I brought the car home with the rebuilt motor tonight. Let me begin by saying that the "new" motor and detailed engine compartment are simply beautiful.

The car ran great when I picked it up. The engine builder had driven the car today for awhile after breaking in the cam yesterday. However, as I drove home (@30 miles), it started having trouble maintaining the idle at stop lights etc.

It sounds like I have a pretty bad vacuum leak somewhere around the carb. I think it is the vacuum advance which is plugged into a 2 inch hose that is coming out of the port on the side of the carb. The vacuum advance line seems to be just kinda stuck into the hose, and the fit does not appear to be very tight. If I remove the line and put my finger over the hose (to stop the vacuum leak), it idles great.

My question is this, if I just disconnect the vacuum advance hose and let the car run without it, what will happen. It was not hooked up before the motor was rebuilt and everything seemed fine.

Thanks

 

I've disconnected my vacuum advance and have no problems. Lots of folks have done the same thing. As far as I know, the only thing you'll screw up is your gas milage, as it will make the engine less efficient at speed.

JB

 

Your Vacuum Advance has a problem! First, it should not leak vacuum ... but second, it should be connected to PORTED vacuum not MANIFOLD vac.

Manifold vacuum is taken from below the throttle plates. Ported vac for advance, needs to be tapped from above the throttle plates. You want the advance to be operative ONLY when the throttle is cracked. :nono:

 

There's a great posting by lars under TOOLS (on the forum tool bar above), then Tech Tips, Then C3 and posted as Distributor Vacuum Advance Control Units. Also Qjet hose routing pix - but sorry for 350. :thumbs:

 

Perhaps I was not as clear as I should have been. Yes, I have a leak as there is not a snug fit where the long hose that runs from the distributor connects to the hose coming out of the side of the carb. I assume I should try and make that connection snug before I just go and disconnect it.

However, I think it is connected to the correct place on the carb. At least, it is at the place that it was before it was disconnected before. I think it is above throttle plates.

Will disconnecting the advance line mess up the timing? SHould the timing be reset without the advance hooked up.

 

No, your timing shouldn't be affected. IMO, it sounds like it is hooked to manifold because you would hardly get any vac out of the ported at idle. It should be hooked to the ported. But your vac advance is probably leaking as well.

 

It is currently plugged into the outlet on the passenger side of the carb up under the fast idle cam, etc. Is this correct?

 

The port in the passenger side of a Q-Jet by the choke linkage is straight manifold vacuum.

You don't want to eliminate your vacuum advance on street-driven vehicle. The vacuum advance improves cruise gas mileage; it improves throttle response at cruise; and it lowers engine operating temperature (more advance lowers temps). Aditionally, the vacuum advance can by used to help smooth out and improve idle characteristics of bumpy-cammed engines. Here is an extract out of my Vacuum Advance Tech Paper:

"Ported" vacuum allows little or no vacuum to the distributor at idle. "Manifold" vacuum allows actual manifold vacuum to the distributor at all times.
Does your engine require additional timing advance at idle in order to idle properly? Radical cams will often require over 16 degrees of timing advance at idle in order to produce acceptable idle characteristics. If all of this initial advance is created by advancing the mechanical timing, the total mechanical advance may exceed the 36-degree limit by a significant margin. An appropriately selected vacuum advance unit, plugged into manifold vacuum, can provide the needed extra timing at idle to allow a fair idle, while maintaining maximum mechanical timing at 36. A tuning note on this: If you choose to run straight manifold vacuum to your vacuum advance in order to gain the additional timing advance at idle, you must select a vacuum advance control unit that pulls in all of the advance at a vacuum level 2" below (numerically less than) the manifold vacuum present at idle. If the vacuum advance control unit is not fully pulled in at idle, it will be somewhere in its mid-range, and it will fluctuate and vary the timing while the engine is idling. This will cause erratic timing with associated unstable idle rpm. A second tuning note on this: Advancing the timing at idle can assist in lowering engine temperatures. If you have an overheating problem at idle, and you have verified proper operation of your cooling system components, you can try running manifold vacuum to an appropriately selected vacuum advance unit as noted above. This will lower engine temps, but it will also increase hydrocarbon emissions on emission-controlled vehicles.

 

Lars, I was wondering if you could straighten something out for me regarding ported vs. manifold vac. The way I think I understand it, ported vac increases as throttle/load increases and manifold vac decreases as throttle/load increases right? So therefore, wouldn't you want the vacuum advance to be hooked up to manifold vac? That way under minimum load you would be at max advance (for max economy) and then under heavy load vac advance is eliminated and mechanical advance is what is left (36 deg @ 2500) for max performance. If your hooked to ported vac, the advance increases with throttle/load and therefore at 2500 rpm, under a heavy load, you are way beyond 36 degrees which is not optimum for max performance. Right? Am I mixed up here? If not, why would you want to go with ported vacuum? I think I must be missing something.

