Holley pump cam help

Bonnewagon

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A friend put a 600 cfm Holley Quick Fuel vacuum secondary carb on his mostly stock Chevy 350 engine. This is in a very stock 1978 Malibu, auto trans. It does have headers. We got the idle nice and smooth, at around 650 rpm, with 20" idle vacuum. The vacuum advance is hooked to ported vacuum. But there is a hesitation off idle. WOT is fine. I know dick about Holleys, and he knows even less. I suspect the accelerator pump. I read up on the pump adjustments. I get the .010" gap at idle, and the .015" gap at WOT. It is the accelerator cam that I don't understand. What color would work best for this set up? My reading indicates the blue cam, in the #2 hole, would give the most volume fastest. I don't know what color is on it right now. I need to check that. If this was your car, what color would you use?
 

melloelky

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i know a just a little as well,not my strong suite-more times than not seeing as you snugged up the acc pump arm to remove slop making a possable delay-my go to is always with a bigger discharge nozzle for a bigger hit of fuel.it was my understanding that the #2 postion delays hitting the peak cam lift on that eccentirc there adding lift later in the throttle postion and the idle range has something to do with this postion also. #1 postion is for idle speeds under 900 rpm
 
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ELCAM

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What happens if you connect the vacuum advance to manifold vacuum?
 
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Sweet_Johnny

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Bonnewagon

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Nothing changes. Which is odd, as the idle rpm only goes up about 100 rpm.
 

Bonnewagon

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my understanding that the #2 postion delays
I read it was the opposite. I found this image below. They sometimes give a chart with the flow in both positions. Part number and -1 is first hole, -2 is second hole. First chart is from Holley. The second is a private test. Lots of testers say Holley is not believable, Most say the blue cam is the best, as I mentioned. I have a LOT to learn here.

"A quick way to enhance the lift curve on any cam is to move the cam mount from the Number-1 to the Number-2 position. This starts the lift curve sooner."
carb-science-series-accelerator-pump-tuning-for-holley-carburetors-2019-11-04_20-31-11_170266.jpg

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ELCAM

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I think you have a vacuum leak to find before you worry about the accelerator pump cam.

Is it a square bore Holley? If yes what did you do to install it on the quadrajunk spread bore manifold?
 
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bracketchev1221

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Well, as you'll come to find out. Its a combo of pump cams, nozzle size, and accelerator pump capacity. Some cams and nozzles require the larger 50cc pump because they run out of fuel too soon even though it may shoot faster and more initial volume. The best I can say is its trial and error at best. But I would try going to full manifold vacuum on the advance and see if that helps it. You may be able to close down on the throttle blades at idle by having more timing in it. More timing may help the transition until the carburetor catches up. Another point to realize is that many times people correlate the off idle hesitation as a lean bog and keep adding fuel. It may be too fast a rate causing a instantaneous flooding until it catches up.
 
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Bonnewagon

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I think you have a vacuum leak
No, I don't think so. It is an aluminum dual plane manifold and the Holley fits without an adapter. The carb that was on was very old and dirty. I would have re-built it, but he has few skills, and just went and bought a new one.
many times people correlate the off idle hesitation as a lean bog and keep adding fuel. It may be too fast a rate causing a instantaneous flooding
I agree with that. I am a Q-jet guy and I cringe when people loosen the secondary spring, causing a bog.
I will be going back today and looking at what cam is on it. Also what the discharge port number is.
Am I correct in assuming that a mostly stock 350 engine in a fairly heavy stock car wants a lot of pump shot?
 
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Bonnewagon

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I went back today to see what was up. We were able to clean off the timing tab enough to see that the timing was set at 6° BTDC. The ported vacuum advance was working well. We also tried hooking the advance to full manifold vacuum. The idle rose about 100 rpm, but the idle got raggedy. It did not cure the hesitation. In fact, if you nailed it, the engine died. We again did the basic adjustments on the accelerator pump linkage. The fuel bowl level I set two days ago, had changed. I had to raise the level a lot. That is normal for Holley, yes? Then I spied it. The pump cam was PINK!!! That is the weakest cam profile available, and that is what Holley included with the brand new carb. That virtually guaranteed you needing to buy the cam assortment kit. I told the owner to do just that. Watching the discharge ports I saw the totally inadequate short squirt come out. Instead of a 10 beer piss, it was a desert dry spit. I feel the blue cam will cure this, as it is the most aggressive profile. If it is too much we can move down the list of cams until it is right. So far, this carb has been a mess right out of the box. Nothing was right. I am being forced to learn about Holley carbs. I guess that is a good thing, but I will never change from my Q-jets.
s-l300.webp
 
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