Rough idle

-

DARTLARRY

Well-Known Member
Joined
Mar 19, 2007
Messages
563
Reaction score
217
Location
FORT WAYNE IN
I have a 30 over 360 with stock heads, small Edelbrock performer cam, and 625 AFB carburetor. It runs pretty good when driving, but will barely idle, it drops from 750 RPM to 400 when I drop it into gear, and barely stays running. There are some days it runs fine, then sometime during the drive it starts idling bad again. I have changed ignition box, coil, plug wires, distributor, cap and rotor, I have even swapped carburetors, still the same. My initial timing is 20 degrees (that is where it likes it), I think it may be a vacuum leak, but i cant find it, i have blocked every vacuum port on the carb, and sprayed around whole base with water, no change. It is taking a lot of fuel to run, my idle mixture screws are almost all out, and when i partially choke it, it seems to run a little better. I am about to change intakes and see if that helps, any suggestion are appreciated.
 
First thing I would probably do is check timing with a vacuum gauge, manifold vacuum to see if your timing mark is accurate or close. If that shows no improvement I would take the top off the carburetor and check float level when holding the top of the carburetor upside down float should be at a 90°, not sagging one way or the other, at that time I would also check for any contaminants and that all parts are moving freely. That's a couple of ideas. Good luck
 
Running exhaust manifolds? Have the heat riser butterfly still?
 
Timing is accurate, it just like a lot of timing. I will probably take carb apart and look at it next. it has to be getting all this air from somewhere, but i cant find it. This is crazy once it is past 1500 RPM it is runs great.
 
Timing is accurate, it just like a lot of timing. I will probably take carb apart and look at it next. it has to be getting all this air from somewhere, but i cant find it. This is crazy once it is past 1500 RPM it is runs great.


Where IS the timing...initial and total?
 
350 rpm drop is huge. See if you can check timing at 700 and in gear at 400. If it's consistent at 20*, look for a vacuum leak somewhere. Power Brake boosters are notorious for leaking. Isolate the intake tract, blocking anything that draws vacuum and see if the drop improves. If it does, hook up one thing at a time and see if it changes.
 
Check any linkage on the carb for interference with your intake. Plenty of times the smallest interference can lift the corner of the carb where it appears to be tight, but is actually not flat on plane with the gasket/intake.
 
I’m thinking you have blown intake gaskets !!
I was thinking the same thing. If you're cruising on the freeway, do you get a dusting of oil on the back of the car? If so, maybe the heads have been milled and the intake doesn't match anymore.
 
That may be it, I am running a Weiand stealth right now. I have an Edelbrock performer intake, I was going to try it next.
 
That may be it, I am running a Weiand stealth right now. I have an Edelbrock performer intake, I was going to try it next.


Sounds like a good plan you have there ! I think your going to find the intake gasket will be blown on the bottom of the port most likely !!
 
I figured that what it was, i was just seeing if someone else had any suggestion. Is the Performer manifold as good as the stealth?
 
I figured that what it was, i was just seeing if someone else had any suggestion. Is the Performer manifold as good as the st
ealth?

As far as I know the eddy is a better manifold ! Someone else may have a different opinion ! Hard to go wrong with the eddy !
 
proving a vacuum leak into the valley is easy; Just put a vacuum gauge on the dipstick, flip the PCV out of the valve cover and seal the covers, then start her up and rev it up a lil, maybe 1200rpm.Then wait a few seconds
But I really doubt that's your problem.

Do not let the pressure rise above 3 or 4 psi, cuz it will want to blow something out, maybe the cam plug in the BH!
If you don't have a vacuum/ pressure gauge, you can tape over the hole where the PCV was. If the pressure blows it off, yur good to go. If the tape gets sucked in, that's bad.
 
Double and triple check the firing order. I've seen 5&7 get crossed up and not pop at all but idle rough as a cob. 1 8 4 3 6 5 7 2 clockwise.
 
When all else fails; Try this;
Step 1
crank the rpm up to 1200
reset the mixture screws to 1.75 turns,
and then BACK THE TIMING UP, to in the range of 8 to 12 degrees.
Finally, reset your idlespeed to 650 in gear, and reset the mixture screws to lean best idle plus 1/4 turn, which should be ~1.75 turns.
If it's not in the range of 1.75 to 2.5 turns,
then the transfers are still not right.
If the mixture screws are at less than 1.75 turns, the engine wants LESS transfer fuel.
If they are at more than 2.5 turns, then the engine wants MORE transfer fuel.
Do what you gotta do to get the mixture screws into the range of 1.75 to 2.5 at about 700 in Neutral/Park. I'd shoot for 2.25 turns on that AFB
Step 2
To set the idle speed, you use idle-timing. Do not be overly concerned about the actual idle-timing number, just give the engine whatever it wants to satisfy the idle speed, and freedom from either stalling going into gear, or banging hard. The less timing she has, and to a point, the slower she idles; the smoother and with less drama, she will engage a gear.

When the ratio of transfer fuel to idle-mixture screw fuel is right; your engine will, or should, idle just fine, down to 5* idle timing and 550 rpm. I know this because mine does, and it is nearly the same build as yours, except I have Edelbrock heads.

Step 3
Tip-in hesitation; you will know you are getting very close to the right ratio of transfer to mixture fuel, because at a slow rate of throttle tip-in, this does not result in a hesitation, and you might be able to reduce your pumpshot; then you know that you are very close.

Step 4
you will have to re-engineer the POWER-timing and the RATE of advance to regain the lost response due to the retarded Idle-timing.
Your biggest hurdle will be to give the engine just the right amount of advance at stall; and then to ramp it up to all-in at the lowest rpm your engine will take it without detonating. Lemmee suggest 34* at 3400 for a safe starting number. Whatever the timing turns out to be between idle and stall is of very little consequence because you can augment that with Vacuum-advance timing; oh sorry, your D does not have a V-can/your loss. IMO, you need to fix that.

OK here's the theory;
At 20* idle-advance you have basically nearly shut off the fuel supply from the transfers, and are idling almost exclusively on the mixture screws. Then when you put it into gear, the engine cannot get enough fuel under load to continue running at the chosen idle speed.

Go back to post #1 and revue what you wrote
It is taking a lot of fuel to run, my idle mixture screws are almost all out, and when i partially choke it, it seems to run a little better.

This here is telling me the engine wants more transfer fuel. The only way to do that is to increase the throttle opening.... which increases the idle speed ....... which makes it slam into gear harder. Your only other choice for idlespeed control is to reduce the idle-timing. This is why I said "I can see your problem from here".
BTW
That "small" Edelbrock cam works best with headers to wake your bottom-end up.

My apologies to @yellow rose for jumping in, but this thread was IMO, rapidly decaying.
 
Last edited:
Thank you, that makes real good sense, I will try that tonight. I had headers on it, but got tired of fighting them to work on anything underneath. That is why I put the manifolds on, they quieted it down, and made it easier to work on.
 
-
Back
Top