Holley 750DP Bog

Discussion in 'Troubleshooting & Diagnosis' started by Pilot53, Jul 23, 2009.

  1. Pilot53

    Pilot53 Veteran Member

    Nov 30, 2008
    New City, NY
    Whenever I floor the pedal from a complete stop, the engine bogs badly, it got worse after changing the main jets to a size 68 from 71. I want it to run leaner because im getting pretty bad gas mileage, about 10mpg, but I dont want it to bog, any ideas?
  2. muscl car

    muscl car Veteran Member Lifetime Gold Member

    1] what's your vacum @ idle in gear

    2] what size is your accel pump squirters

    3] what color pump cams
  3. hhott71

    hhott71 R.I.P 11/19/18 Lifetime Gold Member

    Mar 30, 2001
    Joplin Mo. 64801
    BOG = TOO much AIR vs Fuel.
    Go back to stock jetting.
    And then make sure your Ignition timing is correct.
    MSDIGNITIONS.COM is a good source.
    Make sure the plugs and wires are up to snuff.
    You need about 34-40º Full advance with 10-16 of it initial, Centrifugal all in by 2600-3200 (depends on converter, gearing etc.
    THEN add in as much Vacuum advance on top of that until it pings, then back off the vacuum advance 2-4º

    If you still have a bog, you need to get a larger squirter, no not that one, the ones for the Accelerator.

    HP Books on Tuning and Rebuilding Holley Carbs is a good source of Carb Info.

    You'll get a lot more responses in th Performance or Engine section.
  4. SS402

    SS402 New Member

    Jul 13, 2008
    Lakeland, Fl
    more info would be nice.. like engine size, manual or automatic transmission, stall converter rating, camshaft duration.

    Most bog conditions are caused by low velocity airflow, lean a/f mixtures, too much carburetor, wrong type of carburetor for selected application..
  5. 73RATTYZ28

    73RATTYZ28 Veteran Member

    Feb 23, 2001
    White Plains NY USA
    With double pumpers secondary bog is usually a result of the secondary acellerator pump problems. Have you adjusted the pump to have the squirt hit exactly when the throttle hits the secondary's? I just had a similar issue on my 850DP and that cured it.

    Good luck --
  6. Pilot53

    Pilot53 Veteran Member

    Nov 30, 2008
    New City, NY
    The engine is a 383 stroker with 10.5:1 compression, running about 12 degrees initial advance at idle, idle vacuum is about 8-9 inches at 600rpm in gear, the cam is a comp 270h cam with 270 duration and .470 inches of lift. Im not sure what accelerator pump is on the carb right now, its a model 0-4779. I want to be able to run it lean enough to get halfway decent gas milage because it drive the car every day. But I also want it to make power when I mash the pedal and not have any kind of a bog. Right now if I drive it like my grandmother im getting 10mpg on average, seems kinda low compared to what I hear other people getting.
  7. SS402

    SS402 New Member

    Jul 13, 2008
    Lakeland, Fl
    I'd add around 4 more degrees of initial, use a #7 power valve and run ported vacuum for the vacuum advance..
    Leaning the mixture enough to improve mileage will surely net you a set of burned pistons, if you want performance and mileage then swap out for a vacuum seconday carburetor, check out the summit brand 650 or 750 vacuum for $249..
    In all honesty the 750DP is the wrong carb for your combo, a 650cfm unit would fill the bill just fine..
  8. markw

    markw Veteran Member

    Jun 6, 2009
    A big factor in getting mileage out of a double pumper is the idle circuit. Most of your cruise fuel comes from the idle transition circuits which are controlled by the idle feed restrictions (IFR). You can put a .018 wire in the IFR (item #25 in this picture) http://i18.tinypic.com/61klx5s.jpg. It's a common fix for carbs set up for very radical cams (like double pumpers are usually used with) but used with less radical cams. The main jets can then be set for a safer mixture when you need more power. The PVCR (item #17) can also be enlarged which will allow smaller main jets while keeping WOT mixture the same, but you have to know what you're doing before you drill on a carb. A good Holley book will explain all of this better.
  9. Bikefixr

