need help with my vacuum adv. hose

[crm318]

i need help putting on my vacuum advance hose on the carb. right now, i have it on the passenger side port (a tube that sticks out beyond the choke linkage and diaphrams). the truck kinda starts hard as if its to advanced (its on 11 degrees with many mods). should it go on the front? what port should it go on and what is the difference in the two types of vacuum on a carb?

[piecesparts]

Most carbs have two different ports, one is a direct vacuum signal and the other is what is known as a "PORTED" vacuum signal. The direct is exactly that, it sees the same vacuum htat is in the intake, with all of the changes that it sees. The ported vacuum connection has a small orifice on it to limit the amount of vacuum signal it sees and the time it takes for it to accumulate. Most Rodders will go for the non-ported vacuum fitting, so that they are getting the advance signal as soon as they get the engine started. The SMOG masters want the other one so the the dizzy will not advance too soon and the smog runs more effectively. You can find out which is which by putting a vacuum gauge on the carb and watch for the change as you change your motor's RPMs.

As for starting, the vacuum doesn't come into play until your engine has started and the vacuum builds into the intake manifold, so that should not be affecting your starting. I would wonder if you are properly advanced for your mods to your motor. Are you absolutely sure of your advance setting? Was it set with the vacuum hose OFF of the distributor vacuum pot? I set my timing on my engines with an advance timing light and I adjust my engine for "TOTAL" Advance, rather than the initial timing. The number I shoot for is 36 degrees total advance with the engine at about 1500 RPM and the vacuum disconnected from the distributor. This gives me an initial timing setting of about 18 degrees. I think you may be set to low and need to advance your distributor---I may be wrong, but it is worth a try.

[bryanw1968]

Hmm....

Based on what I've learned, read and heard I believe the opposite about the two vacuum ports. There are in fact two ports, one ported and one direct. I've always closed the direct port. It is used for tuning. The other vacuum port will not draw any vacuum at idle. It will only start to draw vacuum when the primary throttle blade is clacked open. That will start to suck the vacuum advance in your distributor and pull more advance out of it. Most vacuum advance distributors have an allen head screw inside the module itself to determine just how much you want it to give you. I've always heard that this is the setup people run to allow you to not have to have so much initial timing on the street and avoid overheating and get better gas mileage for just cruising around. But when you romp on it you get all the timing you need. Again, this is just what I know and not law. I actually only run a centrifical advance only in my distributor because of the way my motor is set up. However, 36 total degrees of timing on a smalblock chevy does sound about right to me. I know you don't want to go over 40 degrees total. Timing depends on how big your cam is, how heavy your vehicle is and how your motor is set up. For example, the alumunim heads I'm running don't like a bunch of timing and 33 degrees total is recommended.

What are the specs of your motor and what kind of distributor are you running?

[crm318]

my specs are in my signature. so i should be usning the ported vacuum? i also forgot to say that im using a quadrajet carb. where is that port located? also, when yall are taking about 33 degrees timing, is that with the vacuum line off and pluged? how will i know when its at 33 or so when my tab goes to 18? (sorry for all the inexperiance)

[poorboy]

Yeah use the ported vac. That's why it's called vacum advance. If you are unsure which port it is start the truck and hold your finger over the port. The ported you will hardly feel until the rpm's are brought up, the other port you will feel the constant vacum. If you use the non-ported vac it takes your 11 degrees and turns it into say 28 degrees at idle. Your perf and fuel mileage will both suffer from this.

[piecesparts]

My performance works fine with the NON-ported vacuum connection, so it is how your dizzy is adjusted in the vacuum pot. I have a 383 stroker getting 15 MPG and runs hard.As for the timing setting, to get the 33 degrees setting, you would need an advance timing light, Sears sells them for a reasonable price.

[Solver]

I always use the full vacuum at idle. I know the ported doesn't start sucking till the rpm's are up. So if the ported doesn't operate till higher rpm's why do they always say to plug the vacuum hose when setting the timing. With ported there would be no affect on initial timing. We went through this back in 2002 on this board and there was alot of opinions. I copied this info but not the users ID . It is kind of long but a good read
Here is what was posted then:

I am not a motor head but I don't understand why you would want full vacuum at idle. Is'nt the whole idea of vacuum advance to increase timing at higher speeds (ie higher vacuum)?
If you hook it up to full vacuum port isn't that the same as just advancing the timing to that point initially and skipping vacuum advance unit?Like I said I am no motor head but was just wondering about this
Reply:
not quite..if you have your initial at 10 deg. then add full manifold vacuum limited to 10-12 degrees this gives you 20-22 degrees at idle...where your motor likes it...then when you accelerate the vacuum drops off and the mechanical picks up..giving you 20-22 degreees of mechanical advance plus the initial of 10..giving you a total of 32 at 3000 rpm..now if you use ported vacuum..you will have 10 deg initial plus 20 degrees of vacuum plus 20 degrees of mechanical thats 50 degrees at 3000 rpm..can you say detonation??? GM used ported vacuum with real stiff springs on the mechanical for emissions...they would have 2-4 deg. of initial with the vacuum doing all the work until 5000 rpm...you will find more performance with non proted vacuum..

