carb???

[CO5.0BRA]

when i give my 84 gt full throttle it feels like i have to wait for a turbo to spool before it goes i cant just floor it cause it wont go anywhere its a 4 barrel holley with a edelbrock performer 302 intake manifold does my carb need cleaning?? whats goin on

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[67mustang302]

It may. It could be the accelerator pump as well, or ignition timing.

[Gary H]

Depending on the spring, the secondaries might be opening to soon too. Timing may not be advancing either.

[HACKGT350]

ORIGINAL: Gary H

Depending on the spring, the secondaries might be opening to soon too. Timing may not be advancing either.

depending on that spring the secondaries probably arent opening at all. go out and get a spring kit new accelerator pump and power valve. shortest yelllow spring will open up the secondaries at about 2500 rpm not fully open just start opening they are fully opened at 5500. the stock black spring opens them at 5k which is pointless. the bigger accelerator pump will push more fuel and the bigger power valve will squirt more fuel when you first hammer the gas this should get you feeling more from your car. i did all this and my car runs perfect. a little rich but runs great. if i had a scanner i wouldscan a picture of my dyno sheet for you...actually i can take a picture with my phone

[HACKGT350]

this is with a stock engine 192k miles valve springs are bad float the valves at 5100, full exhaust. tuned up carb, 16* timing at the time backed it down to 14 because it was pinging pretty good. no ac ps or smog and an electric fan

[67mustang302]

Secondary spring depends on signal, which depends on the carb and engine combo. I'm running the black spring in mine and the secondaries are open way the hell before 5,000rpm.

[HACKGT350]

yeah it starts to open but doesnt come close to opening all the way. the secondary does not rely on signal is relys on primary throttle posision and engine vacuum. if you dont have your foot the the floor the secondaries cannot open because of machanical restriction. wot will not open till minimum vacuum pressure is in engine with given rpm

starts to open fully opened
yellow short spring- 1620 5680
yellow 1635 5750
purple 1915 6950
plain (std. spring) 2240 8760
brown 2710 8750
black 2720 does no fully open at maximum air flow


would you like me to take a picture of the holley instruction sheet where i got this from?

[imnotmy77stang]

a) even in the closed position there is *some* room for the secondaries to start to open (which they wouldnt because of the spring. hack when 67 said it relies on the signal from the motor you said it relies on the vacuum from the motor..... its the same thing.

b) you dont just up the power valve size. its not a size anyways, the number is at what vacuum it will close the valve. this way when there is a low vacuum condition (like when you stomp the gas from an idle and the vacuum dips low for a sec) the power valve springs open and gives the motor a shot of gas. at an idle, you want that power valve closed. now, i have head some people say use a PV with half the vacuum you have @ idle (say a stock holley 600 has a 6.5 they are figuring your motor will have 13 inches of vacuum @ idle). other people say to make it a little higher than that, like a 8.5 if you had 13 inches @ idle. me, right now i get about 10-11 @ idle and i've been playing with a 4.5 and 6.5

c) i noticed at some point in my tinkering that the short yellow spring wasnt stiff enough to make sure the secondaries stayed closed all the way even when the motor was off. this was causing random pain-in-the-ass-ness when trying to dial in the carb. i have a tall yellow in my 570 now IIRC, but even a purple would be ok since i shift @ 6800 when im being an asshole

[67mustang302]

The secondaries get their signal from the same place as the fuel, the carburetor barrel. There's a little hole inside the barrel that's a vacuum port for the secondary diaphragm, the faster air flows through the barrels, the lower the pressure in the barrels, the stronger it draws fuel and pulls on the secondary diaphragm. As far as what rpm the secondaries open, that depends entirely on the carburetor and engine combination(and vehicle setup as well). The rpm range that Holley lists on those springs is true, but only for the engine they tested with the carb they tested and the way they tested it. If you put a smaller carburetor on the same engine the secondaries will open sooner from a stronger signal, if you put a larger carburetor on then the secondaries will open later from a weaker signal. More engine for the same carb size opens quicker and so on and so forth. Yes, secondary opening is limited mechanically by the primary throttle position, but the rate at which the secondaries open and the point at which they open depends on the particular setup.

[HACKGT350]

yes i know the power valve is rated at what vacuum pressure its spring is rated at. as for stomping on the gas thats nothing to do with the power valve right away. the accelerator pump/sprayer needs to bridge the gap between the idle circuit and the power circuit. the acclerator pump, cam, and sprayer are what give you the umf when you smash the gas. it bridges the gap till the vacuum has dropped off enough for the power valve to open this way you dont get hesitation or bogging back firing all that good expensive noices. right now my car has a little hesitation because i have not bridged the gap enough. i either need to go with a bigger accelerator pump circuit or a power valve the opens with faster so i dont get that split second where the car holds back for a second. mine happens at about 3000rpm and then takes off like a bat out of hell because the power circuit is now completely open