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Discussion Starter #1
I have a digital wide band air/fuel ratio meter and am looking to find the proper air/fuel ratio. From what I have read 12.5:1 A/F is close to optimal for WOT. Does everyone agree? I do not know what is best for part throttle cruise and idle though. I realize these A/F's can be greater but what exactly. I am familiar with reading the plugs and setting idle according to manifold vacuum etc. but I am also using the meter as an aid. Thanks for your help guys and gals!

Edit: Just want to through out that this is a naturally aspirated carburated engine.
 

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No. I have heard that that is not necessarily the case. It is a good baseline, but sometimes the engine may want more or less fuel.

Again, reading the plugs does not tell you how well the carb is metering unless you check them for the various circuits. You can't just go by an overall reading.
 

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Yes, that should be what you are shooting for at WOT. You will find that when tuning it is going to float around a little bit anyway. You just want to maintain an averag of 12.5:1. To be on the safe side you can leave it a little leaner, but you will get the best power around 12.5:1 on an NA engine. I have been working on tuning my turbo engine with WBO2 and I can tell you that you want to make sure and get something that you can do some datalogging with. I would even recommend a few sensors to know RPM and manifold pressure if possible. AFR alone does not tell you too much and it is really hard to watch what the AFR is doing all the time without being able to play it all back.

Chris
 

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Discussion Starter #4
Ok, now that we have confirmed that 12.5:1 A/F is a good baseline for WOT...what about idle and part throttle cruise. I don't expect it to be 12.5 also. I've seen larger A/F values for both of these which I would expect considering the the engine is under little to no load and making very little horsepower. I would guess up to 14.7:1 for idle and possibly close to that for a part throttle cruise condition with little load. Any takers?
 

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The idle mixture is a little easier to determine wether of not it is close. I don't know the mixture ratio. The way I tell if the idle circuit is sufficient is with a vacuum guage. When the mixture is set it should be within 1 to 1 1/4 turns on a 2 corner idle, an 3/4 to 1 1/4 on a 4 corner idle. This applies to Holley carbs.
 

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For idle and cruise 14.7:1 (stoich) can sometimes be used. This number really depends on the engine. Some engines can actually go leaner than stoich and some will not run at stoich. When trying to tune my turbo engine, I found that if AFR got over 13.5:1 when cruising below 3500 RPM it would lean pop. You also want to keep in mind that the idle and cruise AFR can only be leaned down when the engine is at operating temperature.

Chris
 

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Discussion Starter #7
I have a mighty demon 750 and have read several other posts regarding tuning these. From everything I have read they run rich at idle and lean at WOT which I would agree with. I have leaned out the idle mixture screws to within 3/4 of a turn from closed on all 4 corners and the A/R is around 14:1 along with the hightest manifold vacuum. The idle is smooth and the car starts decent so I think I'm close there. Initial timing is 18 and total 36 by the way. I've been tuning in the main jets and have a A/F of about 13.1 WOT. Applying enough throttle to open the primaries only but not the secondaries gets me about 12.8.

My problem is part throttle cruise where I'm running an A/F of 10.8 to 11.2. This seems rich to me even though the car runs fine. Does this sound rich to you guys? Is there any way to lean this out? Thanks.
 

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You can lean the jets down until you just start to feel a surge at steady speeds between 20 adn 70 mph. As soon as you feel it surging jet up two sizes.

You want to tune your jets for cruising. That will give you an efficient cruise. Then, if your WOT mixture is off you can tune it by modifying the PVCR area. Modifying the PVCR can be dangerous. You can't drill a hole smaller.
 

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That is way too rich at cruise. You want to shoot for 14.7:1 if you engine will run there. Some engine dont like to run at stoich, but it is where you will get the most mileage and 100% burn. I would try leaning down the primarys a few sizes to see how close you can get. Then go up a few sizes in the secondaries. Another thought would be to adjust the air bleeds since they control the fuel curves. I have never messed with this since most Holleys dont have adjustable bleeds. If your demon does, I would actually start there.

