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Carb Q

This is a discussion on Carb Q within the General Help forums, part of the LSx Technical Help Section category; Would a carb thats too large cause a motor to run and rev find when there is no load on ...

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    Carb Q

    Would a carb thats too large cause a motor to run and rev find when there is no load on the motor but when driving it starts out fine but boggs when you floor it. Its a 750cfm on a 400ci pontiac, stock internals, no cat. Thanks

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    Senior Member INMY01TA's Avatar
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    What's a carb?

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    Quote Originally Posted by INMY01TA View Post
    What's a carb?

    ....

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    Simpsons Did it!! 98formy's Avatar
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    Quote Originally Posted by irbaboon View Post
    Would a carb thats too large cause a motor to run and rev find when there is no load on the motor but when driving it starts out fine but boggs when you floor it. Its a 750cfm on a 400ci pontiac, stock internals, no cat. Thanks

    What kind of carb is it?

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    Quote Originally Posted by 98formy View Post
    What kind of carb is it?
    Edelbrock not sure if its a performer or thunder series but its 750cfm

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    Senior Member bigrondownhiller's Avatar
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    how is it at part throttle? the size seems just right unless you are making a lot of power.

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    Member ls1speedster's Avatar
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    on a 400
    a 750 should be fine..

    i would take it and have the carb tuned..unless u know how to do it..it may just need a good tune..

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    Racer/Tuner San Diego
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    Sounds like the accelerator pump has failed.
    Common failure if it has been sitting or just old.
    Otherwise it needs to be set up to match the engines needs.

    Gary

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    Im told the carb was just put on a few months ago. So i dont see how the accelerator pump could have gone, unless this new gas they are putting out gummed something up?

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    Member Since 1998 Mecinoid's Avatar
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    It might be the accelerator pump is not set up correctly. There are adjustment holes on some of them. In the past, I just had to adjust mine for a quicker shot of gas and I lost the hesistation on mine. You might need a larger or smaller accelerator pump. It's all in the tuning of the carb. There is a lot more to it than that.... But, that's a good place to start. Then you'll want to get the right metering rods and jets to get it dialed in just right. It's an art that isn't quite yet lost. ;-)

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    Member ls1speedster's Avatar
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    i just had my 305 4barrel rebuilt i did it and it seemed to help some...
    but i had no problems with it before..

    but i dunno i would just make sure all vacumn lines are hooked up..

    with a acceleratior pump it may be the problem, but if it is somwhat new (a year or three)...it should be fine..

    try a rebuild kit and clean the jets

    repost if u get it solved..

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    Senior Member bigrondownhiller's Avatar
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    the accelerator pump has a spring. if it is the wrong size it will dump too much fuel and make the engine bog. find out what size you have and get a lighter one. they probably have a kit with several sizes available. i havent worked on an edelbrock before, only holley. i thought that edelbrock didnt have an accelerator pump, only metering rods and jets.

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    Moderator Firebirdjones's Avatar
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    I am not a fan of the Edelbrock carbs, I am more of a holley and Q-jet person but the Edelbrock can be a good carb when dialed in properly.

    A hesitation when you floor it could be a few things. Most of the time the carb needs more pump shot,,,either the accelerator pump cam needs to be changed to a more aggressive cam that gives more shot or a longer shot,,,,or,,,the pump shooter needs to be enlarged.

    Both would be easy to do, Edelbrock sells all the tuning parts needed for these carbs.

    The other possibility would be the secondary portion of the carb. Not sure if you have a vacuum operated secondary or a mechanical secondary but in any event,,,,,opening a little too soon or too late could cause a bog as well. Here is where things can get confusing for some,,,going back to the pump shot possibility,,,,when you floor it and all 4 barrels open at once,,,you are creating a big hole,,,this big hole has to be covered up with fuel to compensate for the rush of air coming in,,,,,,this is where your pump shot comes in,,,,,and if the pump shot is not sufficient enough to cover the big hole you just created you get a hesitation or stumble,,,,sometimes this can even occur at part throttle driving as well.

    Hope this is simple enough, without going into writing a book here on the subject, because this can get really involved. As far as size,,,,the 750 is just fine for even a stock 400,,,,it's what the 4 barrel pontiac 400's came with from the factory in Q-jet form.

    Something else to check would be to make sure your timing is all in where it needs to be and the advance mechanism is working properly. This can cause stumbles that makes people think the carb is junk,,,when in fact the timing curve was either too slow, non existent, initial timing too low,,,etc....
    Not sure on your combination so I can't give you an area to shoot for.
    Hope that helps. Larry.

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    He told me today when he is in 4th on the highway at low rpm's and floors it, it goes good then flattens out. Hopefully this helps you guys a little

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    Moderator Firebirdjones's Avatar
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    That sounds like a classic symptom of running the carb dry on gas. Those Edelbrock carbs have a very small fuel bowl in them,,,,and don't hold much fuel in reserve,,,,very easy to run them dry if you don't have a fuel pump that can keep up with the demands,,,,,this symptom is most noticable when in 4th gear as you describe,,,and the motor has a long hard pull in top gear.

    You need to hook a fuel pressure gauge up in the car where it can be read while going down the road,,,,go through the same procedure and watch the fuel pressure. It should hold a steady 6-7 lbs. if you have a good pump. If it starts falling off to 3-4 lbs. then either, fuel filter is clogged,,,,pump is going out,,,,or the pump is simply not big enough for the application.

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    Thanks jones, ill try that

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