming off just below the carb. The hose swoops down and goes into the booster in my case. This was on the OEM intake manifold but the Offy is similar (bung is at the rear of the manifold though). Look for a 3/8" NPT threaded hole. Probably is plugged now. 
Last edited by gbauer; 03/29/16 03:59 PM.
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Gbauer, Thanks a bunch man! That is what I was hoping I could do! Do you have a charcoal canister in between the manifold and brake booster? I heard the exhaust gases from the manifold kills the diaghpram in the booster.
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You shouldn't get exhaust gases in your intake manifold... If you do you have bigger problems than your booster!
I've run mine like this for about 5,000 miles.
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You shouldn't get exhaust gases in your intake manifold... If you do you have bigger problems than your booster!
I've run mine like this for about 5,000 miles. Sorry didn't mean exhaust gases. I meant fumes from the gas. LOL!
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You shouldn't get exhaust gases in your intake manifold... If you do you have bigger problems than your booster!
I've run mine like this for about 5,000 miles. Sorry didn't mean exhaust gases. I meant fumes from the gas. LOL! It's sucking, not blowing.
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I noticed the "ported vacuum" source was intended for the distributor advance on the Holley instructions.
But on my assembly manual for my 1978 250cid Camaro, the vacuum advance is connected to the vacuum port on the bottom of the carb along with a tee to the THERMAC air cleaner. The ported source higher up on the Monojet goes to the EVAP canister valve. The PCV and EVAP lines are hooked to a source on the integrated manifold. If it had power brakes there is a plug on the integrated manifold below the linkage where it would connect.
I'm wondering why if the distributor vacuum advance needs a ported vacuum source, why is the stock one connected to full manifold vacuum? Is it because there is a delay valve in the vacuum line to the distributor that is controlled by the thermal valve on the water neck? If I hook the vacuum advance to ported vacuum, should I remove the thermal valve and delay valve? I've seen instructions on EVAP canisters to not hook the purge valve to full manifold vacuum, so should the vacuum canister line and advance line be on a tee together off the ported vacuum source?
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You shouldn't get exhaust gases in your intake manifold... If you do you have bigger problems than your booster!
I've run mine like this for about 5,000 miles. Sorry didn't mean exhaust gases. I meant fumes from the gas. LOL! It's sucking, not blowing. Yes I understand how it's working, but why then do they sell a charcoal canister for these setups?
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...because people will buy them thinking they do something.
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Sorry to bring this up from the dead, haven't been on for some time. That filter that goes between the engine and brake booster IS for gasoline fumes when the engine is not running. We installed them on cars in 1977 as part of a recall at the Olds garage I was working on at the time. Apperantly a couple of boosters failed and caused accidents. If you don't have one I wouldn't worry though, boosters went without the filters for years. Jay
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I noticed the "ported vacuum" source was intended for the distributor advance on the Holley instructions.
But on my assembly manual for my 1978 250cid Camaro, the vacuum advance is connected to the vacuum port on the bottom of the carb along with a tee to the THERMAC air cleaner. The ported source higher up on the Monojet goes to the EVAP canister valve. The PCV and EVAP lines are hooked to a source on the integrated manifold. If it had power brakes there is a plug on the integrated manifold below the linkage where it would connect.
I'm wondering why if the distributor vacuum advance needs a ported vacuum source, why is the stock one connected to full manifold vacuum? Is it because there is a delay valve in the vacuum line to the distributor that is controlled by the thermal valve on the water neck? If I hook the vacuum advance to ported vacuum, should I remove the thermal valve and delay valve? I've seen instructions on EVAP canisters to not hook the purge valve to full manifold vacuum, so should the vacuum canister line and advance line be on a tee together off the ported vacuum source? Don't use the ported vacuum for ignition advance. Somebody once had a very nice link to a writeup on the details behind it. Basically, the porting was intended to improve emissions and doesn't really apply to our applications. My car runs far better using the unported line than the ported line. It was interesting, I followed somebody's guideline here for setting the timing. Unconnect vacuum, advance to max manifold vacuum (mine was 36BTDC) then back off until manifold vacuum drops 2". Mine was 16BTDC on premium gas. Runs awesome. Well so far, I've got some other issues I'm attending to before I can take her on the street again.
Mark
'67 Camaro L6-250
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I noticed the "ported vacuum" source was intended for the distributor advance on the Holley instructions.
But on my assembly manual for my 1978 250cid Camaro, the vacuum advance is connected to the vacuum port on the bottom of the carb along with a tee to the THERMAC air cleaner. The ported source higher up on the Monojet goes to the EVAP canister valve. The PCV and EVAP lines are hooked to a source on the integrated manifold. If it had power brakes there is a plug on the integrated manifold below the linkage where it would connect.
I'm wondering why if the distributor vacuum advance needs a ported vacuum source, why is the stock one connected to full manifold vacuum? Is it because there is a delay valve in the vacuum line to the distributor that is controlled by the thermal valve on the water neck? If I hook the vacuum advance to ported vacuum, should I remove the thermal valve and delay valve? I've seen instructions on EVAP canisters to not hook the purge valve to full manifold vacuum, so should the vacuum canister line and advance line be on a tee together off the ported vacuum source? Don't use the ported vacuum for ignition advance. Somebody once had a very nice link to a writeup on the details behind it. Basically, the porting was intended to improve emissions and doesn't really apply to our applications. My car runs far better using the unported line than the ported line. It was interesting, I followed somebody's guideline here for setting the timing. Unconnect vacuum, advance to max manifold vacuum (mine was 36BTDC) then back off until manifold vacuum drops 2". Mine was 16BTDC on premium gas. Runs awesome. Well so far, I've got some other issues I'm attending to before I can take her on the street again. Yeah, I saw that same article. The engineer stated HEI advance was always meant to run on full manifold vacuum, not ported. The only thing that seems to need a ported source is the valve on the factory THERMAC air cleaner assembly.
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