Now, for the rest of us: Vacuum advance.
Ported vacuum advance was, in fact, used by many makers of performance and stock engines in the past. Reason, the fuels were so good that the burn temperatures of the combustion chambers actually lowered as upper rpms were reached, and the added timing from ported was needed to heat the burn properly. In those days, the amount of degrees added in the cannister were fairly large, with some engines having 20+ degrees, but this number was usually not attained, as the total amount of vacuum added by the ported port was lower than that needed to get the vacuum advance to pull all the way in.
Today's pump vended fuels, and their additive packages, or lack thereof, have just the exact opposite of the earlier fuels, they always burn much too hot, even stuff like the Sunoco 280 have this problem. Add vacuum advance into the lace most succeptable to detonation, upper from mid to upper rpm ranges, and you can take detonation to the bank, you're gonna get it.
The only time today that ported timing is beneficial, to a point, is on Exhaust Gas Recirculation designed engines. These engines recirculate partially burned gasses from the exhaust system into the intake tract again to be re-burned and reduce emissions output, and the partially burned fuel needs the added combustion chamber heat to more completely burn the exhaust fuels recirculated.
In today's non-EGR performance engines, stock to performance, we do not require any upper rpm added timing for burn temperature control, in fact, it is detrimental to proper running.
So why use a vacuum cannister at all? Well, there is a simple silver lining to vacuum advance, engines as we are working with like timing at idle in the 18 to 24 degree range, but this isn't all that easy to do with some parts packages. How do we do this? Well, first, we need to think of the vacuum advance as not that, but an idle timing supplement to the initial timing.
As we now know, the idle timing can now be raised to a decent level at idle, to help with a stable idle, pull against a bigger than normal cam, pull against a converter and help cool the engine better from more complete idle fuel control. There is a catch, though, too many degrees of supplemental idle timing can be very counter-productive.
If we use the 24 degree figure, we can do things like set the initial timing at, say, 14 degrees, starts easily, then add the 10 extra with the cannister. OK, how do we get the 10 crankshaft degrees into the 20 degree cannister? We use either a Crane scroll plate to stop the advance pin that conects the diaphragm to the point/pickup plate to restrict plate movement down to the number of degrees we wish different from the static initial timing and the total idle timing.
OK, now that we have the number of degrees restricted, do we want to be able to change the vacuum level the canister comes in goes out with? Yes. We do this in two different ways, we look at the information of the stock cannister pull rate, and select the proper one for our vacuum levels, or we add the adjustable cannister from Crane, with the scroll plate that comes with it. Adjust away, you will find the correct amount of added timing for idle to make your engine run correctly.
Well, doesn't the idle added vacuum timing stay in the system when the gas pedal is hit? No, actually. It is immediately dropped as the off-idle vacuum goes away, and the centrifugal advance acts as usual . Only 180 on this is an engine that has no idle vacuum from a realy radical cam duration, and then, the vacuum actually rises after the engine is brought off idle. We just wouldn't use any kind of vacuum advance on an engine this radical anyway. Static idle timing would also be jumped up to 18 or so with the redical engine anyway, to try to stabilize the idle quality. Once again, adding initial timing helps the idle, just depends what you have available to help.
All HEI distributors manufactured for GM vehicles as production ignitions have way too much advance, mechanical degrees and vacuum degrees, and usually cause mucho problems in our performance and stock engines until they are set up correctly, and have full intake manifold vacuum sourced.
99 percent of all vacuum advances have way too many degrees of timing available and need to have the total number reduced to work with full manifold vacuum, as the manifold vacuum will pull all the degrees available into the cannister, not like the partial vacuum pull available with ported vacuum sources does at rpms.
Now, to dispell some myths about dyno tests and ported vacuum advance. Dynos always use more jetting to keep the chamber, engine and water temps cooled off, and ported timing just counteracts the rich mixtures of the rich jetting meeded for the dyno. In all the experiences I have had with crate and built performance engines, ported vacuum advance has been simply not productive, no matter what the engine did on the dyno.
If you are forced to use ported vacuum for your supplemental timing surce, then the curve and initial are not correct, and/or the total amount of vacuum supplemental timing is too large foruse as an idle supplemet from full intake


OK heres the deal. When you hook up your distributor to manifold vacuum ( ported ) you give it full idle vacuum which boosts your timing at idle. That allows you to set it at 10 degrees static or so for easy starting with a stock starter. Once it is started you develop vacuum and your timing jumps up to 25-35 degrees or so . That gives you a smooth idle with anything over a stock cam. It also gives you excellent throttle response off idle. When you gas it your vacuum level drops since your throttle plates are opening up and with it the amount of vacuum advance you are gaining. So your vacuum advance goes down while your mechanical advance begins to take hold . That allows you to avoid detonation under full throttle while still giving you higher than normal advance under cruise conditions. I run a very mild cam and high compression for a street motor. With 92 octane I can run almost 45 degrees of total timing under cruise conditions. That drops to 32 or so under WOT so I avoid detonation .

For a motor you want to run good. Use 12 or so degrees timing with the distributor disconnected . Hook the vacuum line to the distributor up to manifold ( vacuum at idle ) vacuum.