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Technical Head scratcher. GM crate engine BOG/cutout problem

Discussion in 'The Hokey Ass Message Board' started by Cubzy, Aug 4, 2016.

  1. onthefritz
    Joined: Oct 29, 2014
    Posts: 41

    onthefritz

    Do not forget to check if your balancer has spun. If it has all of your effort trying to time it would be a waste.


    Sent from my iPhone using The H.A.M.B. mobile app
     
  2. Yes, mechanical linkage opens the secondary throttle plates. When opened, manifold vacuum then opens the secondary air valve. But on your carb, as well as Qjets and some Holleys there is no secondary accelerator pump to temporarily richen the mixture when the air valve opens. When the air valve opens too quickly you'll have a lean bog condition the same as you would on the primary side if you snap the throttle open and the accelerator pump wasn't working.

    By slightly delaying the opening of the seconadry air valve, when you open the secondary throttle plates the secondary throttle bores (below the air valve) "see" manifold vacuum instead of atmospheric pressure. With the air flow restricted momentarily the vacuum will begin to pull fuel thru enrichment transfer ports and the secondary main fuel nozzles. With the air valve adjusted properly, before the engine goes too rich the air valve opens to mix air with the incoming fuel and the secondary main nozzles continue to supply the proper amount of fuel.

    Have you adjusted the tension on the air valve to the 1 1/2 turns baseline setting recommended in the video? If you have and it still bogs, try tightening the valve another 1/2 turn at a time and see if there's any improvement.

    Here's a video of a HAMBer that seems to have the air valve on his Qjet adjusted just right. Notice that it doesn't snap open instantly like the throttle does. And zero bog!


    quadrajet carb in action 400 ram air 2
     
  3. Cubzy
    Joined: Jul 13, 2016
    Posts: 44

    Cubzy

    Thank you for the explanation of how the secondaries work. I haven't touched the air flap yet. For all I know its not even set to factory specs currently. I will start with that then add an extra half turn and see how it responds.
     
    gordy80128 likes this.
  4. DeucemanLt1
    Joined: Aug 15, 2014
    Posts: 143

    DeucemanLt1

    Have you made any progress yet?
     
  5. Cubzy
    Joined: Jul 13, 2016
    Posts: 44

    Cubzy

    No not yet. Won't be able to make any adjustments for few more days. Back to work.. I plan to adjust air valve next, then try manifold vacuum for vacuum advance. I'm waiting for a dial type timing light to check total timing. Then do some digging into why my fuel pressure is swinging between 3 psi and 9 psi. Even though the pump is suppose to be internally regulated to 7 psi?
     
  6. Start with general tune up stuff- cap&rotor fresh plugs and wires, new fuel filters, and absolutely check the fuel pressure.
    There should be no carb work done until the input fuel pressure is verified.
    You guys do know that realizing the designed out put of a carb is very dependent on that carb having the correct and designed in put of fuel pressure.
     
  7. bonzo-1
    Joined: Oct 13, 2010
    Posts: 325

    bonzo-1
    Member

    That would be an oil leak not a vacuum leak. The intake charge runs thru the intake directly to the head.
     
  8. bonzo-1
    Joined: Oct 13, 2010
    Posts: 325

    bonzo-1
    Member

    If you dont have a dial back timing light just extend out the marks on the dampener.
     
  9. DeucemanLt1
    Joined: Aug 15, 2014
    Posts: 143

    DeucemanLt1

    Get that dial timing light and by all means follow 31Vicky's advice
     
  10. canning
    Joined: Jan 22, 2012
    Posts: 63

    canning
    Member

    I was having a similar problem. I could make slight improvements, but could never eliminate a bog condition. Then one day I filled up with a tank of non-ethanol gas...different engine instantly. Can't hurt to try. I'm still scratching my head over this one.
     
  11. Cubzy
    Joined: Jul 13, 2016
    Posts: 44

    Cubzy

    Friday I've got a apt with mechanic to look things over. Would like to see why my fuel pressure varies between 3 and 9 psi on gage by carb. Would love to see rock steady pressure of 4.5 psi. I would also love to go to mechanical pump vs the noisy unreliable electric holley red pump. Will keep you posted. May take awhile to get to the bottom of this.
     
  12. Cubzy
    Joined: Jul 13, 2016
    Posts: 44

    Cubzy

    I took car to the muscle car mechanic in town yesterday. No magic fix. He is thinking its fuel related. Engine not able to get enough fuel at WOT. He suspects that what's causing the bog/cut out. The fuel system may not be built properly, fuel filter restriction or electric red Holley pump failing.

    I was looking into the possibility of trying to bypass the filter and stainless 3/8 tubing on the suction of the pump with rubber fuel hose to see if that is the bottleneck. If no improvement I may try new electric pump. Possibly Delphi inline. Or buy 20 feet of braided line off eBay and run new 3/8 braided from tank all the way to mechanical pump on engine. I have 2002 crate vortec engine. Dealership said all crate engines should WILL have lobe on camshaft to drive mechanical fuel pump. I took delete plate off yesterday and there is hole drilled in the block.

    I crate steel tank in trunk that has suction of system coming off the top. What are the internals? Is there just a little tube running down off the top fitting? The current fuel system has a 90 elbow off top of tank and another one just before filter. There are two couplings along the length of the roughly 12-13 foot run. Is this a potential bottleneck or reason to switch system over to braided line.

