edelbrock dual carb setup need help

First let me say that I have read the archives and look through alot of the posts on here. I understand that my question has been asked before, but I had no luck finding a clear answer. I hope this post will provide me with what I am after. I have a 91 corvette, I installed a 434ci small block chevy stroker motor. The car has 11 to 1 comp, brodix track 1 aluminum heads with 2.10 1.60 valves and 220cc intake runners. The cam is a solid roller with 254int and 260 exh @ 50, thelift is .680 with a 106 seperation. Had to have a small base cam to clear the stroker assembly. The ignition is a mallory unilite system with mechanical advance. The timing is set at 38 degrees at 3000 rpm. I am running shorty headers and a flowmaster exhaust. My intake is a edelbroc low rise dual 4 barrel setup. I have 2 500 cfm carbs on with a 1 to 1 linkage and electric choke on rear carb only. The problem starts with the car running very rich. My first step was to put an air fuel ratio gage on the car. It also agreed the car was running to rich. I am running an electric pump with a regulator and fuel pressure gage. I set the pump to 6 lbs of pressure, which is good for the setup. The car has to idle around 1000 because of the big cam in it. I measured the vacuum at the carb on idle and got a reading of 7-8 inches at idle. Jegs advised me to put in from a orange (5") to a blue (3") stepup spring which is the lightest they have. I think this is where my problem might be. My next step was to change the main jet to from 086 to 083. Still ran rich on idle. Next changed the metering rods to the 1448, the car is a little better at idle not much but at a constant speed of 3000 rpm runs so lean my gage stops working!! Big problem, I think that the stepup spring is so light it is keeping the rod in lean mode when the car is cruising. Does anyone know exactly how to tune an edelbrock carb?? I know the first thing to do is measure the vacuum and determine which spring you will need for the stepup. But what vacuum equals what spring?? Instead of changing your main jets cant you just change the metering rods? I understand that full throttle is all about the jets, if your lean at full throttle, fatten the secondary jets and vice versa. Just having problems at idle and cruise. Any yideas, tips or anything else would be appreciated. Already talked to edelbrock, they said just keep experimenting. THANKS EDELBROCK lol

 

can't help much but I will be watching this w interest as i am doing the same thing as soon as my intake gets here, I am going w progresive linkage though and blocking all the vac.ports on the bottom of the secondary carb,It may be that the cam (if I am understanding the specs right ,I think roller cam profiles may appear much wilder to a old solid/hyd lifter guy) and compression are over the top for a street motor and I agree that the spring and metering rod areas are where your problems are, That cam setup and BIG valves are vacumn killers, actually from those specs the motor is setup WAY to hot for a street motor ,if that is your intention , great setup for 1971 , but on todays gas , WAY too much compression- its going to SUCK drivin it at traffic/cruise speeds anyway-not slammin your setup just a reality check + JMO .BUT if your going to race it- your set- set the idle high and BURN EM

 

I've put dual Edelbrocks on several customer cars and haven't had a problem with them. The jetting was real close out of the box. The first thing I would suggest is to get a Edelbrock owners manual. It has all of the specs you're looking for in it. It also has a graph on jets/metering rods that would be pretty hard to figure out without the book.
Is your setup rich at idle or all the way thru the rpm range? If it's just at idle, you should be able to adjust it out with the mixture screws. If you can't, check and see if the boosters are dripping gas at idle. If they are, could be fuel pressure, float level, or throttle blades cracked open to wide. I guess it could be an air bleed plugged too.
If you can't figure it out, you might try Edelbrocks Tech Line.

 

First thing I would do is figure out where you are... A 91 Vette hardly qualifies for "traditional"... Maybe an intro and tell us about your old cars. Maybe ask on the Vette forums..... Great first post, NOT!....

 

one other thing i have heard of from several owners, this setup sometimes has a fuel percolation problem when hot, I precluded this by ordering 1" plasitic insulating carb spacers.

 

Ok, I just reread your post and looks like I skimmed over some stuff the first time, sorry 'bout that. Guess Edelbrock can't/won't help.
Is your distributor curve all in at 3000 rpm's? most Mallorys I've worked on keep advancing way past that unless you go into them and fix the stop. 38 degrees should be total.
It doesn't have anything to do with low speed flooding, but are your carbs a matched set. The carbs with electric chokes are jetted a little leaner from the factory and they have different emulsefier tubes on the secondary boosters.

