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Technical The dreaded carburetor cough..

Discussion in 'Traditional Hot Rods' started by AGELE55, Sep 16, 2021.

  1. AGELE55
    Joined: Jan 4, 2018
    Posts: 398

    AGELE55
    Member

    350 SBC. 300 hp. Edlebrock 650 cfm carb.
    I’m chasing my tail. I get the carb cough randomly on acceleration, especially rapid acceleration. Plugs read pretty good. I cannot find a vacuum leak. Timing set at 12 btc.
    I’ve tried setting the timing to 10 btc. No help, actually seems worse. I’ve adjusted the idle air screws by tach and with a vacuum gauge. I can get it to run pretty damn good…except the cough returns at random.
    Would re-jetting or metering rod spring swap help?
    Ideas?
     
  2. carpok
    Joined: Dec 29, 2009
    Posts: 512

    carpok
    Member
    from Indy

    Couple things accelerator pump, make sure it is spraying fuel the full range of throttle movement most important at the beginning.
    What type of choke electric or manual if manual pull it out part way maybe a third way. See if that helps if electric loosen up the spring a bit so choke stays on a bit to test.
    Sounds like when you open the throttle it’s a bit lean to me. Hope that helps
     
  3. AGELE55
    Joined: Jan 4, 2018
    Posts: 398

    AGELE55
    Member

    Hmmm…I just moved to an electric choke with the new carb…. I’ll try easing it off a bit.
     
  4. carpok
    Joined: Dec 29, 2009
    Posts: 512

    carpok
    Member
    from Indy

    To be clear just using the choke to trouble shoot not a permanent fix.
    Check the accelerator pump with engine off eyes on discharge nozzle hand moving throttle.
     

  5. jaw22w
    Joined: Mar 2, 2013
    Posts: 1,407

    jaw22w
    Member
    from Indiana

    The most likely scenario is when you open the throttle and let in that extra gulp of air, the carb goes lean until the fuel catches up. I don't know a thing about Edelbrock carbs, but if it was a Holley, I would tell you to put the next size up squirter in it.
     
  6. Doublepumper
    Joined: Jun 26, 2016
    Posts: 1,105

    Doublepumper
    Member
    from WA-OR, USA

    A faulty vacuum advance can sometimes cause a similar symptom.
     
  7. sdroadster
    Joined: Jul 27, 2006
    Posts: 385

    sdroadster
    Member

    Yup, check the vacuum advance diaphragm. My 327 coughed on acceleration. I found the diaphragm ruptured.
     
    AGELE55 likes this.
  8. guitarguy
    Joined: May 26, 2008
    Posts: 578

    guitarguy
    Member

    Edelbrock makes different squirter nozzles, I think they call them clusters. Several above have touched on this issue and I too believe it is a case of the carb going lean on acceleration. A larger accelerator squirter nozzle will let more fuel into the primaries to help "cover the hole" of missing fuel when the throttle blades are snapped open sharply. Also, the pump arm had three positions which control the duration of the fuel going into the squirter nozzles.

    Edelbrock last time I had looked had all of their manuals online, so you should be able to find the carb tuning section in them, but be forewarned it is written for very beginners and for knowledgeable souls, but not really for the guys in between in my opinion.

    Back-ordered at Summit: https://www.summitracing.com/search/department/air-fuel-delivery/section/carburetors-accessories/part-type/carburetor-discharge-nozzle-kits?N=brand:edelbrock
    But this is what you are looking for. They are a pain to change with the choke plate in the way (do not attempt to remove it though)....don't drop the screws down the carb. it is possible to lay a piece of paper towel below the chock plate, just don't forget to use some needle nose pliers to remove it before startup.

    The vacuum advance as mentioned could rupture the diaphram, but how it operates also depends on how you have it hooked up. it can help with tuning, all carb tuning takes time. Having it hooked to ported or manifold vacuum makes a big difference in how the car can perform. There are so many factors that go into tuning, the carb is just a small part....but to get you going, I say swap the squiter nozzle to a larger size first and a longer duration setting on the pump arm.
     
