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Thread: Holley 750 Double Pumper Help

  1. #1

    Default Holley 750 Double Pumper Help

    We need help figuring out what the problem our Carb is having. If I am at idle, and go to full throtle instantly, when it gets to about half throtle the car dies out. It will kill the motor if I stay in it. I can rev up slowly and it will work fine. I checked the clearance on the accel pumps (per holleys website) and that is ok. I checked the pumps themselves and all gaskets and materials look great. there was a minor blockage from the bowl to the metering block (the tiny hole that the gas goes through). Got that taken care of. We went from 31 size squirters to 35s and it didnt really get better. I put the original 31s back on it and it seemed to get a little better. I also went up two jet sizes in the back when the 35s were on it. The distributer is locked out and checked the timing while trying this and everything is fine with that. Does anyone have any ideas or direction for us to go? It did seem to be running a little rich during all this, but we dont have a A/F meter to really tell. Plugs look great also. We have had the carb for 5 years and this is the first time this has happened. Its a holley 750 double pumper with the power valves taken out. Thanks to anyone that can help us on this matter.

  2. #2
    Join Date
    Apr 2010
    Location
    Bowling Green, KY
    Posts
    39

    Default

    Assuming this is a carb problem and not a vacuum leak, ignition or valvetrian problem. The first thing that you need to do is adjust your float level, no fuel should run out the sight plug with the engine running. Check to make sure that your fuel pressure is not over 7 PSI. I suggest 6-6.5 PSI Make sure the acc. pump is properly adjusted. It should be at zero lash at idle, and have at least .015" gap at Wide-open throttle with the pump lever pushed all the way down. Then try turning in the idle air screws all the way in with the engine running.
    If the engine continues to run with both screws all the way in, then the carb is letting fuel into the engine from someplace other than the idle
    circuit. We have to find out where from and correct it (with the screws all the way in the engine should die) Next you want to check the manifold
    vacuum with the engine idling in gear. This will tell you which size power valve to use. If your vacuum is 11" and under divide by two, and that is the size you need. If it is 12" and over use a 6.5 PV. You may have to replace it with the correct size. If you get all of this corrected and the engine still runs rich then you will probably have to remove the carb and adjust the secondary throttle plates open to the bottom of the transfer slots. This will allow you to close the primary plates down some. If either plates are open too far then fuel will be pulled in to the engine from the transfer slots in the base plate. By adjusting those plates like I said, the carb will get the same amount of air at idle, but it will get it evenly from the
    front and rear instead of just the front. Once you get it so the engine dies with the idle air screws, then you can continue with the rest if the adjustments. Next you need to adjust the idle air screws. Turn them in until they seat lightly then back them out 1.5 turns. Start the engine and
    let it warm up. Hook up the vacuum gauge to manifold vacuum. Check it at idle in gear. Adjust the idle air screws evenly until you get max vacuum
    (the vacuum will drop off if you go to rich or lean) With that done you can start working with your jetting unfortunately I cannot tell you how to jet it. Each engine is different and has different requirements. It is a trial and error process. Start with the stock jetting and go from there, Run the engine under a load and read the spark plug color to find out which way you need to go. Black is rich and white is lean. It is best to tune the primaries first at part throttle and the secondary's at WOT run the vehicle at least a quarter mile and shut it down under throttle in order to get an accurate plug reading. This should be accomplished on a track or a long stretch of road with a wide shoulder that it can be done safely. Adjust it until you get a nice light brown color on the porcelain of the plug. When you get this adjusted, if you have an off line hesitation that is less than a second Increase the shooter by one size. If it is over a second increase the shooter by two. If it makes the hesitation better you can increase accordingly if it gets worse drop the size.
    Thanks Tom

  3. #3

    Default Thank You!!

    Wow, thank you for the in depth answer. I will work on that. I know the fuel pressure is good, I have multiple gauges on it. But I will start like you said and go through everything. I will get back with you when I get it all done. Thanks again so much. We have been scratching our heads for awhile.

  4. #4
    Join Date
    Apr 2010
    Location
    Bowling Green, KY
    Posts
    39

    Default

    Your welcome,
    Let me know how you make out.
    Tom

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