50 cc Accelerator Pump on a 4150...

Discussion in 'Engine Topic' started by EricsZ28, Aug 29, 2012.

  1. EricsZ28

    EricsZ28 Veteran Member Lifetime Gold Member

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    I'm setting up my "new" (rebuilt craigslist find) Holly 4779 / 4150 - 750 CFM dp.

    I've been increasing my primary accelerator pump discharge nozzle size, and I'm at a 35. I still need some more, so I'm thinking that I need to switch to the hollow screw and the 50cc pump diaphragm.

    Can I just order the larger diaphragm, or is accelerator pump housing physically different for a 50 cc pump diaphragm?

    The car is coming ALIVE with the bigger pump shot :cool: It's the best it's ever run :bowtie:
     
  2. Musmoke

    Musmoke Veteran Member

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    Holley part # 20-11. 50cc pump kit.
    The housing, diaphragm, spring and arm are all different from the 30cc pump.
     
  3. EricsZ28

    EricsZ28 Veteran Member Lifetime Gold Member

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    Thanks for the quick response and part number.

    What's another $50 in the quest for hidden HP and a better A/F ratio... sigh...
     
  4. Damon

    Damon Veteran Member

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    Many Holley carbs came with 50cc pumps on the secondary side but never on the primary side (Dominators excluded). I've never found it necessary to use a 50cc on the primary side. Quite frankly, I usually take the 50cc off the secondary side as well.

    Are you absolutely certain your accelerator pump capacity is the real issue?
     
  5. Jim Mac

    Jim Mac Veteran Member

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    some intake manifolds, the 50cc pump housing will hit the intake. jim
     
  6. EricsZ28

    EricsZ28 Veteran Member Lifetime Gold Member

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    Hmmm. Just realized this carb came with a white cam on the pump linkage. It was set on the second hole. I thought it was the orange cam. I'm going to try a few different pump cams.

    You always get me thinking Damon. Appreciate it.

    I have an rpm intake - I think it will clear because my Pro systems carb fit previously. I never would have thought of that.
     
    Last edited: Aug 29, 2012
  7. Rich Schmidt

    Rich Schmidt Veteran Member

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    How old is the carb? The stock diaphram tends to harden if the carb is run with modern pump gas. The pump wont compress as easily as it should,and will compress the spring too much and cause a lean stumble that will drive you nuts. Since your carb is used,try a new pump diaphram first. Set the arm up properly too,the right way is to open the carb to WOT,and then see if you can compress the pump more to get it off the pump arm(if this makes any sense). You should be able to compress the pump itself about another .030" past the maximum cam travel. When at idle there should be no space between the pump and the arm. If you crack from closed to wide open(engine off of course) the spring shouldnt compress more then about the thickness of 2 coils (some of that depends on the squirter size),and should should not be traveling more then about a second longer then after the throttle hit wide open. Slow pump travel is a always mistaken for too small a squirter.

    After you check all those things,you can tune the cam and the squirter. I would always use a hollow screw,it wont change the flow of a small squirter anyway,and why have a restriction that you dont need.

    Good luck.
     
  8. EricsZ28

    EricsZ28 Veteran Member Lifetime Gold Member

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    Thanks guys. Fresh rebuild on the carb. The diaphragm was replaced.

    Update: Hollow screw installed, #35 squirter, and BOTH pump linkages adjusted (primary was fine, secondary was a little slow to engage). I switched to an orange cam as well.

    Results: A slight improvement, but I'm not quite there. I'm a little fat everywhere except when I stab the throttle.

    When I go WOT and hold it from cruise, the engine goes lean (from13.1 to 17+) with an occasional miss/sputter until the secondaries kick in. If I hit the throttle a bit less, and hold it, it does do a little better (15.1 A/F ratio sustained fairly evenly) and seems to recover as I hold it down and the primary/secondaries take over from the transition. I would blame the fact that I removed the vacuum advance, but I'm usually north of 3k RPM's when this happens.

    Any thoughts? I have a 6.5 PV already.
     
    Last edited: Aug 31, 2012
  9. EricsZ28

    EricsZ28 Veteran Member Lifetime Gold Member

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    Here is a little follow up for future searches...

    I installed the 50 cc pump on my 750 DP with a 37 squirter up front, and it was a bit rich, so in the process of swapping squirters the threads stripped on the 750's body :screwup: So, I ended up going back to my 650HP carb.

    I installed the 50 cc pump, a 35 squirter, hollow screw, the pink cam, and I'm running ported vacuum advance... it runs much better. It idles fine and the throttle response is nice and crisp. The torque has clearly increased - I'm now having to let up at the top of 1st to keep the tires from spinning under hard acceleration :)

    The A/F ratio is still showing a bit lean on the pump circuit (no lean misses though). I swapped in the orange cam, but I have not driven the car yet. I'm waiting for dryer roads... If the orange cam doesn't do the trick, I'll try the cams that came with the 50cc pump.
     
    Last edited: Sep 5, 2012
  10. Musmoke

    Musmoke Veteran Member

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    The cam that came with the 50cc pump is meant to be used with it. You will not be utilizing the full effectiveness of the bigger pump if you use any other cam.
    .....and that 50cc cam is not meant to be used with the 30 cc pump. (too much travel for the smaller diaphragn)
     

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