 

With that kind of cam profile, your "cruise" should be at about WOT. Enjoy tuning that thing, it's gonna be a nightmare on the street no matter what you do. Should be a blast at the track though.

EFI probably would have been a better choice here

 

yes the carbs are a matched set, i put the choke onthe rear carb myself, as far as the engine being streetable, thats why the cam is so big, it bleeds off some cylinder pressure and allows me to run on pump gas. I blocked all the vacuum except for the one for the power brakes. The machine shop that built the motor also took my mallory distributor and adjusted the inside of it with the mallory kit. All the advance is in at 3000 rpm 38 degrees total. With a big cam like this all your advance should come in early from what I understand. I have the adjustment manuals from edelbrock, the leaner jet and spring combo I used was in their chart and is as lean as the chart goes. We just got a foot of snow so will be a few days b4 I mess with the car again. I work on it in my yard. People who want to ask on my mechanical extent, I will say I am above average. I am 23 and have rewired the car including all autometer phantom gages, changed the gears in the rear to 375, swapped the transmission from auto to a 6 speed and fabricated a whole true dual exhaust for the car, all out in my back yard. I guess I have an idea what I am doing. Any other questions just ask. I think I will put the carbs back to stock a restart all over.

 

c4vette let me start out by saying that I'm not an expert carb tuner but I have had some experience with this multiple carb deal. I've ran two and three carbs on a flathead and had all kinds of problems, most of them stemming from the fact that the throttle plates were not opening the same amount. This is critical. What I did was to disconnect my linkages to the carbs and back the idle screws out until the throttle plates were completely closed on all the carbs. Then turn the screws in until they just start to move the throttle plates. It WILL NOT IDLE with both carbs closed off like this. Turn each screw in an equal amount, say an 1/8th or a 1/4th turn until it will idle. Be careful connecting your linkages that you do not move either throttle arm to get them hooked back to the carbs, you don't want to get them back out of adjustment. Someone above suggested leaving the jets in that came with them, with your guage telling you that it is running way lean at speed you'd have to agree that would probably be a good idea. I don't know about the springs. Hope this helps. Good Luck Stafford

 

First off, is there a specific reason your running the low-rise dual in this application? Hood clearance, perhaps? I would suggest something more along the lines of a Victor with an 850 Holley(or even bigger). That being said, I've run that combo (2x4) on a 350, and had it run like a stocker until I flopped the secondarys open. Make sure everything is equal between the two carbs. You're doing right by using the rear carb for the choke, but also use that carb's idle circuit. You might even consider the progressive linkage that is sold for that intake/carb combo. With that large of a cam, do you have any vacuum for the AVS-type carbs? My advice is ditch the duals. They're cool on a hot rod, a pain in the ass on most everything else.

Karl

 

man that has got to be one of the most MISMATCHED combos i've seen in a long time.

the booster signal to the carbs at idle has GOT to be fucked up.
a 2.10 valve coupled with the big runner coupled with the big duration will have NO velocity at idle. divvy that in 1/2 trying to work 2 street carbs on a flat intake & you're fightin a bear with nothin but a stick. not good.
that's why a holley race carb gets 4 corner idling, bigger idle bleeds, & power valves that either open at 3.5 " or blocked off entirely.

the shorty headers can't be helpin either.

i'm not sayin you can't keep tinkerin & get it decent but i am sayin that you are fightin a BIG battle. too much contradiction in the combo. i can't see it ever being "happy" all the way through the operating range.

 

Let me say that I am totally agreeing with all the people who said ditch the low rise intake and 2 carbs, I have to run a low rise because of hood clearance issues and still had to buy special low rise air cleaners, the intake is the lowest edelbrock sells, the closest single carb intake would be the torker 2 and that wont even fit under my hood since I want to run an aircleaner. I am considering buying a mighty demon 850cfm carb after a little more messing around with them some more. If you ever ran long tube headers on a c4 you would agree shorty headers are better. I hated my long tubes. I will figure it out after some more time tinkering.

 

find a late model Vette board