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Synchronizing dual carbs can be a bitch, and I've found that one problem is one or the other will start to leak (overflow). I don't know if it's float adjustment or a fuel pressure difference

In a closed system (i.e. with both fuel chambers full and the needle and seats closed) hydraulic (fuel) pressure is equal at all points so that's not likely the problem with one carburetor overflowing; it's almost certainly assuming correct float levels, either a leaking needle and seat or a sunken float, or less likely a cracked casting on the inlet (Rochester used a poorer quality alloy than Carter, and generally thinner walled castings!).

Following either Rochester's or Packards procedure (both more or less the same) for setting up the carburetors can get you extremely close to "spot on" as long as you pay attention to any slop in the connecting rod link between the carbs. Carefully done, all you should have to do is adjust the idle speed (front carb only), and tweak the idle mixture screws in the sequence recommended. Setting up the fast idle speeds is a bit of a pain because you can only access the screws with the throttles wide open (meaning, when the engine isn't running), thus it's a bit of a trial and error process. Different fast idle cams and recommended speeds on front vs rear carbs. Don't forget that the dual Rochesters are quite different between 1955 and 1956, though I'd guess interchanged in pairs would give reasonably satisfactory results.

I've enclosed a copy of the Rochester installation instructions. The only real difference between them and Packards relates to the idle speed, I believe Packard recommended 450 rpm in drive but I've not been successful in getting a good smooth idle at that low a speed, I find about 480 rpm about ideal.

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You learn something new every day around here!

Any more pics, AL?

I have one question to ask about the Pacakrd 374 with dual four barrel carbs. What is the cfm rating on each of the carbs?



John F. Shireman

55 and 56 4GCs were different. IIRC, 55 is 375CFM and 56 is 425CFM. Both could be +- 25CFM. Today, one would use a pair of E-carb 500 CFM AVS (adjustable valve 2nd-ary).

Add on edit: One more thing to remember about the 2x4 intake is that it is a 360-degree design, not 180-degree like the 1x4. That means that all 8 cylinders "see" all 8 throttle bores, or at least all 4 idle circuits at the same time. That adds to the complexity of tuning across the board.

Craig

John, the 55 and 56 dual Rochesters are quite different. The 55 has an antipercolator valve on the float chamber, in 56 it was an internal vent. In 56 they added a set of air velocity-activated baffle valves in the secondary that only allowed the secondaries to function when there was sufficient air flow. The 55 and 56 duals had different main jet sizes, and as well the single Rochester 4 bbl was jetted differently that the 2 x 4 Rochesters. For example, the single Rochester-equipped 5680 had primary and secondary main jets of 0.052 and 0.065, whereas the dual setup had 0.048 and 0.054. I believe the corresponding numbers for the 55 Caribbean were 0.050 and 0.058.

Craig, you're certainly right that setting 4 idle mixtures is a bit tedious and I really don't know if the GM or Packard-recommended sequence makes it any easier or not. But simply getting easy access to the mixture screws is also a pain, especially the rear left.

Any multi-carb setup is a PITA.

That's why a Rochestor QJet is so brilliant of a design, before computer controlled FI, of course.

Be that as it may, nothing looks cooler than trips or 2x4. I have a 2x4 for my Panther and that's what I'll run when the time comes.

In the meantime, on my current hot rod, I have a QJet. See:

originalho.com/1963LeMans.html

Scroll down a couple of pix to see the air cleaner covered version. Nobody complains about how that looks.
Craig

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Or, as I think i said earlier, "a bitch." Having only personal experience with a marine setup, albeit on an automotive V8 using Carter 1bbl side-draft units, I can't give a lot of technical detail.

However, I recall, (how I wish i still had the Cars and Parts article)-- it dealt partly with the difficulty of keeping the dual quads properly set up and synchronized on the the milestone Rivieras when they came out with the 64-'65 425 ci "GS" Rivs. It said that a few went on to twist off cranksshafts. Reason being (they suggested) was some very ill-balanced carbs.

Evidently, it takes some expertise to get these right.

Once again, tell me I'm a dreamer if I'm dreamin', but is not there some danger of serious consequences from poorly synchronized multiple carbs?

Maybe you're right, Craig, unless you want the look to be "just so," you can get the job done with one nice big 4-barrel.

Thanks for the pix on your project. Will you keep us posted here or on the Panther site? BTW, was that '63 Tempest one of those that originally featured the rear transaxle? Thank you for rescuing it, if so.

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Yes, it has an IRS rear transaxle. The torque converter sticks out behind the rear axle in front of the gas tank. It also has the "rope" drive which is a 5/8" steel braided line enclosed in a small rectangular "tube", allowing a flat passenger floor. On this model, the starter is in the front attached to the bell housing, like normal. On the 421 Super Duty ones, the starter was in the back along with an extra 2-speed tranny (for 4 forward gears). The starter would twist up the "rope" drive a couple of turns before causing the engine (12:1CR, IIRC) to spin.

It's an interesting car from an engineering viewpoint, just like the T-L Packards, which is why I like it. Other people who've seen it also like it a lot.

It has a 2.73:1 rear gear, so it shifts out of first at about 95MPH. Recently on a 65 mile drive (some in-town, some hill, mostly freeway), it got 19.4MPG. No high gear lock up on the converter, though.

Craig

Bitchin'! Can I say that here? I believe the rear trans setup was strictly for f/r weight distribution.

No wonder you're having trouble getting to the Panther project! Taking off after a Poncho with a tranny in the caboose! ( Just needlin' you. I respect your work ethic.)



I know, I know, I'll shut up now!

Back to AL and his carbs.

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1956 Rochester 4GC's are 486 cfm, the '55's were probably around 425 cfm.