    Last edited: Sep 16, 2021
    AGELE55 and footbrake like this.
  9. Joe Travers
    Joined: Mar 21, 2021
    Posts: 494

    Joe Travers
    Member
    from Louisiana

    Accelerator pump linkage has three settings. I'd try #1 and see if it covers up the bog, if it's not there now. Come stock in 2nd position.

    Joe
     
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  10. saltflats
    Joined: Aug 14, 2007
    Posts: 11,073

    saltflats
    Member
    from Missouri

    Put a vacuum gauge on it, then check the color of the power piston spring.
    There are charts that show the inches to color.
    You can change the metering rods to tune it in.
     
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  11. 97
    Joined: May 18, 2005
    Posts: 1,959

    97
    Member

    90 % of carburetor problems are in the ignition!
    The tune must be perfect before you adjust carburetor at all . I insist all techs follow a full procedure every tune and tick it off at each step. includes oscilloscope to check ignition pattern ...probably not possible at home but the basic procedure is .

    Start with oils and coolant level, battery condition and charging, air leak and compression tests, and all ground and power connections (voltage and voltage drop, resistance across ground), make sure all cables and hoses are in correct place, not leaking .....check all plugs , plug leads, (check resistance... ohms). Air cleaner element (clean or new?) Start engine ,... air leaks, again ? hoses for cracks including vacuum modulator diaphragm and hose , booster diaphragm and hose ... Recheck timing and advance , both vacuum and mechanical, fuel delivery volume and pressure , float level, observe inside of carb while running , making sure there is no fuel pouring through mains when car is idling ( indicates faulty float valve). Once all is correct to spec then its time to adjust the carburetor.
    There is even more to do if the car is going on the dyno!
     
  12. guitarguy
    Joined: May 26, 2008
    Posts: 578

    guitarguy
    Member

    Something else I also thought of, I am assuming if it's a 650 CFM then you are dealing with a AVS2 style Edlebrock. You might also be experiencing the secondary air door opening to quick. This is pretty common on alot of Edelbrocks I find. If it is a AVS2 or Thunder series, you can adjust the opening rate of the air door and slow it down. Again, refer to the carb manual. It will warn of tightening the adjustment too much, but I have successfully ran them tighter than specified without any ill effects.
     
    AGELE55 likes this.
  13. BillSchmid
    Joined: Jul 21, 2012
    Posts: 99

    BillSchmid
    Member
    from Ohio

    The 350 in my Nova would backfire through the carb on acceleration and lacked power. I chased the problem for a good while until I found a few loose rockers. Three lobes on the camshaft were wiped out.
     
  14. moparboy440
    Joined: Sep 30, 2011
    Posts: 754

    moparboy440
    Member
    from Finland

    Sounds more like a fuel pressure problem than a carburetor problem. Edelbrocks are very sensitive to fuel pressure. Installing a fuel pressure gauge would be a good idea.
     
    Last edited: Sep 17, 2021
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  15. '34 Ratrod
    Joined: May 1, 2019
    Posts: 94

    '34 Ratrod
    Member

    This is exactly what my SBC 300hp 350 with Edelbrock 650 cfm carb is doing! Thank you for the info, I love this place!!

    Edit: the engine in my '62 Chevy II..
     
    Last edited: Sep 17, 2021
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  16. Happydaze
    Joined: Aug 21, 2009
    Posts: 1,215

    Happydaze
    ALLIANCE MEMBER

    At 12 degrees btdc the timing sounds a little retarded to me. What's your 'all in' timing at 2500+ or so rpm (vac disconnected and plugged)?

    I had a similar issue following a distributor swap that I did in a hurry and subsequently learned I had it retarded some, but only a little. Thought I had carb issues, chased my tail a bit, then did a careful retiming and problem ta-da gone! :)

    Chris
     
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