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Thanks, you are a wealth of knowledge on these.
Do you know the reason why the 750 Edelbrock is jetted 113/107 and the 800 is 113/101? Seems lean for a 800. Do this have to due to the booster/venture being bigger on the 800?
"reason" ??

I have no idea why Edelbrock carburetors are calibrated the way they are. There doesn't seem to be much reasoning involved in calibrations in the Edelbrock AFB clones. Not much "reason" involved if whoever is the "engineer" in charge of calibration expects the secondary metering cluster to have A/F anything but engine damaging lean with .073" main air bleed and the main well tube restricted with a .073" orifice in the tip.

Skilled in the art or not, everybody who works on carburetors is extremely fortunate gasoline is flammable over a wide A/F range.

Unfortunate for those not so skilled in the art, for a particular engine to run as good as it can in each specific condition of speed and load, the A/F range is very narrow in each specific condition, idle, cruise, light acceleration, heavy acceleration, WOT, each has a different specific A/F requirement for least fuel consumption and best power. But hey, if fuel gets in the engine somehow and the ignition is hot enough, soot or not, it will more or less burn.

If there was no such thing as CD ignition, there wouldn't be as many people who think they are carburetor tuners.
 

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"reason" ??

I have no idea why Edelbrock carburetors are calibrated the way they are. There doesn't seem to be much reasoning involved in calibrations in the Edelbrock AFB clones. Not much "reason" involved if whoever is the "engineer" in charge of calibration expects the secondary metering cluster to have A/F anything but engine damaging lean with .073" main air bleed and the main well tube restricted with a .073" orifice in the tip.

Skilled in the art or not, everybody who works on carburetors is extremely fortunate gasoline is flammable over a wide A/F range.

Unfortunate for those not so skilled in the art, for a particular engine to run as good as it can in each specific condition of speed and load, the A/F range is very narrow in each specific condition, idle, cruise, light acceleration, heavy acceleration, WOT, each has a different specific A/F requirement for least fuel consumption and best power. But hey, if fuel gets in the engine somehow and the ignition is hot enough, soot or not, it will more or less burn.

If there was no such thing as CD ignition, there wouldn't be as many people who think they are carburetor tuners.
Agree, Bought some $1000 carbs that would not out run a BG Demon out of the box.

Stepped these up to .104 Secondary to start. Closed up the Main Airbleeds. Well tubes are the large style already. Hopefully a slight improvement over the 750's I am running.
 

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Discussion Starter #23
These modifications have helped a ton. I put .026 bleeds in the secondary side. I cut the the .073 restriction off the emulsion tubes on the secondary. I have the fuel inlet passage drilled out to 3/8 with the .110 needle and seats. The brass tubes that go through the booster are drilled out to .134. I am currently running 116 jets in all four corners with a .037 rod on the primaries. Should I just remove the brass tubes from inside the secondary boosters? I'm afraid of the carbs not passing enough fuel. I'm thinking a .116 or .119 jet plus the air that gets introduced is more than the .134 passage I have it drilled out to can pass. Any thoughts?
 

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These modifications have helped a ton. I put .026 bleeds in the secondary side. I cut the the .073 restriction off the emulsion tubes on the secondary. I have the fuel inlet passage drilled out to 3/8 with the .110 needle and seats. The brass tubes that go through the booster are drilled out to .134. I am currently running 116 jets in all four corners with a .037 rod on the primaries. Should I just remove the brass tubes from inside the secondary boosters? I'm afraid of the carbs not passing enough fuel. I'm thinking a .116 or .119 jet plus the air that gets introduced is more than the .134 passage I have it drilled out to can pass. Any thoughts?
To find out if the carbs have any headroom to go richer you can remove the secondary jets and see how much effect that has. It should be obviously richer and from that point you can put the jets back in, start at .128" and then step at a time smaller to whatever gives best MPH. When I get home in an hour or two I will check notebooks for old school nozzle sizes, but I think you are large enough.
 

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Did you have some OE 409 carbs to look at to arrive at the .134" sec. nozzle? I have some secondary boosters here in a coffee can of AFB stuff from 50 years ago and the nozzles are .134". I don't remember if they are out of 3361-3362 409 carbs or 3721 300-340 hp 327, but one or the other. These do not have main well tubes, but have a tip-in discharge feed tube with a pinch in the bottom .042", the discharge orifice is .055 with a .026" bleed in the end of the cross-channel at the end near the pickup tube. The main has a .028" bleed. The tip-in has the usual discharge location at the lower edge of the casting where the Edelbrock carbs have the little stub of brass tube. This one is stamped 620 on top of the booster leg but I don't remember what carb it is out of.
 