Thanks
Anthony

 

"The way I think I understand it, ported vac increases as throttle/load increases and manifold vac decreases as throttle/load increases right?"

Ported vac is produced by high velocity air passing directly over a drilled port in the carb venturi... when this condition is not present, no vac. This momentarily occurs when (and only when) the throttle disc passes directly over the hole. It doesn't increase linearly with engine rpm.

Manifold vac is present (or maximized) when the throttle is closed.... even at 160 mph.
It is produced because of the difference between outside air pressure and inside engine air pressure. If you're driving along at 60 mph and you snap the throttle shut, manifold vac will spike. It's not just an "idle" thing.

 

KAT & Tom -
Here's the deal on the ported vs. manifold vacuum:

Manifold vacuum is just that: a direct, unobstructed sensing of the low pressure that is present in the intake manifold. This vacuum is high under light engine load, and approaches atmospheric pressure under heavy load.

Ported vacuum is also manifold vacuum. However, the location of the port for the vacuum hole is a location just barely above the edge of the throttle plate when the throttle is at idle. Thus, at idle, the port is not exposed to the manifold vacuum below the plate, so vacuum is "turned off." As the throttle opens, the port gradually is exposed to the manifold vacuum below the opening throttle plate. Once the throttle is cracked open about 1/8" or more, the port is fully exposed to manifold vacuum, and the "ported" source will read exactly the same as the direct manifold vacuum. The ported source has nothing to do with air velocity, and the two different ports do not read "opposite" each other. The only difference is that the ported source effectively has a "switch" by using the throttle blade to block the vacuum when the engine is at idle.

Tom -
When talking about air velocity causing more vacuum, this is true of "venturi vacuum." Holley and Demon vacuum secondary carbs utilize venturi vacuum as a flow metering signal to open the secondaries. As we know, the venturi area of the carb is the part of the carb that causes a low-pressure area based on engine mass airflow. Due to the low pressure area created in the venturi, fuel is drawn out of the fuel bowl and into the airstream. The low pressure in the venturi is directly related to the total mass of the air going through the venturi, and this is completely independent of manifold vacuum (at hi rpm & wide open throttle, manifold vacuum is non-existent, yet venturi vacuum is very high due to the mass of air passing through the venturi). On vacuum sec carbs, there is a small hole drilled right into the narrowest part of the venturi on the passenger side - this is where the low pressure occurs due to mass flow. This venturi vacuum is applied to a diaphragm opposite a spring, and when venturi vacuum is high enough (indicating that a large amount of air is passing through the primaries), the secondaries are pulled open by the primary side venturi vacuum. Note that manifold vacuum cannot be used to open the secondaries, since there is no manifold vacuum at WOT. Other than the drilled passage running to the secondary diaphragm, there are no venturi vacuum "ports" on a carb, so there are no ports that increase in vacuum as airflow through the throttle increases.


[Modified by lars, 3:58 PM 4/4/2003]

 

Lars... better check your Holley references... ported vac on a Holley is not for the secondaries... there is a separate passage for that.

Holleys have a specific port for ported vacuum advance that is not for the secondaries.. if you want, I can provide reference materials, diagrams, and even digital pics.

I agree with you on the manifold vac, but not the ported vac statements.
Some cars came from the factory using the ported vac setup. Most used manifold vac.

Holley manufactured carbs for a lot of different car companies, all having their own idea of the best way to utilize a vac cannister... they were not all in agreement.

A specific example is the BB Chevy TriPower carbs... the secondaries operate just as you describe... off their own port. There is, however, a separate "ported vac" port in the center 2300 carb that is used for "ported vacuum advance". I'll send you pics if you would like, denoting the separtate ports.

Here's how it works... air passing over a port at a fast enough velocity collides with and picks up molecules of air that are hanging around at the end of the port. These molecules are dragged down stream. The void created by the displaced molecules (inside the end of the drilled hole/port) is now at a lower pressure due to a lowered density of molecules... meeting the definition of a vacuum. If the high velocity air continues for a long enough time (only while the throttle disc is nearby, restricting the air flow past the drilled hole), useable vacuum is present at the tube extending from the carb body... where you attach your hose.

Try using your compressed air blow gun to blow across the end of piece of tubing immersed in a fluid... it creates a vacuum and draws the fluid up the tube.

Siphon spray guns use the same principle.

A Holley 2300 is the simplest, most basic Holley carb I can think of as an example...but there are plenty of others.

I studied this stuff pretty heavily when I was 14 years old because the first engine I ever rebuilt received an Offy manifold and 3 deuces... they had ported vacuum, and no vacuum secondaries.... all mechanical linkage, no provisions for vacuum secondarties.

