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Discussion Starter · #1 ·
While having an issue with an off idle stumble, I was checking the jetting of my QFT 580 VS carb and noticed that the power valve had 4 windows on it. Now I have a question regarding the difference from a Holley with only the two windows. I have changed it out for the Holley and will test it. Not sure if it will make a difference or not. Now does the QFT provide more fuel. My jetting is 66 primary and I think 72 secondary. Just changed the Pink accelerator cam from the stock QFT to an Orange Holley unit. Set it at #1 and did a adjustment on the accelerator arm, with the spring adjustment. Looks like this may help and may try the upgrade to a 35 squirter next.


Greg
 

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The stumble is a lean condition. Power valve has nothing to do with off idle. What size engine? You most likely have the idle mixture screws to lean. Richen them a little say 1/2 turn more and check.
 

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4 hole power valve if for plain jane street , 2 window if for performance.

the PV does help off idle or any increase in throttle opening

forgot , 35 shooter , thats big and can make it worse
 

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the PV does help off idle or any increase in throttle opening
Depending on the OP's combination it may or may not have an effect. Most "street" setups, the manifold vacuum is high and the PV rating is low so no effect. But if the manifold vacuum and PV rating are close the PV can start to crack open.
 

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Depending on the OP's combination it may or may not have an effect. Most "street" setups, the manifold vacuum is high and the PV rating is low so no effect. But if the manifold vacuum and PV rating are close the PV can start to crack open.
any drop in vacuum below PV , street or perf. , even a big spread , vacuum drops quick and below the streel/stock carbs . .0 to 30 in 59 seconds it might not open
 

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the 4 hole PV is for street can restrict flow, The 2 window for performance less flow restriction. The QFT has 4 windows and is the highest flowing of the bunch. How much flow is needed depends on the PVR. To small of a PV window is bad to big dosent hurt.
The QFT has some advantages its cheeper price, Stronger diaphram to help with backfire protection, a more accurate and quicker opening point.

I use them exclusivly in my carb tuning. Was intoduced to me by a famous carb calibrater Nickerson.
 

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the 4 hole PV is for street can restrict flow, The 2 window for performance less flow restriction. The QFT has 4 windows and is the highest flowing of the bunch. How much flow is needed depends on the PVR. To small of a PV window is bad to big dosent hurt.
The QFT has some advantages its cheeper price, Stronger diaphram to help with backfire protection, a more accurate and quicker opening point.

I use them exclusivly in my carb tuning. Was intoduced to me by a famous carb calibrater Nickerson.
thanks for the QFT p/v info wasnt aware of them . plus less cost , thats ok by me
 

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Discussion Starter · #11 ·
I have yet to try the orange cam on the carb. I was under the impression that the pink had less flow on the cart and the orange was in the middle of the ratings? I will give the carb a try with the 6.5 Holley PV and the orange cam. If this doesn't solve the issue this QFT is going into the recycle bin.



Cheers

Greg
 

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Discussion Starter · #12 ·
I have a Comp 270hr camshaft in the car and it idles steady at 15 on the vacuum gauge. The 4 corner idle was a bit touchy but when I set it up after a while it had a very sweet idle quality. Smoother than the 570 Street Avenger with the 2 screw. Now this 570 is a lean carb , but does it's job out of the hole. Very well. Thought the QFT 580 has larger jetting would do a better job. Timing works well at 14-16 btdc and I think it might be a cam/ squirter issue. When the rain stops I will tell the story.



Greg
 

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This chart helped me dial in my accelerator pumps.




Unfortunately, setting the pump cams and nozzles is all trial and error with only some general rules of thumb to help. I tried 20 or 30 combinations before I found the right set up for my engine.
 

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I run the same carb on a 302. I found you can really dial in the idle but it will be to lean off idle and at cruise. More squirt fuel will not help or PV. You need more fuel at the transfer.
 

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Increase the idle mixture by 1/4 to 1/2 turn each and drive. See if better. Report back.
 

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Most of the time its a bit better to have it a bit rich at idle for off idle performance. I tend to set it up for highest speed cruse in the idle curcit close to 14.7. Timing also plays a part in the A/F too. Have to trade it off back and forth with metering of air and fuel. Some what depends on goal of the build. Drag only maybe a bit more timing. Street and road race mabe lesss timing.
 
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