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edelbrock idles too fast... help please!

Discussion in 'The Hokey Ass Message Board' started by acosta, Nov 12, 2009.

  1. acosta
    Joined: Apr 27, 2008
    Posts: 37


    Hello HAMBers,

    I'm at my wits end with my carburetor. I got it used and installed it on my 60 El Camino about 4 years ago, and got it "close enough" so that I could drive it. I feel guilty for driving it so long without it being right, but I was finishing my doctorate (yes, for four years) and just never had the time. It was usually a tough start, idled rough, and wasn't too responsive. Not to mention the lousy mileage. About three weeks ago, I decided to make the time to really dial it in... and now I'm pulling my hair out.

    The motor is a plain stock 305, with an Edelbrock Performer manifold and carburetor (model 1405, 600 cfm). Here is the issue I can't get past: If I use a vacuum gauge to set the idle mixture to give maximum manifold vacuum, I have to back the idle mixture screws many, many turns out (four and half to five). When I do this, the motor runs strong with 20" vacuum-- but the motor is also running really fast, around 1400 rpm. I can't bring the idle any lower because the I've already backed the idle set screw out all the way. My initial timing is right where I think it should be (2 degrees BTDC, using a timing light when it is cranking, so no mechanical advance and vacuum disconntected), and at this RPM the mechanical advance has kicked in to contribute another 20 degrees, for a total of about 22 degrees BTDC.

    My hypothesis is that there is a vacuum leak. I figure it's a pretty big one, since the engine likes it when I back out the idle mixture screws and deliver more fuel (as told by the 20", which I think at 5000 feet is about as much as I could hope for). But I can't find the supposed leak... not at the base of the carb (new gaskets, twice), not around the throttle shafts, not in the vaccum lines, not in the brake power booster, not along the intake perimeter... I was searching for the leak with propane. Then with WD40. Then with soapy water and a sponge. Then with soapy water in a squirt bottle. It bugs me that I can keep backing out the idle mixture screws and not be able to get the engine to stumble because it's too rich... it just keep speeding up.

    Has anyone else experienced this (PLEASE somebody say yes)? Is there something internal that can be causing this problem? I've checked and rechecked my float height (high and low positions), the needle valves are good, and have checked my fuel pressure (stock mechanical pump, 4psi). I have the Edelbrock calibration kit, but it doesn't seem to matter what jet and rod combination I use, it still can't get good mixture and correct idle speed.

    Thanks ahead of time,
    I appreciate your input,
  2. Your running ported vacum i hope. Other than that i'd point my finger at the carb. Edlbrocks are usualy pretty good as a rule on a engine like that
  3. Deuce Roadster
    Joined: Sep 8, 2002
    Posts: 9,519

    Deuce Roadster
    Member Emeritus

    I looked at a friend's car with a Edelbrock like yours and it had the same problems. It had a leaking port at the rear of the carb. It takes a NPT plug.


    Make sure the plug is secure and not leaking :D
  4. 333 Half Evil
    Joined: Oct 16, 2006
    Posts: 1,440

    333 Half Evil

    Use carb cleaner or if you are sure you have no exhaust leaks or spaking going on use starting fluid to check for leaks. If you spray everything external and cannot find one, using carb cleaner pull off one of the oil fill/breather caps on the valve covers and spray into the valve cover with carb cleaner. This will look for leaks at the bottom side of the intake manifold....not always, but sometimes that is where the leak is. More than likely, you do have a vacuum leak. Good luck.

  5. 333 Half Evil
    Joined: Oct 16, 2006
    Posts: 1,440

    333 Half Evil

    Just another thought....if you have an automatic transmission with a vacuum modulator, check the line feeding it and the modualtor itself for leaks....

  6. if the modulator was leaking it would suck it up into the intake and smoke out the pipe killing all wildlife. Big time smoke.
  7. I had this once and it turned out to be a bad base gasket.
  8. Rickybop
    Joined: May 23, 2008
    Posts: 7,257

    from Michigan

    Absolutely true, but as Evil said, the line itself could be leaking. - Rick
  9. First off, kudo's on providing lots of information.
    It is a help.

    Your problem sounds exactly what happened to the BB Buick engine in my 32.

    The rear carb - of a dual 500 cfm Carter (same as the Edelbrock) setup had a metering rod stick open allowing excess gas in at low speeds and idle.
    Fast idle, ran rough etc.

    Both carbs were new ones from the factory and one metering rod was bent . . . by them.

    Loosen the metering rod plate retaining screws, swing the plates to the side so as to access the metering rods.

    Push the metering rod down gently and see if it goes down ok.

    If you find one that's stuck, more that likely it has a slight bend.

    You can find out by pulling the metering rod - they come up and out once the plate is out of the way.
    Remove the piston, pay attention to how it's attached to the rod.
    Lay the rod on a known to be flat surface - piece of glass, surface plate etc. - let the short leg hang over and roll the metering rod back and forth.
    If bent you'll see it easily.