    When running the engine fuel pres at the carb starts at 9 psi even though Holley is suppose to be set to 7psi then as the car runs the gage drops off to 2.5-3 psi. What's the reason. Funny car still has the bog problem when fuel pres is at 2.5 or 9 psi? I'm a little puzzled whether I'm to the root of the problem. I'm unable to tell what fuel pressure is at WOT.

    Do you guys have some insight for me. Mechanic didn't see problem with timing or the running of the engine.
     
  13. Again 14 year old components aren't helping in the diagnosis.

    1st step, compression test to elimate or rule out internal mechanical issues. Do a piston stop test here too so yo know the timing light isn't lying later.

    2 nd step, good general tune up with quality parts. This should include timing advance and total timing discovery.

    3rd, fuel pressure gauges are finicky, there's countless discussions about engine mounted oil filled gauges giving false readings.

    4th hook up a test fuel pressure guage and route it up to the windshield wiper and drive the car. A WOT plug reading will tell you a lot.

    5th, wot fuel related bog will show one of two signs. It will blow black smoke if it's too much fuel or it will backfire if it's too lean.

    6th certain brass fittings certainly do have restrictions. Almost any hose barb fitting for instance.
     
  14. Cubzy
    Joined: Jul 13, 2016
    Posts: 44

    Cubzy

    Looks like ClayMart nailed it. Today I increased tension on the secondary air valve(flapper) by 0.5 turns to make secondary valve open later than stock and now it doesn't cut out. I still have a little hesitation after flooring it, when its coming off idle. I think I will adjust the accelerator pump up one setting to get bigger off idle squirt. Its currently in the center hole. I will move it to the hole closer to the carb. Thanks so much to all for the help along the way. I will keep you posted. This forum is a life saver.
     
  15. Thanks for reporting back. I'm definitely not right all the time (especially trying to diagnose something like this online) so I'm going to give myself a little pat on the back for this one. :rolleyes: The problem you described just sounded so familiar to me.

    Part of the beauty of these AVS style carbs is that it's usually pretty easy to troubleshoot and experiment with them. If it were me I'd try loosening the air valve spring a 1/4 turn and see if the problem started to come back.

    You may not need more accelerator pump shot. You may just need to adjust the pump timing just a skosh earlier.

    I'm not sure if all AVS carbs are alike, and I've never tinkered with the Edelbrock version. But I've seen the Carter versions have accelerator pump shot from idle thru wide open throttle. If there's a dead spot just off idle check to see if there is a good pump stream the instant you move the throttle off idle. It'll be easy to see looking down the carb with the engine off. If there's a slight delay before the pump stream starts you'll end up with a lean flat-spot.

    If I remember correctly I think possibly straightening the accelerator pump link a bit may start moving the pump plunger just slightly down in the pump bore at idle speed and give you an earlier pump shot when you crack the throttle. ;)
     
  16. Cubzy
    Joined: Jul 13, 2016
    Posts: 44

    Cubzy

    The only adjustment on the accelerator pump I'm aware of is the 3 holes. There maybe a slight hesitation on the squirt when touching the throttle. The adjustment you talk of, would it require I bend the link bar that attaches to the accelerator plunger? I tried going to the third hole today allowing for larger squirt and the bog maybe a little worse again and definitely notice a lot more soot build up on the side pipes. It did seem to have a little more gusty on acceleration. I have resorted back to the middle hole on pump arm and going to look for sweet spot on secondary air flapper. I imagine if to much tension performance will start to fall again?

    On a side note, how would you set your timing? I only have a basic light and no marks of the indicator. I'm at 12 degrees initial now was at 18 before and had slightly hard starts and no pinging. Stock spec for motor is 10. Should I make any more changes? I hear some people set ignition timing with vacuum gage. I have a vacuum gage. When going from 18 to 12 degrees I when from 17 down to 16 inches of vacuum on manifold vacuum.

    Thanks.
     
  17. No... Don't bend the link between the plunger and pivot arm.

    carb-diagram.jpg
    The rod I'm talking about adjusting is the one that connects the accelerator pump lever on the throttle shaft to the pump pivot lever that's mounted to the bowl cover. This rod should have a slight bend in the middle. By straightening this bend a little bit it effectively lengthens the rod. This will move the pump plunger slightly down in the pump bore at closed throttle and take any slack out of the pump linkage. It should also help to bring the pump shot in bit earlier when opening the throttle.

    Also look at the link that connects the pivot lever to the pump plunger shaft. When you look at it from the front of the carb it should be "S" shaped. If it appears "Z" shaped the link has been installed backwards which can affect pump travel as well.
     
    Last edited: Aug 16, 2016
  18. ground pounder
    Joined: Feb 16, 2010
    Posts: 71

    ground pounder
    Member
    from ontario

    Make sure u run diesel oil or zinc additive...or say good by camshaft.
     
  19. Todd M.
    Joined: May 24, 2009
    Posts: 482

    Todd M.
    Member

    What you are describing sounds like a vacuum leak somewhere. Check intake manifold, carburetor base, If you have automatic tranny, the vacuum line to that, and if you have a power booster it could be the vacuum line or a torn diaphragm in the booster.
     
  20. I was taught this old school mechanic method of setting timing:
    Run the motor at 1500 rpm, rotate the distributor until you get the highest manifold vacuum reading at that rpm.
    Test drive the car, if it pings at WOT, back the timing off a degree or two.

    I've used this method with some success on several cars when I couldn't find a timing spec, or where the engine was modified significantly from stock.
     

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