Carbs
 

hi guys.im new to this site and looking for some help.im running a 428 in a cougar eliminator.have a edel 7106 cam,edel heads,msd6,unilite dist,ford dual 4 lowrise with two holly 450 mech second carbs.i was told to use these carbs. i cant get them to run good.i dont have a lot of carb know-how.will these carbs work ? i see guys running all different carbs. can you guys help me? thanks

most dual 4bbl carb setups use vacuum secondary carbs, not mechanical secondary carbs. Your motor willhave a tough time getting enough air velocity past the boosters will all 8 barrels open all the time.

how does one figure the right size carbs for a dual set-up?what is most people using?

Most all multi-carb setups overcarb the motor. 450's would be fine for your application, but vacuum secondaries will make them work better.

2X4 Vacuum Secondary Carbs
 

When installing vacuum secondary carbs that were not originally designed to be used in pairs, it's important to change out the secondary diaphram springs to lighter units as the air flow through the carbs cuts in half when using two carbs instead of one. Ford used the long yellow springs as the starting point in their two four carb setups. An extra touch is to install vacuum secondary covers that allow both carb secondary diaphrams to be connected by a common vacuum hose so that the secondary opening is synchronized.

Hi,
You have 236* of duration so you will not have a strong vacuum signal for the carbs to work properly. I would turn the idle up to 1,000 rpm [hot] and adjust the timing for 14* initial[disconnect vacuum advance] Late timing will not be good for air velocity. . Try and set both air screws for 1 1/2 turns out from all the way in. Check for any other vacuum leaks,etc. If your intake is a dual plane manifold your carbs should work fine.If it's an open plane manifold then you should go to vacuum secondary carbs. does your distributor have a vacuum advance? If not then a change to one with it set for 10* of vacuum advance would help alot.
Perry.:cool:

my 450's work great.

I will throw in my two cents worth here. I agree with the statement about using vacuum secondary carbs for a dual quad setup. First, you rarely if ever see any factory dual carb setup using mechanical secondary carbs and for probable good reason. Most of the cars that used dual carbs were relatively heavy vehicles and didn't rev up all that fast. The vacuum pots were much more forgiving in that respect. Secondly, the vacuum secondaries only open as fast as the engine needs the extra CFMs. If you look around you will see that it is mostly the drag racers using mechanical secondary carbs. The majority of our engines do not displace a gazillion cubic inches and even a total of 780 cfm on a whoopass FE is plenty.

Setting up a pair of vacuum secondary carbs isn't really that complicated. First, each carb should be initially run on a single carb motor and adjusted for best performance. That way, when the carbs are installed together on the dual setup motor you stand a better chance of being "in the ballpark" from the git go. From experience I can tell you that the carbs in a dual setup must be adjusted to a finer level than single carbs because any misadjustment will be magnified (1 + 1 = 3) in a dual carb arrangement. One of the main things to do is be sure that the butterflies of each carb are properly adjusted to provide full closure in the throttle bores. Any misadjustment here will result in the inability to properly adjust idle speed. Also, be sure to purchase Holley kit #20-73 which is a pair of quickchange vacuum pot covers each having a small hose nipple. You want to connect these two vacuum pots with a length of vacuum hose to equalize the vacuum secondaries. These covers also allow quick and easy vacuum secondary spring changes.

IMHO, the best Holley carbs to use for a good running dual quad setup are the #8007s which are rated at 390 cfm. I run a pair of them on my 351W with S&H air cleaners and get not only good gas mileage but blistering acceleration. I ended up using them just the way they came from the factory, no jet or spring changes. In addition, I no longer smell like exhaust fumes after a drive like I used to with a single 650 DP Holley carb. I also know a guy with a 427FE who installed the same carbs and he loves them, saying his motor runs stronger with the 390s than with the original 600s. Overcarburetion is a real detriment to good performance. Don't even try to adjust the idle mix screws the same as on a single carb setup. Your one engine will now be drawing idle mixture from two carbs instead of one so each carb will supply roughly half the requirement. This means that the idle mix screws will most likely be turned inward more than if a single carb was used. Once you get everything adjusted you will probably never want to go back to a single carb setup. The duals just look so right in a Cobra, surpassed only by a set of Webers or Wayne's Stacks.

the way i have seen it done with vac secondaries is to wire the secondaries shut with both carbs and makes runs and adjust it to where it runs good with the primary jets. then you can add the secondaries into the equation and use the different springs to adjust when the secondaries open. you're supposed to go lighter and lighter spring pressure until you lose power then go to the previous spring. that's probably the best way to do it if you don't know anyone that is familiar with them. hope that helps