Vacuum Problem?

[goldenmopar]

Alright well i just put a 4 barrel carb and intake on my 318, and am encountering some issues. Normally while driving when you take your foot off the gas the rpm's wind down, mine don't. The rpm's fall and i can't even tell that the engine is running, till i give it gas and it revs. Also, when this happens my power brakes work a little then runs out of vacuum amd i can't stop. The carb is too big but would it affect my motor in this way?

 

[AJ/FormS]

Well, lets back up the bus.
Firstly, what car,year and trans
What carb. What intake. What bracket and cable?
When did this start?
Was it fine before the swap?
Was anything else done at the same time?

 

[goldenmopar]

Well, lets back up the bus.
Firstly, what car,year and trans
What carb. What intake. What bracket and cable?
When did this start?
Was it fine before the swap?
Was anything else done at the same time?

1973 Duster, 904 tranny, 318, Holley 750 (i know it's too big) on a Edelbrock Performer intake, stock bracket and cable. The brakes have never been "awesome" but this happened after the swap.

 

[AJ/FormS]

Now we are cooking
#1) the 4bbl carb needs a 4bbl bracket and a 4bbl cable. I bet you located the black part of the throttle cable into the clamp, instead of the long metal sheath termination?
#2) it would appear that the engine is pulling in air from somewhere its not supposed to, and the curb-idle screw was cranked in to compensate for a very low idlespeed
#3) I bet you havent hooked up the KD rod yet.

Okay. The first thing you need to do is unclamp that throttle cable.You are looking at the potential for a runaway engine. Then you need to answer the questions.Then we will dig into it.

 

[67Dart273]

Just what did you do at the time of this swap in the way of..........

Adjusting the idle mixture?

Setting the timing? Where is the base or initial timing?

Do you have a hotter cam in the engine, or "stock" 318?

Does it seem to idle at a reasonable speed, IE when you put it in gear, It "acts normal" or not, or die, etc?

 

[goldenmopar]

Just what did you do at the time of this swap in the way of..........

Adjusting the idle mixture?

Setting the timing? Where is the base or initial timing?

Do you have a hotter cam in the engine, or "stock" 318?

Does it seem to idle at a reasonable speed, IE when you put it in gear, It "acts normal" or not, or die, etc?

I have played around with the idle mixture, and as the for the timing i'm not sure i had my neighbor help me with that one. Stock cam too, and it idles at a reasonable speed

 

[67Dart273]

Well you need to "whut" and to SET the timing. You SET the timing FIRST. If for example "you used to" run it some what advanced, and now your friend has set it "more stock" (IE retarded) that might well be part of the problem. "The thing" about timing is if your car has a factory distributor then it is a "smog" advance curve, not ever ever designed for performance. Even so, I would bump the timing up to at least 8 BTC

It "could want" more initial advance, but then might OVER advance unless the distributor has been "re curved." ALL FACTORY DISTRIBUTORS suffer from this problem in the "smog" years 68 and later, and "gets worse" in the mid -70's and later.

And you don't "play around" with idle mixture. Mixture is adjusted by a PROCEDURE. After setting the timing, you get the car fully warmed. A good mechanic sometimes uses "by ear" but you should use either or both a tach and vacuum gauge. Adjust first one, then the other MIXTURE adjustment for maximum vacuum and idle RPM. RE adjust the idle SPEED screw to keep the idle where you want. The mixture screws should be "touched" last "towards" lean "just at" the point that vacuum or rpm just starts to drop. "Back and forth" on the screws is often needed