View Full Version : which vacuum port to use on carb

07-27-2002, 08:34 PM
i keep getting mixed opinions about this.. i have an edelbrock performer quad #1406 600cfm on my 72 satellite 318 and i'm not sure which vacuum port to use , the "timed" left one or the manifold right. some people say left, some right. can anyone tell me for sure? thanks

07-27-2002, 09:38 PM
Use the "timed" fitting. Check the search area for "vacuum advance."


07-27-2002, 10:45 PM
If it is for vacuum advance, use the metered (timed) port.

07-28-2002, 12:22 AM
Ed and Maniac are right.

This topic has been discussed several times here. The bottom line, is that manifold vacuum is NEVER used on vacuum advance.

The timed port is "ported" vacuum.

The people who are telling you to use manifold vacuum, simply have no idea what they are talking about. I don't usually say stuff like that, but this is a black and white/cut and dry, issue.

Look in a service manual if you have any doubts.

07-28-2002, 09:11 AM
I am wondering about this same thing. I have a 69 383 w/ an Edlebrock Performer 600 carb, and the instructions specificly say to used the manifold vacuum port fort for "non-emission controled" motors, and the timed port for "emission controled" motors. Itried both on mine, and it backfires on decelleration real bad w/ using the timed port. I do have a miss under hard acceleration, that I have not been able to locate, so that may have something to do with it. I have called Edlebrock to ask them, but keep getting put on "Intergalactic Hold". Never have talked to anyone.

07-28-2002, 01:15 PM
that's exactly the same thing with mine.. my timed port backfires on deceleration and it seems like i get less power. i seem to get more power with the manifold vacuum but i'll constantly stall out and backfire when accelerating from a stop. edelbrock responded pretty quickly to me the only time i dealt with them, but it was about something else (though this may be related). 600cfm sounds a little lean for your size engine.. that may be a problem for you

07-28-2002, 02:40 PM
Dave571 was correct as well as the others he mentioned. There was a lengthy discussion on this , finally ending up with a wrench head that had been working on cars from the early 60's till now. If you think about it for a moment, lets say your base timing that you set for a 318 is 10 deg. btdc as a nice round number. To set it you pull the vacuum line off the distributor and plug it and set it at 10 btdc. If you slowly increase the rpm's and with an advanced timing light you will observe that the distributor’s mechanical advance starts to kick in and advance the timing to a point that the mechanical ends, usually 30-34 deg btdc @2500rpm's. Many old racers swear buy the tried and true method of setting the distributor to the maximum allowed mechanical advance, for a small block that is 34 btdc and taking the car for a run to see if it detonates. If it does than the distributor timing is reduced by 2 deg to 32 deg. Btdc and repeating the test until it no longer detonates. Once this is achieved then they set up the vacuum advance to add too the mechanical advance, usually another 14-20 deg btdc depending on what the engine likes. If you had the distributor hooked up to manifold vacuum, as soon as you put the vacuum line back on your distributor the vacuum advance would kick in and your off the line acceleration would be much lower. As you increased speed the manifold vacuum would decrease and thus your vacuum advance would decrease and kind of defeat the purpose of vacuum advance, to increase power and mileage. With ported vacuum it is 0 at idle and as you increase rpm’s it slowly starts to increase the vacuum and thus slowly adding advance to the mechanical advance when needed. I had a neighbor that had a 1978 Dodge Van and he was constantly complaining about the gutlessness of it and the poor highway mileage. After listening to him bitch for about 2 months on how crappy dodges were I decided to look at his engine for him. After removing the dog house the first thing I checked was the distributor vacuum advance to see where it was coming from, sure enough it was manifold vacuum on a “vacuum tree”. After rearranging to ported vacuum and adjusting the timing to actually work, he couldn’t believe the difference it made. He said he noticed immediately with off the line power and picked up 4 mpg on the highway. Another satisfied dodge man was created. Don’t believe me, look at the Mopar Performance Instruction sheet, they tell you how to set up your dodge engine exactly the way it should be. You can find it at http://www.dippy.org/main.html under upgrades and print it off. It is for converting the old points system or the Leanburn system over to the regular Electronic Ignition style. The first two or three pages walks you thru the conversion and then the remaining pages tells you exactly how to set up your timing for all Mopar engines in black and white right from Mother Mopar, no further discussion n

07-28-2002, 06:01 PM
I've been playing with the advance on my 440 CUDA for a while now. The motor has the usual, cam, Performer RPM manifold, 750 Holley,10 to 1 pistons,MP electronic distributer,727, 3000 stall and 3.91 grears. I went with full manifold vacuum, ported and no vacuum advance. Setting the full advance to about 38 deg. each time. While I have not checked the milage, the motor pulls MUCH better with no vacuum advance at all. I just set the timing at 38 deg total advance which is about 10 before top dead center at idle and plug of all the ports including the vacuum canister on the distributer. Runs the best that way.

12-27-2002, 11:01 PM
Just bumping this thread back to the main screen, as this seems to have become an issue again.:argue:

12-27-2002, 11:29 PM
I've always associated backfiring on deceleration as a lean condition. Fatten the carb a little and that goes away. Sometimes an exhaust leak will exacerbate the problem.