Carb tuning question - mixture screw vs. idle air bleeds

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Carb tuning question - mixture screw vs. idle air bleeds

Post  4604X4 on November 1st 2012, 12:39 pm

I just got my new QFT HR780-VS installed (replacing a Holley Street Avenger 870) on 466 with mild cam.
The old carb ran rich at idle - 12:1 according to my wide-band O2 sensor. It had 76 pri and 82 sec jets and 6.5 pri PV and a 3.5 sec PV.

The new QFT is even richer at idle - 10:1. It has 72 pri and 83 sec and a single 6.5PV.

I adjusted the mixture screws to lean it out some (got to 12:1), but then it backfires with a quick WOT stab. If I set the idle mixture screws to about 1-1/2 turns out (the 10:1) mixture, it goes WOT nicely without backfire.

I have not driven it yet. Just trying to get it all setup first.

My question is - if I was to go to larger idle air bleeds (has 71/71 now), that would lean out the idle, but would that be same as leaning the mixture screws (and give the backfire at WOT again)?

What is the relationship to the idle air bleeds and the mixture screws? Do you only need change air bleeds when the mixture scres have no effect - or do the air bleeds do more?
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Re: Carb tuning question - mixture screw vs. idle air bleeds

Post  rmcomprandy on November 1st 2012, 2:33 pm

It is probably rich because to much of the transfer slot is showing below the throttle plate at idle. adjust the secondary "idle speed" screw about another 3/8 of a turn more.

The idle screw adjusts ONLY the mixture at idle; (though what the idle IS will have somewhat of an effect on the off idle mixture as well) ... the air bleed adjusts lean/rich over the entire idle CIRCUIT which also includes "off idle" and "cruise".

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Re: Carb tuning question - mixture screw vs. idle air bleeds

Post  4604X4 on November 1st 2012, 5:32 pm

Thanks for the explaination on air bleeds.
I just took the truck out for a road test, and the difference between the QFT and Holley is amazing. The QFT is putting down 3x the rubber (my favorite testing method). The seat-of-pants gauge is registering high too. I'm still richer than I'd like to be. I'm at 12.7-13.0 at cruise, at WOT maybe 12-13 (hard to keep eyes on the meter), and at idle, 10:1. Maybe I need to drop to a 70 pri jet.
If I didn't have this O2 meter/display, I wouldn't change know or want to change anything. Laughing For now I'm going to try to get throttle closed a little more (I'll see if I can get secondaries open a crack at idle to increase idle speed then close primary blades a little to cover transfer slot more (I belive that is what you are suggesting).

Oh, and from a standing start to WOT I have a huge bog. No backfire, just a huge bog, then she really takes off. the QFT ships with a purple VS spring. I'm going to go a little stiffer (I'll try the "plain" color spring). I think the secondaries are opening too fast. Or maybe my pump shot is too small (I had a 50cc pump on the Holley and the QFT is a 30cc pump). Well, one thing at a time.

My hot start backfire (mentioned in previous post) is gone too.

I am definately a beliver in QFT. Thanks for suggesting them. cheers
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Re: Carb tuning question - mixture screw vs. idle air bleeds

Post  rmcomprandy on November 1st 2012, 6:34 pm

If it goes a few feet from a standing start then lays down a bunch ... you need a bigger shooter size. (Maybe bigger pump too but, I doubt it).
If it bogs immediately ... you need a more aggressive pump cam or both.

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Re: Carb tuning question - mixture screw vs. idle air bleeds

Post  4604X4 on November 2nd 2012, 5:28 am

It bogs immediately. So I'll move the 50cc pump, shooter, and cam from the Holley to the QFT.
Thanks again.
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