This video is like 6 months old, but it's a video of a friends big block Olds race car on a chassis dyno. We had seen this in the dyno put never took a video of it. I finally got off my *** and am posting this for viewing. Its a shot down the carburetor during a dyno pull. You can see when the pump shot happens and then you can see for the ENTIRE pull that fuel is being sucked out of the accelertor pump nozzles the entire time. This pretty much happens on every carburetor. The Edelbrock carburetors seem to be a little worse for this than a Holley. If this is consistant, you can adjust your jets for this, but if it's not, then what?

jerry

 

It’ll also pull fuel from the idle circuit at WOT.

 

Maybe increase the weight of the discharge needle or add a spring to prevent pullover.

 

We tried adding a spring. The problem is all the springs we tried, even one from a pen cut down, are too stiff. They don't allow fuel to come out of the nozzles and makes the throttle very, very hard to operate. Then the accelerator pump circuit gets pressurized and it starts leaking everywhere due to the high pressure, since it doesn't come out the nozzles very much. If you remove the accelerator pump arms you have no problem at all, as the system is not pressurized.

How do you add weight to the discharge needle?

jerry

 

I am wondering if it really matters. if it is consistent in doing this, then you still can tune for it at WOT.
I'm not saying its ideal, but it is manageable.

 

So let me add these thoughts about it being managable. When we removed the accelerator pump arms fuel stop coming out the pump nozzles. This means that if there is fuel in the circuit and that fuel is under pressure (from opening the throttle) what happens when the fuel runs out? The Dominators had 50cc pumps. I think a standard Holley is 35cc. Since no arms resulted in no fuel, I will assume that once the pump circuit empties it will also result in no fuel, and then the engine is leaner. I'm not saying I'm right, and I'm more than willing to listen to other opinions, but thats what I was thinking. Marks comment about the idle circuit is valid, but that won't run out of fuel, so it's always there. But the accelerator pump circuit could run of fuel pretty easily. I could even test it if I wanted to.

jerry

 

Maybe use lead or heavy metal. Have you contacted Holley's tech reps? I'd be interested in what they have to say.

 

That's a good point on the pump volume, I did not consider that.
Maybe enough for the 1/8 mile but Bonneville would be a problem. :-)

Thanks for posting video, would have never thought of the fuel from that circuit adding to the mix other than during pump operation. I wonder if the nozzle design would impact?

 

Nozzle size and style make a difference. It's completely normal and well known. Holley actually had what's called an "anti pull over" style nozzle to stop that. Other manufactures also make them different to try and reduce the effect.

 

Can you remove the carb, or even do it on the engine while off, and hit the accelerator pump with something else and hold it down? I am curious to see if it empties with no venturi flow at all. I'm not familiar with the holley accel pump circuit, but, if the pump is held down, is the circuit permanently open and subject to whatever? Or is it closed behind the pump and won't refill until the pump is released and it recharges?

 

Makes sense, thank you.