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Discussion Starter #28
So the factory edelbrock was .107. I opened it up to about .119. Then I got to a point where increasing the jet didn’t make a difference so I drilled it to .134. Pure coincidence that the early factory carbs were the same. I don’t have any factory 409 carbs even though this is a 409. It’s a stick car and I leave with it wide open on the limiter. No secondary air valves.
 

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Have you done anything about the primary boosters? They are too short in the 750 Edelbrock. The bottom edge of a carb booster should be slightly below the minor diameter of the main venturi, about 1/8" or 3/32", to be in the location of highest velocity flow. The 750 Edelbrock units use the same booster casting as the 600, but in the 600 carb body the minor diameter is almost 5/16" higher than the 750. The old OE carbs with that venturi size (single 4bbl 409, cross-ram 426 Wedge and 375HP 440 Mopar) have longer boosters that extend down to where they should be located in the "vena contracta".

https://en.wikipedia.org/wiki/Vena_contracta

On a street vehicle the short booster causes the carb to have erratic A/F depending on throttle position and RPM, rich~lean~rich~lean :rolleyes: as the throttle is progressively opened. In your application the main purpose is WOT and high RPM over a relatively narrow range so it might be a satisfactory situation because part-throttle isn't important. You might consider modifying some boosters by adding sleeves extending down to the desired location, about 1/8" below the minor diameter. Look at and measure as many OE carbs as you can for clues.

Another thing to consider is the Edelbrock bowl vents are not the same as the old-school OE carbs. The old carbs have the slash-cut tubes projecting into the air flow above the venturis with the openings facing into the flow, thus providing impact pressure to the bowls and increasing metering pressure differential across the jets.
 

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Sorry, post high jack. My N/SS car runs the 750 Eddys with the previously mentioned mods except the booster mod. I have a pair if Carter Comp series 750s. Worth the swap? 572" drag motor on an Indy cross ram. Automatic on foot brake leaving at 3000. 1.25-1.30 60ft depending on how far its slowed to run index.
Doug
 

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The boosters interchange, fit either carb, Eddy or Carter Comp. If you have the longer boosters give them a try.
 

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Discussion Starter #32
Have you done anything about the primary boosters? They are too short in the 750 Edelbrock. The bottom edge of a carb booster should be slightly below the minor diameter of the main venturi, about 1/8" or 3/32", to be in the location of highest velocity flow. The 750 Edelbrock units use the same booster casting as the 600, but in the 600 carb body the minor diameter is almost 5/16" higher than the 750. The old OE carbs with that venturi size (single 4bbl 409, cross-ram 426 Wedge and 375HP 440 Mopar) have longer boosters that extend down to where they should be located in the "vena contracta".

https://en.wikipedia.org/wiki/Vena_contracta

On a street vehicle the short booster causes the carb to have erratic A/F depending on throttle position and RPM, rich~lean~rich~lean :rolleyes: as the throttle is progressively opened. In your application the main purpose is WOT and high RPM over a relatively narrow range so it might be a satisfactory situation because part-throttle isn't important. You might consider modifying some boosters by adding sleeves extending down to the desired location, about 1/8" below the minor diameter. Look at and measure as many OE carbs as you can for clues.

Another thing to consider is the Edelbrock bowl vents are not the same as the old-school OE carbs. The old carbs have the slash-cut tubes projecting into the air flow above the venturis with the openings facing into the flow, thus providing impact pressure to the bowls and increasing metering pressure differential across the jets.
I haven't played with the primary boosters other than some stream lining and detail work. I'm going to look for some carters at the swap meets and see what I can see. I like the idea of extending the boosters. Anything to aid a/f ratio consistency has got to help. Thanks! I wish the original factory carbs flowed more air. I would be all over those. What about enlarging the needle and seats? Edelbrock only makes the .110 but I know Carter used to sell larger and there must of been a reason. It looks like Rochester ones would work. They have the same thread and appear to be only .100 taller. Thinking they could be made to work https://quadrajetparts.com/quadrajet-needle-seats-c-128_30.html
 

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Have you noticed the bottom of the booster is above the smallest part of the venturi in the body?

