Which carbs... Zenith-Stromberg or Dellorto? And if Dellorto, what throat and choke sizes?
From what you describe, it does sound like you're fighting an air leak. There's no simple "They all do that" answer, yours is a fluke, you're just going to have to find the leak and remedy it.
The obvious possibilities are the manifold to head joint, and the soft mount O-rings between the carbs and the manifold. But don't overlook any vacuum hose connections. Make certain hoses are secure, and that they haven't dried out and cracked/ split. If you have disconnected any emissions vacuum hoses, or the distributor vacuum retard hose, make certain you have plugged the vacuum port spigot.
When you spray the brake cleaner up under the rear carb, are you aiming it more at the manifold to head joint, or at the carb soft mounts? Or just spraying for effect, everywhere? Use the slender red tube wand, and be very specific about where you aim it.
At the manifold to head, make certain the gasket didn't get folded over or torn. A light smear of Hylomar on 'both' sides of the gasket is a good thing.
The soft mounts can leak in several ways. It's necessary to manage a number of loose parts during installation, and gravity is not your friend. If one of the O-rings slipped out of position during installation, that could produce a significant air leak. It's normal procedure to oil O-rings, but I've found that a light smear of Hylomar helps hold the O-rings in place during installation, and helps promote a secure seal. Hylomar never sets up hard, so it won't impair the O-rings ability to move and flex with the soft mount long term.
Even if you get all the soft mount parts installed without displacing any, then over-tightening the joints can either crush the O-rings, or cause them to squirt out of the joint. Either case can cause an air leak. It's critical that the soft mount joint NOT be tightened down hard... metal to metal. It's important to use the springs between the nuts and carb flange... either the rubber grommet type, or the steel coil spring "Thackeray Washer" type. Then tighten the nuts evenly until there is a uniform 0.040" (1 mm) gap on either side of the soft mount's central plate... ie, between the manifold and the soft mount, and between the soft mount and the carb flange.
Hylomar comes in a number of 'flavors', and the best one for this job is Hylomar Advanced Formula. In some markets, it's called Hylomar Solvent Free. It's slow to set-up, so it spreads easily and provides ample working time. The version sold in North America by Permatex is a re-brand of Hylomar Racing Formula. Think fast pit stops. It sets up very quickly and is aggressively tacky. That makes it a royal pain to spread, and it doesn't provide enough time to assemble the soft mounts & carbs. In North America, you can get 'Advanced Formula' on Amazon, from Pegasus Racing, or from Valco Cinncinati... http://www.valco-cp.com/hylomar.htm Valco calls it "Hylomar Solvent Free" and/or "Hylomar Blue, Medium". They also have an aerosol Hylomar, but I've not tried it.
Good luck, and Regards,
Last edited on 02-28-2017 09:54 pm by Esprit2