    Straigten with needle nose pliers and your hand.
    Two sets of pliers may be needed.
    In any even, be careful, you want to straighten it not make it worse.

    Re-install carefully....


    I would argue the point about using a ported vacuum source for your vacuum advance.

    Most times it leads to overheating.

    Lots of good posts here on the HAMB about this particular subject.
  10. acosta
    Joined: Apr 27, 2008
    Posts: 37


    Thanks, Tedley. For all my diagnosis I have the ported vacuum plugged (passenger side of carb base), and my vacuum gauge connected to the manifold vacuum (driver side of carb base).

    Thanks, Duece-- yes, I looked at that port. I use it for my power brake booster, 1/4" npt to 3/8" hose. With each of my leak detection efforts, it didn't appear to be the problem. But to be certain it's not to blame I may go back this weekend with an npt plug and block it off all together.

    Thanks 333, that is a novel idea. After scouring all the possible leak spots for several hours of several evenings, I thought I was going nuts. I had not realized the the bottom of the intake to head sealing area could be leaking, and I would have probably never figure out a way to check them. It won't be as easy as taking of the breather cap, because I really dig those cute "Chevrolet" script covers and am using those (with holes holes at the back for pcv valves, near the firewall... you can't even see them). I have a valve cover I cut the top off for setting valve lash, so I'll dig it out and use that.

    Thanks T, the base gasket was my first thought... I couldn't find a thicker gasket at the parts store, so I made two out of 1/16 gasket material and glued them together using copper gasket spay adhesive (and they ask me why I don't throw out half the stuff cluttering my garage). Sandwiched it in under 88lbs from my weight set and let it sit for about 30 hours. I was pretty happy with myself since it was 1/8" thick and made from stuff lying around-- but it didn't work. I then broke down and bought the edelbrock 3/8 fiber heat spacer and two new gaskets (they're a bargain, by the way) from Summit-- it didn't work either. While I had the carb off I checked with a straight edge to see that the base was not warped from over tightening. The ears were down a little in each corner, but the gap they produced over the base perimeter was nominal... about 40-50 thousandths. The gasket, I thought, would easily take care of that.

    And I also paid lots of attention to the carb base as I leak checked.

    Thanks, 440... it's good to have my suspicions confirmed. I was afraid I'm going nuts. I might still (go nuts), if I can't find it.

    C9, thanks. I've long admired your contributions to helping other HAMBers... I'm going to try not to go all blushy-fanboy on you. In fact, I had read about your bad metering rod (and it was reading your post about timing that made me decide it was time to do this right). I don't think that's the case with me, since this problem has persisted through several jet and metering rod combinations. With each installation I take care not to force anything, and then make sure that they ride up and down the way they should. Also, it appears that my rough idle is because I am way lean-- when I give it more fuel through the idle circuit when I back out the mixture screws it behaves better. If it was opposite-- lots more fuel and it gets worse-- I would think that I'd have a circuit letting gas in when it shouldn't.

    Still, it wouldn't hurt to check the rods again, they come out easy enough. I'll try that when I get home tonight.

    you sure know lots of stuff. Good stuff.

    Thanks so much to everyone taking the time to read about my problem and offer their advice. It really helps me retrace my steps and get ideas for what it could be.

  11. UnIOnViLLEHauNT
    Joined: Jun 22, 2004
    Posts: 4,824


    I have nothing to add, but go you on getting your doctorate!
  12. oj
    Joined: Jul 27, 2008
    Posts: 6,263


    The edelbrock performer intake is a direct replacement for the GM factory and has large secondaries for a quadrajet. The edelbrock carb is setup like a holley, squarebore. There is an overlap at the rear of the carb where the intake secondaries are open to the carb vacuum ports and will make a vacuum leak. Edelbrock sells plate #2732 to fix this problem. Just a flat chunk of steel same shape as an open gasket and it seals off the carb vacuum ports to the intakes' secondaries.
    I too have had a bent metering rod, it does happen.
    Hope the plate thing does it, an easy fix. Good luck, oj
  13. Just gonna toss a few things out.

    The carb is fairly new?
    I know that some Carters - AFB's I believe - had a small core plug at the base that would leak and cause problems.
    Cured by epoxying it.
    Probably not, but if it's an older carb it's worth a check.


    By high and low float positions you mean height and drop?
    I had a small AFB on a 273 engined Barracuda that had the float dropping so far the needle valve would hang open because the float tab was pushing sideways instead of up.


    No grit holding the needle valve open?


    Nitrocellulouse floats?
    (They're black and a form of closed cell plastic.)

    Sometimes they get "water-logged" with fuel and sink or not float as high as they should.
    If they're just barely awash in a test can of fuel and feel overly heavy you need a new one.