    Bikefixr Veteran Member Lifetime Gold Member

    Mar 13, 2006
    markw put you right over home plate. You can get great mileage with a Holley DP. No BS, I had a 350 in a Monte carlo with a 3:07 rear, TH400 trans. I used the old Chevy 350HP/327 cam .447", 222 degree, with 1:6 ratio rockers, a Holley single plane manifold, headers, factory 291 Fuelie heads mildly ported, an HEI and a worked Holley 650 DP. The car ran high 13's in a 4000 barge, and swear to God, 26mpg on the highway and 19 all-around. All day long. The key with Holleys is to understand how they really work, and tweak where it helps you most. On Holley carbs, the majority of your driving is in the idle and isle transition circuit. Up to about 2500 RPM, the idle circuit is what your car cruises on! The idle screws only control how much air-fuel mix gets into the idle feed that sits under the base plate butterfly. DP carbs are waaay rich. The amount of air entering the idle stream is controlled by the idle air feed restrictors which are the bigger of the brass openings, the outer set inside the choke horn in the main bores. Air from there goes into the idle well where it is mixed with gas that comes through the idle feed restrictors in the metering block. It's a super-small, non-replaceable jet. So, those two orifices control the amount of air and gas. From there, the mixture goes past the idle screws and exits under the throttle blades. The screws only affect how much mixture, not the richeness as is commonly thought.

    Idle Circuit

    The idle circuit is fairly simple, yet has a dramatic effect on street manners, idle quality, emissions, and fuel mileage. Fuel enters the idle circuit through the main jet and then travels through an idle-feed restrictor and into the idle-feed well. Once in the idle well, atmospheric pressure pushes the fuel up across an idle air bleed located normally at the top of the barrel. This air bleed mixes air with the fuel, and then this mixture is pushed down an adjacent passage that leads past the idle mixture screw and then out the curb-idle discharge port located underneath the throttle blades.

    There's also an idle transfer slot outlet that's located just upstream of the idle- discharge port. This transfer slot discharges additional idle fuel once an opening throttle blade uncovers the slot. This feature introduces additional fuel into the airstream as the throttle is opened, which prevents a lean hesitation that can occur due to an as-yet-inactive main metering circuit. This transition fueling occurs in the first few degrees of throttle opening.

    Main Circuit

    The main metering circuit consists of a jet that restricts the total amount of fuel introduced into the main metering well. Inside the well is a small tube with even smaller holes drilled in it. This emulsion tube is designed to mix air with the fuel. The air comes from a high-speed air bleed located at the top of the idle well. This air bleed acts as both an emulsifier and a suction break to prevent fuel from siphoning into the engine after the engine is shut off. The air and fuel is then directed to a discharge nozzle that's part of the booster venturi located in the throttle-bore airstream. As the throttle is opened, air moves past the booster venturi, creating a low-pressure area that pushes fuel through the main circuit into the airstream. The high-speed air bleed contributes additional air into the main well to improve fuel atomization and circuit activity

    Power Valve Circuit

    A carburetor doesn't really need a power-enrichment circuit. However, the main metering system is required to supply fuel to the engine both at maximum demand under wide open throttle (WOT), and also at part-throttle. >> WOT metering, dictated by the jet size and other variables, determines the amount of fuel necessary to maintain the proper air/fuel ratio at this maximum demand. But this means that at part-throttle when fuel demand is reduced, the jet will flow more than necessary.

    The power circuit creates an on-demand enrichment circuit that adds more fuel during WOT operation. This circuit uses manifold vacuum to keep a small valve closed during part-throttle operation. When the throttles are opened far enough to lower the manifold vacuum to a certain level, a small spring in the valve opens the valve and additional fuel is introduced into the main metering circuit. All carburetors employ a power circuit, but the way they make this work is slightly different.