You can enlarge the N&S by hand if you don't have access to a lathe. Put the seat in a hand drill motor and hold the drill bit stationary in a hand chuck in the other hand. Put the seat in the drill motor chuck so you drill from the needle side toward the thread side.

Use a piece of wood to deburr and burnish the seat (I use chop sticks from a neighbohood Asian restraunt) and follow that with a twist of red Scotchbrite, then drop a 5/32" or 3/16" steel ball in the seat and tap it using a brass punch (1/4" brass rod), hold the punch against the ball when you whack the punch with a small hammer or the side of your needle nose pliers. The ball will swage the seat round so it fits the needle tip.

If it holds vacuum on your tongue it will hold pressure. Drill the seat .120", .125", .128", .136", whatever ?? size you want.

QJets, with only one N&S, sometimes need up to .159" (or whatever) and with large seats will run well with only 4 PSI.

Keep in mind that as the seat gets larger the liquid level will be higher with the the same fuel pressure and the same mechanical float level setting.

Larger seat allows the use of lower fuel pressure and so reduces the fuel foaming which results when there is a large pressure drop through the needle valve.
 

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Discussion Starter #34
More good stuff! I had to do the ball trick every spring with big holley regulator I had. You could see daylight between the check ball and the brass seat. The fuel pressure would creep. A couple love taps to the check ball cured that for the season. I think I'll go to .125. I don't want to go too big and not be able to control the fuel level. I haven't made it back to the garage to look at the boosters but it is on my to do list for sure. Thanks for all the help.
 

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Discussion Starter #35
I had it with the Edelbrocks. I am switching over to Carter 4760S. This is their race carb from the late 60s early 70s. This is not the street/strip 9000 series super quad from the same time period. I have one decent one and one for parts. Just need another decent one. The question I have is what was the out of the box jet/rod combo on these? The only thing I can find is the Edelbrock jetting people say "they are the same". There is nothing the same between the 4760S and an edelbrock. Well almost nothing.
 

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Not 100% sure, but I think this
Primary Jets 110
Secondary Jets 92
Metering Rods 16-388 .073 X .060
 

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Discussion Starter #37
Ok cool. Thank you. I was wondering if it would be beneficial to add a small step to the bottom of the boosters to help atomize the fuel better? Also, I did this epoxy work to my current Edelbrocks. Worth doing?
DSCN2284.JPG
DSCN2280.JPG
 

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That is a lot of tootsie rolling. I can't believe you went to all that trouble and left the rough parting line in the primary venturi. Looks like that might improve the air flow, but without knowing the effect on booster signal and subsequent metering signal I wouldn't do it. If you can swap the lids between the Edelbrock and Carter main bodies without doing anything to change the Carter it might be worth a try.

Did you notice the primary boosters in the Carter are longer and extend down to below the minor diameter of the venturi in the body? The 4760S Carter is very similar to the 375HP 440 Mopar carb and should run well as manufactured, other than optimizing the jet and rod combination for your engine. The eldenhouser carbs are a dime a dozen but you can't easily replace the old Carter, so I wouldn't screw it up. You might try the Carter primary boosters in your 'airflow improved' Edelbrock. Cutting the discharge nipples out of the boosters probably wasn't the best idea, so don't do that to the Carter parts.
 

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Discussion Starter #39
Yes that was one of my concerns. I might have gained air flow but disturbed how the air flows around the booster. Yes I did notice the primary boosters are quite longer. I'll leave the carter lids alone for now. I don't know yet if the edelcraps interchange but will check it out. It would make an interesting back to back test. Can't wait to dig into these things.
 

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Discussion Starter #40
Back again! So I did my flow improvements and saw a 15% percent improvement over the stock afb. Filling the cavity flush where the weights for the air valve would go lost flow. So I didn't do that. The extra epoxy work like my edelcraps did nothing. Zero. So I didn't do that either. I finally got to a track and having the jetting really close. Right now I am at .098 primary with a .042 rod and 89 secondaries. The edelcraps even after all the drilling and smaller air bleeds were jetted at .119 all around!!!!!!!!! Anyway, my problem now is it is really lean on the idle circuit. It idles nice at almost 12" of vacuum but is undrivable in the pits and nearly impossible to do a burn out. Just putting a little pressure on the pedal causes it to cut out. Holding at a steady 2500 rpm in first it surges. To do a burnout, I had to keep pumping the throttle so the accelerator pump would give it fuel and keep it from crapping out. Wide open throttle is perfect.
 
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