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Re: Car runs to rich at cruise rpm
[Re: sshemi]
#1442854
05/29/13 06:01 AM
05/29/13 06:01 AM
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Joined: Jul 2007
Posts: 1,682
Philadelphia
radar
top fuel
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top fuel
Joined: Jul 2007
Posts: 1,682
Philadelphia
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Quote:
Why not just turn in the mix screws?
Because the term 'mix screws' is a misleading. The idle corner screws don't determine the air/fuel mix of the emulsion that comes out the idle discharge ports, just how much of it can come out. Like an adjustable jet. The same emulsion is delivered to the T-slot which is almost never adjustable. This circuit can be crucial to crisp throttle and not fouling plugs in slow traffic.
Since a strong motor can cruise well at very small throttle openings it is common for the cruise mix to be supplied mostly by the transition slot. The T-slot is non adjustable in most metering blocks, but the emulsion that is supplied to it is tunable by messing with the Idle Feed Restrictions (IFRs).
So if a standard preset of 1.5 turns out from lightly seated is too rich of an idle you can screw in the 'idle mix' screws to limit the emulsion delivered to the idle discharge ports. Your idle is cleaned up but the T-slot is still pig rich- especially with a big motor sucking hard on it pulling air past a mostly closed throttle.
A strategy that worked for me is to choke down the IFRs until the light throttle cruise cleans up, which will also lean out the emulsion that is metered by the mix screws, so now they get unscrewed to fatten the idle back up.
In my limited experience wrestling with double pumpers there seems to be a magic setting for the IFR- when you choke it down to the point where the mix screws end up around 1.5 turns out the t-slot is gonna be close and can then be fine tuned with the low speed air bleed. I started to notice that on carbs with a real fat off idle cruise the 'mix screws' were more like 3/4 turns out before limiting the IFRs, even after opening the secondaries to idle off them too.
This is why drilling the butterflies can be a 'magic' fix but not easy to take back or adjust later. By the time you are on the mains the hole is not leaning out the overall mix much.
I asked a lot of questions here about this stuff but I ended up having to pour over holley books and web articles to figure this stuff out. For some reason this is like secret knowledge. I've learned a ton on here building my a-bodies and I hope I can help give back a little!
Cheers
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Re: Car runs to rich at cruise rpm
[Re: sshemi]
#1442858
05/29/13 12:52 PM
05/29/13 12:52 PM
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Joined: Apr 2005
Posts: 12,271
Overpriced Housing Central
RobX4406
I Live Here
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I Live Here
Joined: Apr 2005
Posts: 12,271
Overpriced Housing Central
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Quote:
But if it is set correct you are right, decrease the ifrs but this will also lean curb idle.
Back the mixture screws out a bit.
The mixture screws don't mix the air/fuel from the IFR/Air bleed. The IFR/IAB is mixed before hitting the mixture screw, at that point you are allowing a designated amount of the pre-mixture in the idle ports.
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Re: Car runs to rich at cruise rpm
[Re: rapom]
#1442859
05/29/13 01:22 PM
05/29/13 01:22 PM
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Joined: Aug 2003
Posts: 43,286
Bend,OR USA
Cab_Burge
I Win
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I Win
Joined: Aug 2003
Posts: 43,286
Bend,OR USA
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What is the List number(including the dash number, IE 9375-3) on the carb. you bought and are trying to make better?
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: Car runs to rich at cruise rpm
[Re: rapom]
#1442862
05/29/13 06:25 PM
05/29/13 06:25 PM
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Joined: Oct 2003
Posts: 11,686
W. Kentucky
justinp61
I Live Here
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I Live Here
Joined: Oct 2003
Posts: 11,686
W. Kentucky
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Quote:
Well for one thing I sure don't have to worry about them opening early and really richening things up. They were in the carb when I got it and I always read that you should have the power valve number 1/2 of the vacuum number.
Thanks for the easy to understand writeup radar 
Ok. I've never read or heard about the power valve being 1/2 of the vacuum at idle. Everything I have had 1 1/2" less than idle vacuum for the power valve. More than one way to skin a cat I guess.
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Re: Car runs to rich at cruise rpm
[Re: justinp61]
#1442863
05/29/13 09:04 PM
05/29/13 09:04 PM
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Joined: Feb 2005
Posts: 4,318
Prospect, PA
BSB67
master
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master
Joined: Feb 2005
Posts: 4,318
Prospect, PA
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Quote:
Quote:
Well for one thing I sure don't have to worry about them opening early and really richening things up. They were in the carb when I got it and I always read that you should have the power valve number 1/2 of the vacuum number.
Thanks for the easy to understand writeup radar
Ok. I've never read or heard about the power valve being 1/2 of the vacuum at idle. Everything I have had 1 1/2" less than idle vacuum for the power valve. More than one way to skin a cat I guess.
The 1/2 vacuum number is a suggested starting point for tuning. It is often portrayed on here as if it is the answer.
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Re: Car runs to rich at cruise rpm
[Re: DodgeCharger]
#1442871
05/31/13 08:00 PM
05/31/13 08:00 PM
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Joined: Jul 2007
Posts: 1,682
Philadelphia
radar
top fuel
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top fuel
Joined: Jul 2007
Posts: 1,682
Philadelphia
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Quote:
What is the purpose of the power valve? I haven't had a carb with a PV in years. My carbs run fine without them.
The power valve is another non adjustable stab in the dark by holley. Ideally you want to be able to lean out the primaries so that under normal conditions (med to high vacuum) the air fuel ratio provides decent power and economy. When you really step on it a little first the acc. pump covers the initial gap when the vac. drops. Then PV opens, providing a boost of fuel where the primary jets would have been too lean.
The problem with this is that the Power Valve Channel Restrictions are another non-adjustable orifice. So when the PV opens it might go pig rich, or pinging lean.
The thing to do is get the idle and transfer slot settings close, then work on the primary main jets with a vac. gauge and AFR meter. Get to where you can drive with a crisp throttle if you drive like a regular old lady. Take note of how low the vac. reading is when the AFR starts to go too lean to give good power.
Then put in a PV that opens around that vac. Go test it. Notice when it opens the mixture will fatten a certain amount. Now you can restrict or enlarge the PVCRs and swap PVs to open earlier and later. This is a balancing act. As long as your regular cruise is good and WOT is giving the best times at the track you will have to decide weather you want a little richer or leaner here or there. Having a good AFR meter really shows you how much of a compromise a carb is compared to EFI.
I personally am happy if I know how to drive my car so that it will have good manners, do what I expect, and not foul out putting around.
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Re: Car runs to rich at cruise rpm
[Re: radar]
#1442872
05/31/13 08:56 PM
05/31/13 08:56 PM
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Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
BradH
Taking time off to work on my car
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Taking time off to work on my car
Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
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Quote:
The problem with this is that the Power Valve Channel Restrictions are another non-adjustable orifice.
Better look again. Lots of modern metering blocks, including the ones that come on all the Holley Ultra HP series, have replaceable jets for the PVCRs, along w/ the IFRs and emlusion well circuits.
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Sorry.
Take the carb. off the motor and remove the metering blocks, both of them and look at the transition slot in the carb, it is the small narrow slot that the throttle blades straddle on the front of the throttle plate 