FYI

Tom

 

Ahhh... I now see what I was missing. So if I were to guage ported vacuum as throttle/load is increased, I'd see zero vacuum initially with a gradual increase to full manifold vacuum and then a drop as manifold vacuum drops at full throttle. Cool. Thanks. That helps alot, and makes sense now. :thumbs:

 

Tom, I think you misunderstood what Lars was saying. He was making the distinction between the vacuum source for the secondaries and that for the vacuum advance system. As you both have said, vacuum advance units are fed vacuum from either ported or direct manifold sources. Until the mid `60's when emissions became an issue, manifold vacuum was used. From then on, mfg's went to ported vacuum for the reasons that Lars stated (better emissions). As Lars stated, once the throttles are open, they both operate the same but many engines will show improved idle characteristics with the extra advance provided by using manifold vacuum.

 

TRhe best way to handle it is to first make sure the advance can on the dist is good and functioning like it should. Next recurve the dist to get the mechanical advance working right. a good example would be 12 degrees initial at idle and 36 total in by 2500rpm. Vacuum advance can only help you if everything else is right. Theres no advantage on a street drivin vehicle by dis-connecting it. Only drawbacks like poor part throttle response and worse mileage. some say thier car runs better with it dis-connected. To me thats an indication that something else is wrong. Something that is less evident with the vc advance unhooked. For instance, If you have poor mechanical advance curve or its too short, some might be running too much initial advance. Then they get pinging with the vac advance connected. So instead of correcting the advance curve, the unhook the can and think the problem is solved. On a 1/4mile drag car that is raced only and never drivin on the street, vac advance is not needed since there is practially no vacuum available at full throttle. On a street car, so long as everything else is right, it can only help.

 

"Other than the drilled passage running to the secondary diaphragm, there are no venturi vacuum "ports" on a carb, so there are no ports that increase in vacuum as airflow through the throttle increases."

I have a lot of respect for Lars... and I think he knows that. We have conversed behind the scenes. But... the above verbage is questionable... so I questioned it. (It's my nature... doubting Thomas)

No intent to rain on anyones parade. Sorry if it seemed that way.

I have 4160/3667, 2300/4055-1, and 4165/6211 Holley carbs sitting in front of me.. they all have "ported vac" vac ports specifically intended to be used for vac advance cannisters. The 1969 Assembly Manual has the 69 TriPower in it... and it clearly shows the distributor vac cannister hooked up to a ported vac tube on the Holley 2300.
Manifold vac is typically drawn through the baseplate (throttle body) on a Holley. Ported vac is produced in a separate port and fed through the metering body to an outboard tube. Vac to control the secondaries is produced via its own internal port, which typically is fed directly to the secondary diaphragm through a direct connect gasketed main body port... no hoses. TriPower is an exception in this area.... the secondary carbs run off a ported vac tube connected by hose. This tube is off the main body. The ported vac (advance) tube is off the throttle body, directly below the choke pulloff. This is "portted vac", used to control the vac advance cannister. Another 3rd port is on the front of the throttle body, and it provides manifold vac to run the choke pulloff.
FYI... my danged 1986 Silverado with the electronic carb also uses ported vac.

 

Mine has been disconected since I owned the car. There are two types of advance centriugal and vacuum. You can control the centrifugal advance by changing the advance weight springs and veriing how fast the advance comes in. The trick is you don't want the engine to ping under heavy acceleration. The vacuum advance is more for economy minded folks, it add more advance when the vacuum is higher and gives you better fuel mileage. :thumbs:

 

See my other post-my headers are now glowing.

I am a novice with this stuff. That is why I rely on you guys.

I have a very simplistic view. With the vacuum advance hooked up to the wrong inlet that was producing manifold vacuum, at idle the timing was advancing, causing the poor idle? Some of you mention above that the timing advancing will help the idle in larger cammed cars. Could the timing be advancing too much at idle causing the poor idle?

Basically, the car would not run unless I stepped the idle up to 1200-1500 RPM or so.

Tonight, I disconnected the vacuum advance. I have found the correct port for the vacuum advance, but the line that I have is too short to reach it, so I just capped it off until I can get longer line. If the initial timing is retarded and the now without the vacuum advance, is this my cause of my glowing headers? WIll hooking it up to the correct port fix this problem?

 

The best way to obtain the proper vacuum port is to use a vacuum guage and check each port. What you want is zero or near zero vacuum at idle and an increasing amount of vacuum as you open the throttle plates. If you don't have ported vacuum connected to the carb then you will not accelerate as fast as you would if you did have it connected, unless you are using some electronic ignition timing advance device, or something like that.

 

Just want to say thanks to lars for coming forward to explain a few concepts. The forum has had a lot of vacuum advance questions lately that had participants stepping on each other. Though I've read the tech tips before, this last posting has cleared things up for me - and maybe corrected a vacuum in my brain too. :rolleyes: I was surprised that I still had some misconceptions.
Again, thanks lars. :yesnod: