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Acceleration Issue
#1
First, this is a '71 stock M-code with Pertronix ignition and Holley Street Avenger 80670 vacuum secondaries and toploader. When I am cruising at any RPM and jam the accelerator the car hesitates for a Brief moment then takes off like a rocket. Steady increasing pressure on the pedal is smooth. I am thinking; carb's accelerator pump, timing or a vacuum leak. I don't think it is a vacuum leak as the car runs well under all other conditions. Just a momentary hesitation when I jump on it.

mike

[Image: 1_11_11_13_11_50_27.png]
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#2
I would agree with the acc. pump or a jet change. Deffinately not a vac. leak from what you are describing. You could also do a discharge nozzle change to help fine tune the pump shot but I would wait 'til after you do the acc. pump or jet change. Key is to figure out if the bog is from lack of fuel or too much fuel then change jets or pump accordingly up or down. Is this when you floor the pedal enough for the sec. open? Could also be a sec. spring change.....it's a carb., it could be anything! lol. Sorry, this is how I learned about carbs, chasing stuff. I had the same issue and had to change the pump, jets, and discharge nozzles before it was good.

As I said, find out if it's too much gas or not enough first and go from there. Be patient and accute to changes as they will be hard to notice at first.

"Life is what you make it......"
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#3
I would start with changing the accelerator pump cam. Try a pink cam it is more aggressive than the stock cam. When you change cams you will need to re-adjust the clearance of the pump lever. I always set it to 0 lash. If that does not work try going up a size with the nozzle. In my experience a bog is usually not enough pump shot. Secondary springs that are too stiff simply delay the opening and a spring that is too soft will accelerate fine and then the engine will bog.

Jeff T.

Low buck, touring style, '73 Convertible "rolling restoration", 351c, 2v heads with a shave and a haircut, Performer intake, Holley 650(ish), roller rockers, screw in studs, guideplates, stainless valves, Duraspark / Motorsports MSD, T-5 conversion. 1-1/8" front, 3/4" rear swaybars KYB shocks and some home brewed subframe connectors. Future plans; JGC steering box, Cobra brakes and... paint, interior, etc.

When I die I want to die like grandpa, peacefully in my sleep... not screaming, like his passenger.

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#4
The secondaries are vacuum so I don't think they come into play. I am also chasing carbs. Have not figured out how they work even after 40 years of trying Smile Another problem is this is my first Mustang ever and has a quench head Cleveland. Another learning curve.

mike


I would agree with the acc. pump or a jet change. Deffinately not a vac. leak from what you are describing. You could also do a discharge nozzle change to help fine tune the pump shot but I would wait 'til after you do the acc. pump or jet change. Key is to figure out if the bog is from lack of fuel or too much fuel then change jets or pump accordingly up or down. Is this when you floor the pedal enough for the sec. open? Could also be a sec. spring change.....it's a carb., it could be anything! lol. Sorry, this is how I learned about carbs, chasing stuff. I had the same issue and had to change the pump, jets, and discharge nozzles before it was good.

As I said, find out if it's too much gas or not enough first and go from there. Be patient and accute to changes as they will be hard to notice at first.

[Image: 1_11_11_13_11_50_27.png]
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#5
I had the same problem with my car. I adjusted the acc. pump so that it was pumping more fuel on initial acceleration and I cleaned and fine tuned the rest of the carb. It worked for me.

71 Mustang fastback
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#6
We adjusted the pump clearance and now if I tap the gas pedal the tires chirp. Cool. Holley makes no mention of adjustments in their manual or online for the acc. pump. I emailed and their response was somewhat cryptic, but the value 0.015" was in their reply. So we turned to the Ford shop manual and under the page on carbs is a Holley 4150 which is your basic 80670 that I have. There is a picture with the acc. pump lever held down and at WOT, the gap should be 0.015". Mine was closer to 0.040". Problem solved by adjusting to 0.015". My mechanic suggested putting a drop of Loctite on the adjusting nut as this is not supposed to go out of adjustment. Probably why there is no mention of it in the Holley manual.


I had the same problem with my car. I adjusted the acc. pump so that it was pumping more fuel on initial acceleration and I cleaned and fine tuned the rest of the carb. It worked for me.

[Image: 1_11_11_13_11_50_27.png]
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