Fuel bubbles?

Clothespins won't get you anywhere; that's an old wive's tale. Metal fuel lines running directly above the exhaust manifold or cylinder head will move you in the wrong direction (more heat transfer to fuel through line).

Fuel line reroute is a good start; try the slant-6 fuel line mod for better insulation of the fuel against engine bay heat. Another thing you may want to do to combat the tendency of hot weather to aggravate conditions like this is to install a vapour-return system. It uses a 3-nipple fuel filter (Wix 33054 or NAPA Gold 3054, Purolator F20030); the 3rd is ¼" and has an 0.060" orifice to prevent bleeding off useful fuel pressure. You run a new vapour hose from that 3rd fitting all the way back to the fuel tank.

Also make sure the carburetor's in top condition. New float is a good idea; the Holley 1920 carburetor has a nitrophyll (foam) float that absorbs fuel over the years and grows heavy. Float is a Walker #100-14. Get the rebuild kit from www.daytonaparts.com , they have a better and more flood-resistant inlet needle/seat design. Carburetor operation and repair manuals (and some training movies) are posted here for free download; also see the threads linked off that post regarding 1920 dial-in.