how do you set up 2x4 carbs

[SuperHart]

Even though this subject has been discussed before here are a few ideas. Bog is usually caused by the throttle plates opening too quickly without a sufficient fuel shot causing a momentary super lean condition. When air flow through the secondary venturiis picks up enough to siphon fuel the condition disappears. This is why nearly all auto manufacturers go with vacuum secondary carbs rather than mechanical ones. With vacuum secondaries, the secondary throttle plates open gradually as engine demand increases. If you get a bog with a vacuum secondary carb when you nail the throttle Holley suggests installing a stiffer spring in the vacuum pot. That goes for dual quads also. With dual quads you must have a hose connection between the two vacuum pots to balance them.

Nearly all dual quad intake manifolds have runners connected to equalize and balance the vacuum and mixture signal to all cylinders. If the dual quads are reversed like on the Ford FE motors, the primary throttle bores of the primary carb are near the center of the motor and do give an adequately balanced mixture to all cylinders. Therefore, you can set up your linkage in a progressive manner so the secondary carb opens later than the primary one. Here again, crisper throttle response without the bog.

When setting the idle mix on a dual quad setup remember the rule of thumb for setting one carb, the idle mix screws turned out 1 1/2 turns from lightly seated. Now that you have TWO carbs, the settings are usually around 1/2 that for a single carb, i.e., turn the mix screws out 3/4 turn from seated on all four idle screws. The idle speed screws are also set the same way and are usually 1/2 the number of turns out. It does take some fiddling to get things right but the end result is worth the effort.

[Argess]

Originally Posted by SuperHart

Even though this subject has been discussed before here are a few ideas. Bog is usually caused by the throttle plates opening too quickly without a sufficient fuel shot causing a momentary super lean condition. When air flow through the secondary venturiis picks up enough to siphon fuel the condition disappears. This is why nearly all auto manufacturers go with vacuum secondary carbs rather than mechanical ones. With vacuum secondaries, the secondary throttle plates open gradually as engine demand increases. If you get a bog with a vacuum secondary carb when you nail the throttle Holley suggests installing a stiffer spring in the vacuum pot. That goes for dual quads also. With dual quads you must have a hose connection between the two vacuum pots to balance them.

Nearly all dual quad intake manifolds have runners connected to equalize and balance the vacuum and mixture signal to all cylinders. If the dual quads are reversed like on the Ford FE motors, the primary throttle bores of the primary carb are near the center of the motor and do give an adequately balanced mixture to all cylinders. Therefore, you can set up your linkage in a progressive manner so the secondary carb opens later than the primary one. Here again, crisper throttle response without the bog.

When setting the idle mix on a dual quad setup remember the rule of thumb for setting one carb, the idle mix screws turned out 1 1/2 turns from lightly seated. Now that you have TWO carbs, the settings are usually around 1/2 that for a single carb, i.e., turn the mix screws out 3/4 turn from seated on all four idle screws. The idle speed screws are also set the same way and are usually 1/2 the number of turns out. It does take some fiddling to get things right but the end result is worth the effort.

Good post. I run twin 600 cfm vacuum Holley's and except for one time when I had an internal carb leak, all plugs look the same indicating the same mixture. I do believe the front one primary being close to center does help.

The front carb opens approx 30% before the rear one but due to the linkage being more agressive on the rear, they both wind up at WOT at the same time.

I havn't found a need to have the transfer slot open at all. If I try for that, my idle is way too high even with the secondaries adjusted just shy of sticking.

I use the lightest vacuum springs avialable (but not the shorty ones) with no trouble.

And Superhart is right on with teh two carbs delivering twice as much fuel as required through the idle passage when at idle. I first used two generic carbs, and really couldn't get the idle to peak with the screws. ALl the way in seemed to be almost right. GUess some fuel must ahve been drawn through the transfer slots even though they wern't uncovered. Restricting the idle paassages in the primary metering block is the answer, and I had that done and wound up with all 4 screws about 3/4 turn out.

Now all that doesn't help if your carbs are mech secondaries. I would guess tghe recommendations to switch to vacuum units is not popular. Does this bog occur when you give it part throttle (primaries only open)? IS the bog gone if you give it WOT at higher rpms? If that is the case, it may be of note that I can only give my car 1/2 throttle in 2nd gear without fear of the losing traction. So you may be able to adjust your driving style for a while until you decide what to do.