    Accelerator Pump Circuit

    The accelerator-pump circuit is also a power-enrichment circuit, but operates slightly different. If the throttle were always operated very slowly, you probably wouldn't need an accelerator-pump circuit. But hot rodders like to mash the throttle down very quickly. This creates an instantaneous and short-term loss in velocity in the system. Since that lower velocity means a lower pressure differential between the booster and the float bowl, this will create a lean condition or a bog.

    To cover up for this short-term loss in fuel enrichment, all carburetors use an accelerator-pump circuit that shoots fuel directly into the venturi to prevent this hesitation. The circuit uses a small diaphragm or piston that is operated with a lever located on the primary throttle shaft. Add all these circuits together and you have the basis for a working carburetor.

    I got my DP working this way. I enlarged the idle air feed .008". This took my idle air mix ratio from 12.5:1 (rich) to 15:1 (leanish). I drilled 2 small holes in the primary throttle blades. 1/16". This allowed me to close the throttle blades tighter in the bores so the metering was more precise. Open blades make the rest of the circuits harder to tune. I also used a 750 baseplate on my 650 because the bigger bores increased airflow to almost 700cfm...but the incoming air had to squeeze through the smaller 650 venturi, so it had a higher itake velocity. Higher velocity equals better mixtures and more torque. (Ask any Nascar guy how they get 500HP from a Holley 400 DP.) . Next I made the primary jets 4 sizes smaller. I set my idle to best vacuum reading in gear, at 600 RPM idle. Ported vacuum to the HEI, 16 degree initial. Once I had the highest and smoothest idle vacuum reading using initial timing and the idle screws, I lock it down and drive. I keep going smaller on the primary jets untill I feel a lean surge at steady speed, then I go up 1 size and that takes care of the surge usually. I am now as lean as the engine can take at cruise speed. Next, I bring it up to cruise and then jump on it. If there is a delay and then it picks up, I know my transition from idle circuit (2500 RPM ish) to high-speed circuit is too lean. That is what the Power Valve is for. Think of it as a vacuum controlled auxillary jet. On Holley DP's, this valve turns on at 6.5" of vac. problem is, on a lean mixture, that flat spot might start at 8 or even 9". So the 6.5 is coming in too late. The Power valve on/off is tunable by getting different ratings. Also, the AMOUNT of extra fuel is determined by the size of the 2 holes underneath. They are very tiny on DP's. As a rule, Holley made DP's so rich that they really don't need much extra fuel dumped in the midrange. But if you lean out the curve, you need to add more for accelleration. So I increased the size of these holes 20%. Now, I have added more mix to the engine only under load. I used a 2-stage power valve. Not commonlyused, this is a 2-stage valve that opens up a little at 9.5" and then all the way at 6". Llike a dimmer switch for a light, but vacuum controlled.

    Now drive the car again, recheck overall timing. Might have to tweak it a bit because the leaner mix needs more lead. Now I tune the accellerator pump. Iused the smallest shooter I could get and the factory pump cam and pump. You want just enough shooter gas to cover up the bog from a dead-stop or a freeway downshift tromping. Any more than the minimum needed is wasted, not enough and the bog goes away at forst, but then shows up about 2 seconds later. Smaller shooters squirt less, but over a longer period of time untill the pump is empty.

    Finally, I found that leaving the secondary jets alone is fine. Sometimes I'd add 1 size, and I'd leave the secondary squirter alone because it is rich already, and that would compensate for the leaner primary. It took a lot of time, but when I got it right, this carb was so silky smooth that people swore it was injected. I got it so tight that if the temp outside dropped 30 degrees, the car would lean surge on the highway and I'd go up a jet size or two for winter.

    Go get the HPBooks on Holley carbs. It's full of tips and tricks and specs.
    Heres a little graph that will show where the gas comes from.
    Last edited: Oct 24, 2011
  10. Pilot53

    Pilot53 Veteran Member

    Nov 30, 2008
    New City, NY
    Wow, thanks fort he great info, that should keep me busy for a little while. Just to be sure, are size 65 jets in the primary and size 71 jets on the secondaries safe? I dont want to burn a piston.

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