    They're probably brass though.

    We just took care of a leaking brass float in my pal's 600 cfm Edelbrock.

    He did a rebuild - which it didn't need - and buffed a hole in the brass floats.
    Dunno why there, I think our mutual friend who's real sharp with carbs had him do it to keep him busy while he re-built the carb.

    Anyway, a pair of new floats ($16.) and he was in business.


    Good plug wires?
    Sometimes #5 and #7 will induce voltage in one another and run rough.
    Again, probably not your problem.

    Check plug wires while the engine is idling in total darkness.
    If they're bad, you'll see the sparks.


    Is your PCV oriented in the correct direction?
    If inline you can get em backwards.

    Usually idle is ok, but the initial throttle opening will shoot some accelerator pump fuel in then the engine will fall flat on its face.

    If the PCV isn't operating correctly you can have some strange idle problems.


    Nuff for now, gotta go out and chase some electrical gremlins in my Ranger's charging system....
  14. I am not familiar with the exact design, but do you have a secondary throttle plate adjustment screw? It could be holding the secondaries open?

    Another thought, is your fast idle adjustment screw possibly holding the primary open, even though your idle speed screw is all the way out? What about something simple like linkage binding up and not fully returning?
  15. acosta
    Joined: Apr 27, 2008
    Posts: 37


    Hello OJ,

    Thanks for the suggestion, I was wondering if there were issues using the squarebore carb on the spreadbore intake. When the carb is off and I see just how thin the mating surface is on the intake, and how wide the shape gets around the secondaries, I was questioning if they were compatible. I took one of my used base gaskets and was looked at the imprint left in it on the intake side while comparing it to another edelbrock carb I have on the shelf... That adapter might be the solution. However, if it is leaking because the carb base does not match the intake correctly, shouldn't I be able to find that when I'm leak checking? I mean, a leak means air is entering the system from the outside, so around the base of the carb I should still be able to find the spot where it's getting in...

    It's a shame no one around here has one on the shelf, I'll have to call Summit again and it'll get here next week (it's a little frustrating, if I'd known a couple of days ago I could have included it in my last order-- aaargh!).

    Hello C9. Thanks for your suggestions.

    Yes, the carb has relatively low hours on it. When I had it off and was checking the base with a straight edge, I didn't see any stains around any of the plugs suggesting a leak.

    I have brass floats, and you are correct, I meant I checked their height and drop. I had read some posts that blamed fuel levels in the bowl being to high to running poor (and even improper heights out of the box), so I made sure I double checked that. I didn't think to inspect the needles for grit, that might be something... I'll put it on the list of potentials.

    My plug wires are pretty good, I think. Jacobs Energy Core, with a Jacobs High Energy Ignition. Lots of voltage, fat wires. Nothing burned, nothing cracked.

    I thought it might be my PCV system working incorrectly, so I disconnected it an plugged the port. Didn't make any difference.

    Hello 38 Chevy,

    Thanks for the suggestion. None of the linkages are binding. The secondary throttle is completely closed... the fast idle is not engaged, and I've even run it with the pedal linkage disconnect to no avail.

    Again, thanks to everyone for offering their ideas and experiences. I have several things to try today after I get back from work (just a few hours). I hope the weather gets a little nicer (Isn't it dumb the way I have a two car garage but have to work in the driveway? but the 59 has to sit somewhere... waiting).

    Thanks again,
  16. acosta
    Joined: Apr 27, 2008
    Posts: 37


    Thanks Unionville-- easily the worst decade of my life, but I try not to think about it. Still, it make my momma proud, so I guess it's a little good :)

  17. budssuperpro
    Joined: Jul 30, 2008
    Posts: 391


    sounds like you need to narrow all avenues I would block off all vac ports and I use WD-40 and spray the intire carb while Running and lisen to see if the rpm change as well as were it bolts to the intake.
  18. If you decide the thin sheet metal adapter isn't cutting it, you could look into using an adapter for square-bore to the Quadra-jet style intake manifold opening.

    Did that with my 32 when I went to one Edelbrock 750.

    Make sure you can handle the 1" increase in height.
  19. de-generator
    Joined: May 7, 2006
    Posts: 15


    I had a similar problem, found that the brake booster had a bad diaphram.
  20. holeshot
    Joined: Sep 18, 2009
    Posts: 1,519

    from Waxahachie

    hey ACOSTA... if i understood you right, you said you turned the idle mixture screws out 4 turns. turn them back in( all the eay) and then turn them out one and a half times. this should fix the problem.(all the way) sorry...POP.
  21. SlamIam
    Joined: Oct 8, 2007
    Posts: 449


    This happened to me also, at first the leak was small and just made the carb idle too fast, and I couldn't figure it out. Then when the leak worsened I noticed the engine ran rough when I stepped on the brake pedal, giving the problem away.

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