Chris
 

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Discussion Starter #10
lluciano,

That's the dilemma. I've been jetting up to get the WOT A/F richer. If I jet down to lean out the cruise then the WOT will be too lean. To me WOT is more important than cruise as overly lean WOT can cause engine damege. I've never touched the PVCR and am not familiar with tuning it. I do know what it stands for though but that's about it. I assume drilling it out is a permanent modification. If so I would not be to keen on doing this yet. Any other way to lean out cruise without changing the main jets.

Edit: Sorry Turbo I must have posted the above the same time you were replying to my previous post. The primaries are currently at 76 while the secondaries are at 86. I guess I could try 74 and 88 and see of that goes in the right direction.
 

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On a street driven car jets are always for cruise. The PVCR + Jets area = WOT mixture.

The best place to lean the cruise is at the jets.

You may need to enlarge the PVCRs slighty. Power Valve Channel Restrictions. That will give you the correct cruise with the "extra fuel enrichment" added as it is supposed to be by the PVCRs. The PVCRs are behind the powervalve. They are the two holes behind the powervalve.

The Holley book made by SA Designs had a formula for modifying the PVCR area.

You will most likely need a quality drill index. Or have a competent shop do it. But, like I said make sure you know for sure you need to modify them.

Drilling out the airbleeds affects the circuit timing too much.
 

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Discussion Starter #12
My car is about 90% street w/ 10% race so I'd say you're on the right track with the jets. I just consulted with my Demon manual concerning the air bleeds and it mentions "should only be done by someone with a deep understanding of carburetion..." which I don't believe I qualify for. I think I'll try leaning out the primary jets and richening up the secondaries and see what happens. I'm not ready to try the air bleeds or PVCR'r just yet.
 

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Messing with the PVCRs is a last resort. Try the other stuff first.

You brought up a point I made a week or so ago. I don't understand the whole Proform body with the changeable airbleeds. I would be amazed if 1% of the Proform owners really know what they are doing when they change the air bleeds. Out of that 1%. I would be amazed if half of them actually made an improvement.

Then there is the new Holley HPs with the changeable everything. If they don't own a flow bench, a dyno, and a track to test 1/4 mile times, I don't know how anyone could tune them effectively.
 

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Discussion Starter #14
Yeah I agree with you on the air bleed tuning. I guess they put those bells and whistles on as a selling point. Do you agree that dropping down on the primary jets to lean out the cruise and increasing the secondary jets to get the WOT correct might work? Seems logical to me considering the secondary jets will have little to no affect at cruise. I don't mind a little trial and error but I'd like to know I'm pointed in the right direction. Thanks.
 

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Maybe a little. I would try and get them both right.

I haven't read any books or heard anything about the true separation between the circuits in the primaries, and the ones in the secondaries. I would imagine the secondary jets would still be activated somewhat at cruise.

I have tried jetting the secondaries richer to see if I could get a leaner cruise and richer top end. It seems that jetting all around works better. I run at steady speeds like I was saying. Tune the primaries and secondaries like you would the mixture screws. Go back and forth between them until you are 2 jets sizes above surging on each.
 

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I think while cruising at part throttle the secondary main circuit should not be active. The progressive linkage should keep the secondaries closed. With the tiny amount of air flow with the butterflies closed, there should not be enough air velocity to "tickle" the venturis. It takes a decent amount of air velocity for the venturi effect to happen.

Chris
 

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Discussion Starter #17
Demon 750 rich

I ended up jetting both the primaries and secondaries up. The final jetting is 78/89 from box 76/83. The A/F at WOT averages out to about 12.5. My cruise A/F is still very rich still though so I'm trying to get a hold of a BG tech rep if I can get home from work in time.
 

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The changeable air bleeds are a big help when using a carburetor in a situation it wasn't designed for. Such as a blow-thru turbo. Only the secondary idle air bleeds are still in use. I have increased the primary idle air bleed size, and decreased both sets of high speed bleeds. I run at ~14.7:1 at idle (when hot), closer to 15:1 at cruise, and between 11 to 12:1 at WOT. PVCR's have also been enlarged. Of course, it has taken a lot of testing to get to this point!
 
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